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Int. J. Mol. Sci. 2018, 19(12), 4089;

Methodology of Drought Stress Research: Experimental Setup and Physiological Characterization
Department of Plant Physiology and Biochemistry, St. Petersburg State University, St. Petersburg 199034, Russia
Department of Biochemistry, St. Petersburg State University, St. Petersburg 199904, Russia
Department of Bioorganic Chemistry, Leibniz Institute of Plant Biochemistry, Halle (Saale) 06120, Germany
Institute of Analytical Instrumentation, Russian Academy of Science, St. Petersburg 190103, Russia
All-Russia Research Institute for Agricultural Microbiology, St. Petersburg 196608, Russia
Department of Genetics and Biotechnology, St. Petersburg State University, St. Petersburg 199034, Russia
Department of Scientific Information, Russian Academy of Sciences Library, St. Petersburg 199034, Russia
Correspondence: [email protected] (A.F.); [email protected] (L.A.W.); Tel.: +49-(0)-34555821350 (A.F.); +49-(0)-34555821301 (L.A.W.)
These authors contributed equally on the manuscript.
Received: 23 November 2018 / Accepted: 14 December 2018 / Published: 17 December 2018


Drought is one of the major stress factors affecting the growth and development of plants. In this context, drought-related losses of crop plant productivity impede sustainable agriculture all over the world. In general, plants respond to water deficits by multiple physiological and metabolic adaptations at the molecular, cellular, and organism levels. To understand the underlying mechanisms of drought tolerance, adequate stress models and arrays of reliable stress markers are required. Therefore, in this review we comprehensively address currently available models of drought stress, based on culturing plants in soil, hydroponically, or in agar culture, and critically discuss advantages and limitations of each design. We also address the methodology of drought stress characterization and discuss it in the context of real experimental approaches. Further, we highlight the trends of methodological developments in drought stress research, i.e., complementing conventional tests with quantification of phytohormones and reactive oxygen species (ROS), measuring antioxidant enzyme activities, and comprehensively profiling transcriptome, proteome, and metabolome.
drought stress; drought models; drought tolerance; oxidative stress; phytohormones; polyethylene glycol (PEG); stress markers

1. Introduction

Being a natural climatic feature, drought occurs in almost all climate zones with varying frequency, severity, and duration, and is one of the most deleterious factors of environmental stress [1,2]. Indeed, even a short-term water deficit results in essential annual losses of crop yields [3,4], impeding sustainable agriculture all over the world [5,6,7]. Due to oncoming climate changes, the frequency and duration of drought periods will increase, making this factor one of the most important threats of the current century [8,9].
In the context of agriculture, drought is defined as a period of below-average precipitation [10], when the amounts of available water in the plant rhizosphere drop below the limits required for efficient growth and biomass production [11]. Such a soil water deficit can be persistent in climate zones characterized by low water availability, or by intermittent and unpredictable water supply during the vegetative period [12]. Because of this, drought is the major environmental stressor, affecting the plant’s growth and development by disrupting its water status [13]. This dramatically affects all key physiological processes, such as photosynthesis, respiration, and uptake of mineral nutrients [14,15]. First, drought compromises stomata function, impairs gas exchange, and leads to overproduction of reactive oxygen species (ROS) and development of oxidative stress [16]. Second, water deficit inhibits cell division, expansion of leaf surface, growth of stem, and proliferation of root cells [7]. In concert, all these factors dramatically reduce plant productivity and might lead to the death of drought-sensitive plants upon prolonged exposure to drought [17].
At the quantitative level, water deficit in the environment can be characterized by a decrease of soil water potential (Ψw) [18]. According to the van’t Hoff equation, it indicates a decrease in free energy of substrate water that makes water uptake from the medium under these conditions thermodynamically unfavorable and loss of water by the plant more probable. Values of Ψw from 0 to −0.3 MPa are characteristic for well-watered plants, whereas values below −0.4 MPa correspond to moderate water stress, and potentials of −1.5 to −2.0 MPa represent severe stress and permanent loss of turgor [19]. However, these values vary among species and drought models. They are based on experience with seeds and seedlings, which are commonly more drought tolerant. Thus, in our experience, Ψw values of −0.3 to −0.8 MPaare more typical for experimentally useful, i.e., recoverable, moderate drought stress in plants beyond the seedling stage (v.i.). In general, leaf Ψw can be determined by several approaches. In the easiest but most reliable way, Ψw can be addressed by the gravimetric method [20]. It can also be accomplished with a Scholander pressure chamber and thermocouple psychrometer [21] or tensiometer [22]. Thermocouple psychrometry is one of the most popular methods, and is usually accomplished with press saps or freeze-thawed leaf disks [23]. Recently, a new method was proposed for determination of Ψw in leaf cell apoplast, relying on the measurement of photosynthetic CO2/H2O gas exchange [24].
It is important to mention that not only the degree of Ψw decrease, but also its duration, can affect the plant organism [25]. Therefore, water stress often develops upon minimal reduction of soil Ψw. To avoid this scenario, plants adopt various strategies to prevent water loss, to preserve water supply even under reduced Ψw, and to sustain periods of unfavorable water regimen accompanied by low water content in tissues [10]. These drought-induced alterations can affect plant morphology, physiology, and biochemistry in degree -depending on plant species, developmental stage, and duration and severity of drought [4,6,7,26,27].
The main strategies employed by plants to sustain water deficit are (i) drought escape, (ii) drought avoidance, and (iii) drought tolerance [28]. Generally, all three strategies impact the development of the state known as drought resistance, which can be defined as the ability to maintain favorable water balance and turgidity under drought conditions. In the escape strategy, plants complete their life or growth cycle before the impact of drought causes harm, i.e., they use a seasonal response [4]. The strategy of drought avoidance relies on enhanced water uptake and reduced water loss, whereas drought tolerance is mediated by osmotic adjustment, extension of antioxidant capacity, and development of desiccation tolerance [28]. On one hand, these strategies represent different steps of drought response (Figure 1). On the other hand, they might indicate different climatic and ecological specializations of plant species [29]. This concept of stress avoidance and stress tolerance proposed by Levitt [30] provides insight into plant responses to a relevant decrease of Ψw at the cell and organism levels [10].
As can be seen from Figure 1, the first response of the plant organism to drought as a drought-resistance strategy relies on avoiding water deficit [31] by maintaining tissue Ψw by increasing water uptake or restricting water loss [32]. At the early steps of drought response, it is mainly achieved by stomata closure, triggered by abscisic acid (ABA). However, according to Muller et al. [33], the rapid expansion of roots and young leaves (as a major C sink) is affected earlier and more intensely than photosynthesis (C source); accordingly, root growth is enhanced to provide sufficient water uptake under drought conditions. These avoidance mechanisms can secure the maintenance of crop plant productivity during short-term periods of water deficiency [18]. However, this is achieved at the price of reduced CO2 uptake, a dramatic drop in photosynthesis rate, and redirection of assimilate transport for enhancement of root growth [33,34]. When drought persists for a long time and adaptive capacities of the avoidance strategy are not sufficient to sustain plant growth and productivity, other mechanisms might be involved. At this step, mechanisms such as accumulation of compatible solutes and protective proteins (so-called metabolic adjustment), cell wall hardening, ROS detoxification, and metabolic changes are involved in establishing drought tolerance [10].
Thus, plant drought resistance is a complex process that requires a global view to understand its underlying mechanisms. Obviously, the majority of molecular events triggered by a decrease of tissue Ψw cannot be unambiguously attributed solely to avoidance or tolerance strategy. Therefore, a complex multilevel regulatory network controlling plant adaptive responses to drought stress is required. Studies of responses to water deficit such as stomata closure, expression of stress-specific genes, accumulation of osmolytes, and up regulation of antioxidant systems recently made considerable progress [17,35,36,37,38]. It was shown that the mechanisms underlying stress resistance are crucial for plant survival and are associated with significant changes in the patterns of metabolites and proteins [10,15,35]. Hence, analyzing the changes in plant metabolome and proteome associated with the onset of drought might be an important step in breeding and engineering plants with increased drought resistance [15,35] or developing plant protectants against drought stress [39].
Recently, Wang et al. [15] comprehensively reviewed drought-related effects on the plant proteome, including changes in signal reception and transduction, ROS scavenging, osmotic regulation, protein synthesis/turnover, modulation of cell structure, and carbohydrate and energy metabolism. These functional patterns of plant response to drought gave access to understanding of fine mechanisms underlying the process of stress tolerance. Apparently, for successful study of plant responses to drought stress under experimental conditions, reliable and adequate stress models are required. Accordingly, various drought models have been established (Table 1). However, the available information is often complex, incomplete, and inconsistent. A comprehensive literature search for drought tolerance research shows great variability and inconsistency in the experimental designs and methods for stress characterization [15]. Therefore, here we systematically address different experimental setups for establishing drought stress models and consider physiological and biochemical methods for their characterization.

2. Experimental Models of Drought Stress

Despite a large variety of available drought models, according to their basic setup, all of these techniques can be classified as soil-based, aqueous culture–based, or agar-based. The common feature of all drought stress models is reduction of the water potential in the substrate or medium surrounding plant roots. However, individual methods have different applicability limitations and vary in terms of the scientific questions they can address. Therefore, the advantages and disadvantages of each model need to be carefully considered prior to experiment planning.

2.1. Soil-Based Drought Models

The obvious advantage of this model strategy is the close similarity of experimental conditions to actual drought in nature and agriculture. In this case, the decrease of soil Ψw is established by gradual decline or immediate interruption of plant watering [40]. Such models adequately simulate short-term drought, which represents the most frequent case in the European agricultural practice due to varying weather conditions [41,42]. However, difficulty controlling the substrate Ψw represents an essential limitation of this approach. Indeed, in this experimental setup, the severity of drought stress is determined by the rates of water evaporation from the soil surface and consumption by the plant [43]. As these cannot be defined by the researcher and depend on multiple factors, reproducibility and predictability of such experiments are always questionable. Moreover, as the rates of water consumption and evaporation are relatively high, this model does not allow probing long-term drought responses, such as accumulation of osmoprotective metabolites or proteins and cell wall modifications [10]. Therefore, many important aspects of plant drought tolerance and adaptation to low Ψw, such as, for example, accumulation of osmoprotective proteins and hardening of cell walls, can be overlooked in this experimental setup, although using large and deep pots might improve this situation [10].
Despite the above-mentioned problems, several improvements can be made to increase the reproducibility and reliability of soil models. First of all, in this type of experiment, the size and structure of soil particles, as well as their water capacity, should be taken into account. Thus, to achieve moderate (i.e., less severe) drought conditions, in an optimized variant of this model, plants are grown in foil-sealed vessels to prevent water evaporation from the soil surface [5]. Thereby, each pot can be equipped with a piece of tubing inserted into the soil to facilitate rewatering of plants. Due to the water supply, in this model, water deficit can be increased gradually, making it possible to address long-term plant responses to drought [44]. Moreover, stability of the water regimen can be improved by an automated irrigation system.
Recently, Todaka et al. [40] introduced an automatic irrigation system to relay monitoring of actual water content in soil. Using this approach, the authors proposed a drought model able to ensure the desired values of Ψw (−9.8, −31.0, and −309.9 KPa). However, this system failed to reproduce the conditions of severe dehydration. Although the optimized method described above is reliable and reproducible enough, repeated measurements of leaf and soil Ψw are laborious and require large amounts of plant material, which are hardly available in long-term experiments under reproducible laboratory conditions. For example, such a restriction can be critical when mutants or transgenic plants are dealt with, in particular those with reduced stomata density or small leaf area [45].
An elegant way to avoid this complication is to culture mutant or transgenic plants in the same pot with the reference plants, e.g., the wild-type (wt) counterparts [10]. In this case, leaf Ψw determination can be limited to the reference (or wt) plants, which are commonly more suitable for assessing stress markers. The obtained results can be extrapolated to the mutants. In this case, both reference or wt and experimental or mutant plants would grow in the same medium and therefore be exposed to the same soil Ψw if they are planted in a suitable scheme and position. The best way to provide a quantitative characteristic of drought stress by this approach is to complement it with a measurement of soil Ψw at the end of the dehydration period. Analogously, this method can be applied to untreated and treated plants in assays for chemical drought tolerance enhancers or other phytoeffectors (v.i.) to be tested.
It is important to mention the setups that rely on inert substrate, such as vermiculite or perlite, as soil substitutes. The advantage of this approach is that the roots of experimental plants can be pulled out easily and without damage to investigate drought-related changes in water potential [46] or oxidative and metabolic responses [47] at the root level. Inert substrates are suitable for studying the effects of drought in legume–rhizobial nodule symbiosis [48]. On the other hand, the certain disadvantage is that watering, unlike soil culture, is carried out not with water, but with a nutrient solution, so the impact of drought by cessation of watering is accompanied by the appearance of another stress factor, i.e., a deficiency of mineral elements.

2.2. Drought Models Based on Hydroponic Aqueous Culture

Despite the high relevance of soil-based drought models because of their similarity to natural conditions, they all have a common intrinsic limitation: the difficulty of adequately controlling Ψw in the root microenvironment. However, this is critical when a precise definition of substrate Ψw is required, as in multiple or long-term experiments comparable over months (and seasons). Therefore, the models, based on aqueous hydroponic culture with predictably decreased Ψw of nutritional solution, might be advantageous for such applications. The easiest way to reduce the Ψw of growth medium is to decrease its level in pots and partially exposure roots to air, as was shown for lettuce by Koyama et al. [49]. To simulate severe dehydration, plant roots can be left under air for up to eight hours [50], allowing the severity of simulated drought to be defined by the duration and repetitions of the dehydration process. This approach is based on the fact that Ψw of leaves, at least to some extent, corresponds to the index of water availability for plants, which in turn depends on water potentials of soil and plant roots [51]. Thus, experimentally affecting the Ψw of roots influences the Ψw of leaves as well. When using this approach, however, one needs to keep in mind that the degree of dehydration and kinetics would strongly depend on air humidity. Further, it is important to remember that in this case the plant response is dependent on root distribution (e.g., long vs. short roots).
Despite the ease of the above approach, most often desired Ψw values of plant rhizosphere are obtained by supplementing nutrient solutions with osmotically active substances (osmolytes, which reduce available water), taken in calculated concentrations. This approach is based on the simulation of drought by application of osmotic stress, i.e., increasing the medium osmotic pressure compared to that of plant tissues [52]. Similar events occur in soil when the water content decreases (due to evaporation and absorption by the plant) and the concentrations of solutes grow, resulting in an increased osmotic component of the water potential [53]. Thus, the described setup corresponds well with natural drought. This strategy allows precise adjustment of Ψw and efficient monitoring of its magnitude, resulting in high accuracy, reproducibility, and interexperimental comparability of acquired data [54]. However, when working with this kind of drought model, selecting an appropriate osmolyte requires special attention. Thus, low-molecular-weight osmolytes (e.g., sugar alcohols and sodium chloride) routinely used in early studies [55] demonstrate strong negative side effects when applied in experimental drought. Indeed, these compounds easily penetrate cell walls and plasma membranes, increasing intracellular osmotic pressure and leading to plasmolysis [56]. Any salts also change ion titers and distribution in plants, affect ionic strength, and trigger the process of ion transport. On the other hand, nonionic carbohydrate-related osmolytes (e.g., sorbitol and mannitol) are readily involved in cellular metabolism themselves, and thus might directly affect the results of the experiment [56], as they are often toxic to plants [57]. They can also increase mold growth under commonly nonsterile conditions. Because of this, the use of biologically inert polymeric osmolytes is preferable and advantageous [58]. Therefore, currently, drought stress models rely on presumably nonpermeable high-molecular-weight osmolyte polyethylene glycol (PEG) with an average molecular weight of 6000 Da or more [55,59].
It is well documented that PEG effectively decreases medium Ψw, thereby disrupting absorption of water by plant roots [60]. In terms of this approach, 5–20% (w/v) [61] or even 40% (w/v) [62] PEG in growth medium enables a stabile decrease of Ψw during any desired period of time [63]. Importantly, PEG-based aqueous models allow the setup of recovery experiments by transfer of stressed plants to PEG-free nutrient solution or exchange of the PEG solution [10]. Therefore, PEG-based models of drought stress represent the method of choice in molecular biology and plant protectant studies and screening experiments [64]. One issue yet underexplored in the PEG model is the complexing ability of PEGs on metal ion species and thus the altered availability of the various ions for the plant. However, also under drought conditions, ion availability eventually changes and decreases.
One of the most promising applications of aqueous PEG-based models of osmotic stress is screening for potential drought-protective compounds. Substances that influence plant performance (without being plant protectants against biotic stress, e.g., from pathogens) in agrochemistry are defined as phytoeffectors and include drought stress tolerance enhancers. Phytoeffectors are able to prime crop plants against short-term drought and ensure that their productivity is sustained under drought conditions with spatiotemporal control, largely independent of the crop species or variety used. Such effects were described for salicylic acid and its derivatives [65], as well as for various fungicides of the triazole [66] and imidacloprid [67,68] families. The drought-protective effects of small molecules on a plant organism are usually mediated by inhibition of enzymes, involved in plant response to stress, as was described for poly(ADP-ribose) polymerase (PARP)in the beginning of this decade [68], although later at least direct involvement of PARP appeared doubtful [69]. If a molecular target for drought stress effects is known, and ideally the active site too, methods of computational chemistry like virtual screening and molecular docking approaches [70] allow virtual screening of thousands of structures with millions of conformers. The most promising candidates for wet lab testing can thus be identified.
For rapid screening of such compounds, a reliable model based on a Lemna minor culture was recently developed in our group [68]. This technique (Figure 2A) relies on a microtiter plate format and assumes treatment of plants with PEG6000 or PEG8000 supplementing the growth medium in the presence and absence of potential phytoeffectors. After a 24 h stress period, plants are transferred to a PEG-free medium, and stress recovery is monitored for further 48 h, before the protective effect is assessed by attenuation of growth inhibition via measurement of leaf peak area increase by means of a 2D-photodocumentation visualization system.
The Lemna system has several advantages over classical spraying systems: Plants are all clones, reproducing by budding, and they are small and can be grown in microtiter plates (6-, 12-, or 24-well format) under sterile conditions. The small scale allows medium-throughput screening with small amounts of compounds. Most importantly, these can be applied in a concentration-dependent manner to the multiwell plate well (while spraying or dumping delivers only uncertain amounts to plants), and both root and leaf uptake is ensured. The leaves are flat and 2D phenotyping is easily done with the respective software [68]. For better reproducibility, initial root length should be unified, and until termination of the experiments, plant growth should not be limited by well size.
Despite their wide use, PEG-based models have some intrinsic limitations that need to be taken into account when planning experiments [63]. First, PEG-containing nutrient solutions are characterized by high viscosity, which compromises diffusion of oxygen to the roots, especially in deeper vessels, and can cause hypoxia [10]. To prevent the development of hypoxia, additional aeration needs to be provided for plants grown in PEG-containing medium. For this, air is continuously supplied by pumps through silicone tubes connected to the culture vessels [71]. Although this approach can be easily established for larger plants (as it was done in our lab with B. napus; Figure 2B [72]), small model plants like Arabidopsis, typically grown in small vessels on large scale for highly replicated biological experiments, cannot be supplied with air individually and are typically grown under hypoxic conditions [73]. Small and flat vessels like the wells used in the Lemna system [68] are usually not prone to such problems.
Another possible issue is absorption and accumulation of PEG with molecular weight 4000–8000 Da in plant roots, which might result in damage [74]. The accompanying partial root dysfunction might impact leaf dehydration in an unpredictable way. Thus, stress responses observed in plant shoots are only partly related to osmotic stress applied by PEG solution. The impact of PEG-related root damage on these responses is difficult to estimate, but is obviously increased when plant transfer on PEG-containing medium is accompanied with wounding of roots, which should be avoided [75].

2.3. Agar-Based Drought Models

In general, growing plants in agar allows avoiding or reducing development of the hypoxic state. As this is especially relevant for Arabidopsis, agar-based models are widely used in plant biology, and specifically in drought stress experiments with Arabidopsis thaliana seedlings [76]. Thus, van der Weele et al. proposed an agar-based PEG infusion model relying on saturation of solidified agar (filled in Petri plates) with Murashige and Skoog medium supplemented with PEG8000 during two days [77]. Unfortunately, PEG affects the solidification of agar, therefore adding it directly to the agar medium under preparation is not advisable [73]. Because of this, generating a desired Ψw of agar medium is achieved by diffusing PEG from a concentrated overlay solution into preformed, solidified agar. Adjusting the concentration of the overlay solution, the equilibrium in Ψw between aqueous overlay medium and agar is achieved after 24 h of diffusion [76]. After decantating the PEG solution, seedlings can be transferred to the now PEG-containing agar medium (stress application), and eventually plants can be replanted to a PEG-free one after a defined treatment period (recovery).
Due to a constant character of Ψw, the agar-based PEG infusion model is advantageous compared to those based on soil or (nonaerated) aqueous culture. Thus, the Ψw of seedlings can achieve equilibrium with the agar medium during treatment time. Under soil drying conditions, this is impossible, as soil Ψw changes continuously along with water evaporation and consumption by the plant. On the other hand, due to PEG interfering with root integrity [78], this equilibrium is also hardly achievable in aqueous PEG solutions (especially when PEG concentration is high). Thus, the agar-based model system currently is an ideal choice to address dehydration avoidance and mechanisms of dehydration tolerance [10]. Most commonly, PEG concentrations in the agar medium do not exceed the values needed for medium to medium-high drought stress, i.e., Ψw < −1.2 MPa [57]. However, based on the solubility of PEG8000 in water, the agar-based infusion model can be established in a broad range of overlay medium Ψw values from −0.47 MPa to −3.02 MPa [79].
The agar-based PEG infusion model was successfully applied to different plants and fungi [75]. Further, a similar setup (10% w/v PEG6000 in the overlay medium) was used to probe the effect of water stress on rape oilseed (Brassica napus) germination and seedling development [80,81]. An essential limitation of the setup, originally proposed by Verslues and co-workers [10], was its applicability to only the early steps of plant ontogenesis, seed germination and seedling development. Thus, this method was inapplicable to mature plants, and corresponding stress responses characteristic of later stages of ontogenesis could not be addressed.
Therefore, to extend the agar-based approach to mature organisms, we modified the method of Verslues and co-workers to 5-to-7-week-old A. thaliana plants [35]. This setup combined germination on agar in truncated polypropylene tubes, growth for 4–5 weeks in aqueous culture, and transfer to agar medium preinfused with PEG8000 solution with a polymer concentration corresponding to the targeted substrate Ψw (Figure 2C).
In general, our observations confirmed earlier data indicating higher sensitivity of mature plants to drought compared to seedlings [10], although stress tolerance varies essentially between species. Thus, in contrast to seedlings, application of Ψw below −0.6 MPa led to reduced survival of plants over a period of 7 days, whereas a drop to −0.4 MPa was accompanied by significant alterations in plant metabolome and proteome, indicating metabolic adjustment and changes in redox metabolism [35]. Soybean turned to be more resistant to osmotic stress applied in an agar-based model and successfully survived osmotic stress applied by 8 and 16 (w/v) PEG for two weeks for pre- and post-flowering treatments [82].
To summarize, compared to other setups, the agar-based PEG infusion model has two fundamental advantages. First, it provides a stable and reproducible decrease of substrate Ψw that cannot be achieved with the soil-based model. On the other hand, compared to models based on aqueous culture, it has higher relevance to the conditions of a real field, as it relies on a solid substrate. Second, the agar-based model allows precise Ψw setting in plant rhizosphere without accompanying hypoxia and PEG-related root toxicity. It is important to keep in mind, however, that this setup does not allow a direct extrapolation of drought effects to the field or ecosystem due to the model’s simplicity, which doesnot consider water gradient in soil and heterogeneity in terms of water holding capacity. For fast (pre-)screening of phytoeffectors, especially if only small amounts of test compounds are available, the Lemna minor aqueous system has advantages [68] but must be complemented later by the solid medium method for validation [35].

3. Physiological and Biochemical Characterization of Drought Stress

Adequate and correct application of an experimental drought stress model requires comprehensive characterization at the levels of physiology, biochemistry, and molecular biology. These experiments deliver objective information on the actual functional state of the plant organism and its metabolic response to stress. This block of data is necessary to confirm the stressed state of experimental plants (i.e., development of stress response) and to estimate the severity of stress-related alterations. Accordingly, a panel of physiological and biochemical markers of drought stress ideally accompanies any study relying on modeling setups. Importantly, these markers can be used for dynamic characterization of plant adaptive responses throughout the whole experiment, i.e., acquisition of stress kinetics (Table 2). Thus, ideally, selection of the markers needs to consider all steps of drought response, starting from drought perception. It is assumed that drought is recognized by roots, which send a chemical message to the shoot [83]. Abscisic acid (ABA) plays a key role in this signaling [84]. This effector is synthesized in response to hydraulic signals in vascular tissues and further transported to leaf epidermis cells. Resulting stomata closure results in suppression of xylem transport, decreased turgor, and root growth arrest [37].

3.1. Water Status and Photosynthetic Parameters as Markers of Drought Stress

One of the first detectable symptoms of drought is dehydration of plant tissues, which is characterized by a decrease of Ψw and loss of leaf turgor [6]. Due to its simplicity, low time expense, and robustness, the measurement of leaf water potential prior to sunrise is one of the most commonly used tests for this marker [85]. Although critical values of tissue water potentials are species-specific, Ψw of less than −0.8 MPa is commonly recognized as a sign of drought stress [86]. On the other hand, the degree of water loss can be reliably assessed by a decrease of leaf relative water content (LRWC) [87]. In the easiest and most straightforward way, this parameter can be addressed by the gravimetric method and calculating dry weight/fresh weight ratios [88]. Despite its simplicity, this approach yields highly reproducible data. An obvious disadvantage of this method is its destructive character, i.e., consumption of plant material for each determination [85]. In this context, a nondestructive technology based on automatic assessment of short-wave infrared irradiation reflected from the leaf surface might be a good alternative [85]. Another nondestructive approach relies on long-term phytomonitoring, i.e., continuous measurement of leaf transpiration, turgor, and xylem flow by means of nondamaging sensors attached to the plant [89].
One of the primary plant responses to dehydration is stomata closure, which is intended toprevent transpiration-related water loss, and is essential for the success of the drought avoidance strategy [90]. Similarly to dehydration itself, this parameter can be quantitatively characterized [91]. Experimentally, it can be done by measuring the rate of gas flow through a leaf surface or the electrical conductivity of the water film (of constant ionic strength) on the leaf surface [92]. Therefore, stomata conductance is usually expressed in mmol/m2/s [92]. Technically, such experiments are based on porometric measurements, i.e., determining times required for increased air humidity in an isolated chamber with a leaf inside [93].
Since stomata closure disrupts the supply of parenchyma cells with carbon dioxide, drought ultimately negatively affects the efficiency of photosynthesis by inhibiting carbon assimilation and light reactions [5]. In the simplest way, photosynthetic activity can be addressed by quantitative determination of pigments: chlorophylls (at least chlorophyll a) and carotenoids [94]. Thereby, decreased chlorophyll level is considered a symptom of oxidative stress and may be the result of pigment photo-oxidation and chlorophyll degradation [95]. Accordingly, as was shown in a comparative screening of barley genotypes, higher chlorophyll content was generally associated with higher drought tolerance [94,96]. This allows us to consider this indicator as an important marker of plant functional state under drought conditions.
Besides degradation of photosynthetic pigments, dehydration negatively affects the whole photosynthetic apparatus [97]. One of the most reliable markers of this process is decreased photosystem II (PS II) activity [98]. Both relative chlorophyll content and PS II efficiency can be easily quantified with pulse amplitude modulation (PAM) fluorometry [99,100]. Thereby, the ratio of minimum (background) and potentially maximum chlorophyll fluorescence (Fv/Fm) is interpreted as the maximum of PS II photochemical activity and might be considered as a reliable marker of PS II photoinhibition and one of the most important indicators of drought stress [101]. Importantly, the chlorophyll fluorescence is registered in vivo, thus it does not require sampling of plant material [102]. Interestingly, in some cases, drought does not cause any alterations of PS II activity. This result, observed with potato leaves, can be explained by photochemical quenching of excess light energy by increased photorespiration [103]. It needs to be taken into account that besides drought stress, the onset of senescence can underlie a decrease in Fv/Fm ratio [104].
In agreement with the described mechanisms, the features protecting the chloroplast photosynthetic machinery from oxidative damage might increase stress tolerance. This was illustrated in a comparative study of two B. napus cultivars grown for 3 weeks in aerated aqueous nutrient medium with Ψw of −0.6 MPa (18% w/v PEG8000) [105]. The developing stress could be recognized in both cultivars by a pronounced decrease in growth and photosynthetic parameters, including PS II activity and chlorophyll a content. However, the cultivar with higher leaf contents of chlorophyll a and carotenoids, as well as higher Fv/Fm ratios, demonstrated a clearly higher drought tolerance. Thus, it could be concluded that the quantum yield of photosynthesis and the chlorophyll a content could be effective selection criteria in screening for cultivars of crop plants with drought tolerance [105].

3.2. Changes in Phytohormone Patterns as Markers of Drought Stress

Plant response to environmental stress is a complex process, precisely tuned by multiple regulatory systems [12,106]. In particular, dehydration triggers activation of signal transduction cascades, including long-distance transport steps mediated by phytohormones [107]. Specifically, drought-induced stomata closure is regulated by abscisic acid (ABA) and relies on ABA-dependent signaling pathways [108]. Upon dehydration, ABA tissue content in Arabidopsis leaves can be increased up to 30-fold [107]. In a time-course study of the drought-avoidance response performed with Arabidopsis, early accumulation of ABA and induction of associated signaling genes coincided with a decrease in stomata conductance, as revealed by a panel of physiological, biochemical, and molecular biology methods [12]. Therefore, increased ABA in leaf cells represents a reliable marker of drought stress in model experiments [35].
Besides ABA, several other hormones and their interaction networks have an impact on the control of stomata conductance during water deficit. Thus, auxins, cytokinins, and ethylene are prone to inhibit the ABA-mediated stomata closure mechanism, whereas brassinosteroids, isoleucinyl jasmonates, and jasmonic and salicylic acids support the effects of ABA [109]. Jasmonic acid and its derivatives play a significant role in plant response to drought in terms of opening and closing of stomata [110], acting in an interplay with ABA and starting ABA signaling transduction [111]. In contrast to jasmonates and ABA, ethylene is involved in the stimulation of stomata opening via inhibition of nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase in the leaves of plants, responsible for the launch of ROS-dependent stomata closure pathways [112], but ethylene also induces senescence. Thus, despite their impact on drought response, the mentioned phytohormones have complex patterns of effects [107]. Therefore, their use as drought stress markers is hardly possible. Similarly, their potential to be applied as phytoeffectors in the field is limited. Apart from cost, bioavailability, and stability issues, it would require an extremely balanced mixture of suitable hormones, adapted in each case to the plant species, developmental stage, and status.

3.3. Metabolites as Markers of Drought Stress

Various abiotic stressors are known to affect the profiles of plant metabolites [113]. Indeed, the process of metabolic adjustment, i.e., accumulation of osmotically active and metabolically neutral solutes, such as different sugars, amino acids (predominantly proline and glycine), betaine, polyamines, and organic acids, under drought conditions is well documented [20]. Metabolic adjustment is the second step in plant adaptation to drought (after stomata closure) and is critical in maintaining the water status and physiological activity of plant cells, especially during relatively short-term drought [5,114]. To address the tissue contents of drought-protective metabolites, different methodological approaches can be employed. On the one hand, each group of metabolites can be analyzed individually (for example, analysis of betaine [115] and inositol [116] levels). On the other hand, entire profiles of primary metabolites can be addressed by comprehensive gas chromatography–mass spectrometry (GC-MS)-based hyphenated techniques, giving access to relative [117,118] and absolute [119] amounts of individual analytes. For a complete understanding of plant response to drought, an analysis of plant hormones and secondary metabolites can be equally essential. In this regard, Ahmed et al. reported upregulation of phenolic metabolites in the leaves of Gossypium barbadense L. under water deficit conditions [120], and Ma et al. demonstrated a drought-related increase of the expression levels of flavonoid genes and upregulation of leaf flavonoids in Triticum aestivum [121].
It is important to mention that accumulation of sugars in the background of reactive oxygen species (ROS) overproduction (usually accompanying plant response to drought) might result in enhanced formation of reactive carbonyl compounds (RCCs) and glycation of plant proteins [72,122], similar to the mechanism recently reported to occur under plant aging [123]. Additional in vitro experiments with peptide and protein models showed formation of various glycoxidative modifications of lysyl and arginyl residues [124,125,126,127] prospectively, with an impact on pro-inflammatory properties of glycated proteins [128]. Hence, these modifications might affect nutritional properties of plant-derived foods. Moreover, the processes of DNA damage and reparation (associated also with the PARP/PARG system [69]) can impact protein glycation as well [129,130].
Remarkably, metabolic adjustment in different plants has both common and species-specific features. Thus, some osmoprotective metabolites, like glycine betaine, are specific for certain plant species, e.g., sugar beet (Beta vulgaris), spinach (Spinacia oleracea), and barley (Hordeum vulgare) [131], while increased proline content, which is apparently a crucial and the most conserved response to drought, is characteristic for a wide range of plants [132]. Obviously, such metabolites can be used as nonspecific and species-specific markers of drought stress. It is important to remember that metabolic adjustment is efficient only on a relatively short time scale, whereas when drought persists for longer times, increased accumulation of compatible solutes can be energy- and resource-intensive for the plant. In cases of severe stress, when soil water content is largely depleted, metabolic adjustment may have only a small effect on water uptake, or even be detrimental by taking too many resources from the plants [18,133].

3.4. Protective Proteins as Markers of Drought Stress

Underlying the long-term adaptation of plant organisms to drought is a pronounced increase in the expression of drought-specific genes, such as Solanum tuberosum DS2 (StDS2) [134], late embryogenesis abundant (LEA) [135]. Accordingly, biosynthesis of a broad array of drought-protective proteins, predominantly chaperones, LEA proteins, and enzymes of antioxidant defense (referred to below in detail), is upregulated. Chaperones form the group of proteins involved in the formation and maintenance of the native protein structure [136], mostly represented by so-called heat shock proteins: the ubiquitous polypeptides, originally described with respect to a heat shock response, but actually involved in an array of stress adaptation responses [137]. Currently, special attention is being paid to the role of heat shock proteins in drought tolerance [138]. Xiang et al. found that overexpression of the heat shock protein Osnsp50.2 in rice leaf reduced water loss and increased resistance of plants to drought-related osmotic stress [139]. It was also shown that increased expression of chaperone-like proteins ERD10 and ERD14 in A. thaliana cells impacted the prevention of luciferase, alcohol dehydrogenase, and citrate synthase inactivation in firefly [140].
LEA proteins represent another class of polypeptides involved in adaptation to water deficiency. These proteins were discovered more than 35 years ago in a study of embryogenesis and germination of cotton seeds [141]. The key feature of the LEA proteins underlying their drought-protective properties is their high hydrophilicity [142]. These molecules are known to prevent mechanical damage of mitochondria, chloroplasts, and other cellular structures by forming a membrane-protecting shield, thereby preventing peroxidation of membrane lipids [111,143]. The constitutive expression level of LEA proteins can be considered as a marker of drought resistance. Thus, it was shown that more LEA genes are overexpressed in drought-resistant Gossypium tomentosum cultivars than drought-sensitive ones [144].

3.5. Oxidative Stress Associated with Drought

Water deficiency results in a disturbance of the balance between ROS (and RNS) generation and detoxification, triggering oxidative stress, and upregulation of ROS production under drought conditions is well documented (comprehensively reviewed by de Carvalho et al. [145]). Due to their high reactivity, ROS are extremely toxic and can damage proteins, lipids, and nucleic acids [146]. Under persistent oxidative stress, this damage can become irreversible and might lead to cell death. Indeed, excessive ROS production is a central process in response to infection (killing the intruder cells or tissues surrounding it).
Although ROS as singlet oxygen can be produced by the energy transfer from triplet chlorophyll to molecular oxygen [147] (Figure 3A), the main reason underlying overproduction of cellular ROS in response to plant dehydration is the overload of electron transport chains in chloroplasts and mitochondria due to overproduction of reduced forms of nucleotides [145,148]. Indeed, even under normal conditions, light reactions of photosynthesis are associated with continuous ROS production [149], and PS II is the main contributor to chloroplast photosynthesis [150]. The superoxide anion radical (O2) is formed on the electron acceptor side of PS II by electron leakage into molecular oxygen (Figure 3A). Due to the drought-related stomata closure and overload of electron-transport chains, the rate of this process is essentially increased [151]. The formed O2can be dismutated to hydrogen peroxide (H2O2), which can further yield highly toxic hydroxide radical (OH·), for example, by the Fenton reaction in the presence of certain transition metal ions [152]. On the PS II donor side, incomplete water oxidation also leads to H2O2 production. Dehydration affects the function of PS II, resulting in higher production of H2O2 and faster transformation into OH radical [145,150].
Another important source of ROS in chloroplasts is the Mehler reaction (Figure 3A), i.e., partial reduction of O2 to O2 (with subsequent formation of H2O2) by components of the PS I–Fe-S centers and reduced ferredoxin and thioredoxin. Under stress conditions, reactions of the Calvin cycle are inhibited by lack of CO2 due to the stomata closure. This situation provokes an overreduction of the chloroplastic electron transport chain, which results in a higher leakage of electrons to O2 in the Mehler reaction [145]. Importantly, the deficit of CO2 might result in enhancement of H2O2 production in peroxisomes, with photorespiration contributing over 70% of the total H2O2 production in C3 plants subjected to drought stress [153].
Mitochondria also can represent an important source of stress-related excess ROS. Normally, approximately 1–2% of the oxygen consumed by plant mitochondria is converted to O2 and H2O2. Underlying this increase in ROS production are complexes I and III of the mitochondrial electron transport chain, which can act as electron donors for molecular oxygen and enhance the generation of O2 and H2O2 [148]. It is assumed that the excess NADH produced during glycine oxidation in the photorespiratory pathway results in an overload of the mitochondrial electron transport chain [154]. Interestingly, the activity of alternative oxidase, and probably rotenone-insensitive NAD(P)H-dehydrogenase, is involved in detoxification of ROS under these conditions and contribute to plant drought tolerance [148].
In general, ROS production correlates well with the severity of drought stress [145]. This allows the use of some compounds associated with oxidative stress as biochemical markers of drought. ROS readily attack double bonds in polyunsaturated fatty acids, resulting in the formation of lipid hydroperoxides [155]. Consequently, shorter and reactive carbonyl products result from their breakdown, such as, e.g., malondialdehyde, known as a reliable marker of lipid oxidative damage [156,157]. The contents of these compounds increase in plant leaves under stress conditions and can be used as drought stress markers. Similarly, H2O2 tissue contents are often used for estimations of drought stress severity in plants, as this molecule represents the most stable and easily measurable form of ROS [158].
The mechanisms of plant drought tolerance necessarily include pathways that reduce ROS content in stressed cells. The most efficient antioxidant defense relies on the activities of specific antioxidant enzymes (Figure 3B). The enzymes of the ascorbate–glutathione cycle play a central role in detoxification of H2O2 under drought stress conditions [5,159]. Ascorbate peroxidase, the key antioxidant enzyme neutralizing H2O2 in plant cells, relies on ascorbic acid as a donor of electrons [160]. The resulting dehydroascorbate can be regenerated (i.e., reduced to monodehydroascorbate) by the reaction with NADPH catalyzed by monodehydroascorbate reductase [161]. The formed toxic monodehydroascorbate is rapidly reduced to ascorbic acid by dehydroascorbate reductase, parallel to the oxidation of glutathione to glutathione disulfide (GSSG). The subsequent regeneration of glutathione (GSH) is catalyzed by glutathione reductase, which plays a key role in maintaining the pool of reduced glutathione required for survival under stress conditions [161,162].
The ratio of reduced to oxidized forms of ascorbate and glutathione is crucial for maintaining a favorable redox status of living cells, being an informative indicator of plant stress adaptation capacity [162]. Therefore, addressing the expression or activity of antioxidant enzymes may be important in screening different plant species and cultivars for drought tolerance. The enzymes of the ascorbate–glutathione cycle were recently considered as targets for the engineering of transgenic stress-resistant plants [163].

4. Conclusions

The comprehensive literature survey clearly demonstrates the importance of an appropriate experimental design of reversible stress induction under reproducible and long-term stable laboratory conditions. Currently, PEG-induced drought stress models in particular are state of the art. Stress characterization methods include a set of standards but also species-specific small-molecule metabolites and enzymes indicative of the elucidation of drought tolerance mechanisms in plants. In this context, multiple modifications of the drought model experimental setups allow monitoring different aspects of plant functional states, in agreement with specific objectives. Currently, the progress of studies focused on improving plant drought resistance is associated with molecular biology and omics techniques, in an effort to eventually understand and genetically or chemically influence plant responses to drought periods.
Table 1. Overview of drought stress model setups.
Table 1. Overview of drought stress model setups.
SpeciesDrought Stress ModelOsmotically Active AgentAge of PlantDuration of StressReference
Arabidopsis thaliana L.Agar system50, 300 mmol/L mannitol7 days2 weeks[164]
Arabidopsis thaliana L.Agar system100, 200, 300 mmol/L mannitol8 days1 day[165]
Arabidopsis thaliana L.Agar system17% PEG80002 weeks3 days[100]
Lemna minor L.Hydroponic system
(Microtiter plate formate possible)
PEG6000 or 8000
Variable conc.
Adult24 h[68]
Hordeum vulgare L.Soil systemNoAdultEvery 15 days until physiological maturity[166]
Zea mays L.Soil systemNoAdultEvery 15 days until physiological maturity[166]
Zea mays L.Hydroponic system15% PEG60005 weeks24 h[167]
PopuluseuphraticaSoil systemNo2 months0, 4, 8 24, 48, 96 h[168]
Solanum tuberosum L.Agar systemSorbitol (0.1, 0.2, 0.3, and 0.4 m) and PEG8000 (0%, 4.8%, and 9.6%)2 weeks3 weeks[169]
Lolium perenne L.Hydroponic system10, 20% PEG60001 week4 weeks[170]
Solanum lycopersicum L.Hydroponic system15% PEG800025 days0, 3, 6, 24, 48 h[171]
Medicago sativaHydroponic system15% PEG600028 days24 h[172]
Pistacia lentiscusSoil system5, 10, 15, 20, 25% PEG60001,5 months20, 23 days[173]
BrachypodiumdistachyonSoil systemNoVegetativestage4, 8, 12 days[174]
Transgenicplum “Claudia verde”Soil systemNo8 weeks7, 15 days[175]
StipapurpureaSoil systemNoTrefoilstage (about 3 weeks’ growth)7, 15 days[176]
Saccharum spp.Soil systemNo2 months17 days[177]
Hordeumvulgare L.Hydroponic system20% PEG600031 days9 days[178]
Brassica campestris ssp.Hydroponic system60, 120%PEG600034 days7 days[179]
Oryzasativa L.Soil systemNoReproductive stage[180]
Cucumissativus L.Hydroponic system2% PEG60002 weeks7 days[181]
Table 2. Markers of drought stress in plants.
Table 2. Markers of drought stress in plants.
ParameterGrowth ModelPlant ObjectMethodReference
Physiological Markers
Leaf water potential (MPa)SoilCotton (Gossypium hirsutum L.)Pressure chamber technique[182]
Relative water content (RWC; %)SoilPotato (Solanum tuberosum L.) RWC (%) = [(FW − DW)/(SW − DW)] × 100, where FW, DW, and SW are fresh, dry, and saturated (turgid)weights of leaf tissues, respectively [183]
Stomatal conductanceSoilTomato (Lycopersicon esculentum Mill.)Abaxial stomatal conductance measurement with diffusion porometer (AP4, Delta-T, Cambridge, UK)[90]
Photosynthetic parameters (chlorophyll content and PSII activity) SoilBarley (Hordeum vulgare L.)Determination of leaf chlorophyll using chlorophyll meter (SPAD-502, Minolta, Japan); measurement of chlorophyll fluorescence with portable fluorescence spectrometer (Handy PEA, Hansatech Instruments, Norfolk, UK)
Fluorescence value Fv/Fm represents maximum quantum yield of PSII: Fv = Fm−Fo
Biochemical Markers
PhytohormonesSoilClover (Trifolium subterraneum L.)ABA analysis in xylem sap by ELISA[184]
SoilWheat (Triticum aestivum L.)ABA analysis by HPLC[185]
MetabolitesSoilTriticum spp.LMW drought stress–responsive metabolites in root and leaf samples of 7 wild and domesticated wheatrelatives revealed by GC-MS based comparative metabolomicsapproach [186]
Protective proteinsSoilRice (Oryza sativa L.)Expression pattern analysis of OsHSP50.2, an HSP90 family gene[139]
SoilCotton (Gossypium tomentosum, Gossypium hirsutum)LEA gene expression analysis and profiling[144]
ROS and antioxidant enzymesWater culture + PEG6000Wheat genotypes[187]
PS, photosystem II; ABA, abscisic acid; LMW, low molecular weight; LEA, late embryogenesis abundant.

Author Contributions

N.O., J.S., A.D., and E.T. wrote the draft; O.A.K. and A.K. prepared tables and figures and contributed to the manuscript; T.G. and T.B. supervised the work of A.D., O.A.K., and J.S.; V.Z. and I.T. contributed to writing the manuscript and critical reading; A.F. and L.A.W. initiated and supervised the whole work and wrote the final version of the manuscript.


This research was funded by the Russian Science Foundation (project number 17-16-01042).

Conflicts of Interest

The authors declare no conflict of interest.


ABAabscisic acid
APxascorbate peroxidase
DHARdehydroascorbate reductase
GC-MSgas chromatography–mass spectrometry
GRglutathione reductase
GSHreduced glutathione
GSSGoxidized glutathione
LEAlate embryogenesis abundant
LRWCleaf relative water content
MDHARmonodehydroascorbate reductase
NADPHnicotinamide adenine dinucleotide phosphate
PAMpulse amplitude modulation
PARPpoly (ADP-ribose) polymerase
PEGpolyethylene glycol
PS IIphotosystem II
RCCreactive carbonyl compound
ROSreactive oxygen species
SODsuperoxide dismutase
StSolanum tuberosum


  1. Drought Assessment, Management, and Planning: Theory and Case Studies; Wilhite, D.A. (Ed.) Springer: Boston, MA, USA, 1993; ISBN 978-1-4613-6416-0. [Google Scholar]
  2. Zhang, X.; Lu, G.; Long, W.; Zou, X.; Li, F.; Nishio, T. Recent progress in drought and salt tolerance studies in Brassica crops. Breed. Sci. 2014, 64, 60–73. [Google Scholar] [CrossRef] [PubMed][Green Version]
  3. Shao, H.B.; Chu, L.-Y.; Jaleel, C.A.; Manivannan, P.; Panneerselvam, R.; Shao, M.-A. Understanding water deficit stress-induced changes in the basic metabolism of higher plants—Biotechnologically and sustainably improving agriculture and the ecoenvironment in arid regions of the globe. Crit. Rev. Biotechnol. 2009, 29, 131–151. [Google Scholar] [CrossRef] [PubMed]
  4. Basu, S.; Ramegowda, V.; Kumar, A.; Pereira, A. Plant adaptation to drought stress. F1000Research 2016, 5. [Google Scholar] [CrossRef] [PubMed]
  5. Farooq, M.; Wahid, A.; Kobayashi, N.; Fujita, D.; Basra, S.M.A. Plant drought stress: Effects, mechanisms and management. Agron. Sustain. Dev. 2009, 29, 185–212. [Google Scholar] [CrossRef]
  6. Jaleel, C.A.; Manivannan, P.; Wahid, A.; Farooq, M.; Al-Juburi, J.; Somasundaram, R.; Panneerselvam, R. Drought Stress in Plants: A Review on Morphological Characteristics and Pigments Composition. Int. J. Agric. Biol. 2009, 11, 7. [Google Scholar]
  7. Anjum, S.A.; Xie, X.; Wang, L.; Saleem, M.F.; Man, C.; Lei, W. Morphological, physiological and biochemical responses of plants to drought stress. Acta Physiol. Plant. 2015, 37. [Google Scholar] [CrossRef]
  8. Trenberth, K.E.; Dai, A.; van der Schrier, G.; Jones, P.D.; Barichivich, J.; Briffa, K.R.; Sheffield, J. Global warming and changes in drought. Nat. Clim. Chang. 2014, 4, 17–22. [Google Scholar] [CrossRef]
  9. Zhao, T.; Dai, A. The Magnitude and Causes of Global Drought Changes in the Twenty-First Century under a Low–Moderate Emissions Scenario. J. Clim. 2015, 28, 4490–4512. [Google Scholar] [CrossRef]
  10. Verslues, P.E.; Agarwal, M.; Katiyar-Agarwal, S.; Zhu, J.; Zhu, J.-K. Methods and concepts in quantifying resistance to drought, salt and freezing, abiotic stresses that affect plant water status. Plant J. 2006, 45, 523–539. [Google Scholar] [CrossRef] [PubMed][Green Version]
  11. Deikman, J.; Petracek, M.; Heard, J.E. Drought tolerance through biotechnology: Improving translation from the laboratory to farmers’ fields. Curr. Opin. Biotechnol. 2012, 23, 243–250. [Google Scholar] [CrossRef] [PubMed]
  12. Harb, A.; Krishnan, A.; Ambavaram, M.M.R.; Pereira, A. Molecular and Physiological Analysis of Drought Stress in Arabidopsis Reveals Early Responses Leading to Acclimation in Plant Growth. Plant Physiol. 2010, 154, 1254–1271. [Google Scholar] [CrossRef] [PubMed]
  13. Sun, W.; Zhao, X.; Ling, Q.; Li, H.; Gao, X. Exotic shrub species (Caragana korshinskii) is more resistant to extreme natural drought than native species (Artemisia gmelinii) in a semiarid revegetated ecosystem. Agric. For. Meteorol. 2018, 263, 207–216. [Google Scholar] [CrossRef]
  14. Chirkova, T.V. Physiologicheskiie Osnovy Ustojchivosti Rastenii; Izdatelstvo Sankt-Peterburgskogo Universiteta: Saint Petersburg, Russia, 2002. [Google Scholar]
  15. Wang, X.; Cai, X.; Xu, C.; Wang, Q.; Dai, S. Drought-Responsive Mechanisms in Plant Leaves Revealed by Proteomics. Int. J. Mol. Sci. 2016, 17, 1706. [Google Scholar] [CrossRef] [PubMed]
  16. Kar, R.K. Plant responses to water stress: Role of reactive oxygen species. Plant Signal. Behav. 2011, 6, 1741–1745. [Google Scholar] [CrossRef] [PubMed][Green Version]
  17. Gill, S.S.; Tuteja, N. Reactive oxygen species and antioxidant machinery in abiotic stress tolerance in crop plants. Plant Physiol. Biochem. 2010, 48, 909–930. [Google Scholar] [CrossRef] [PubMed]
  18. Boyer, J.S.; Kramer, P.J. Water Relations of Plants and Soils; Academic Press Inc.: Waltham, MA, USA, 1995; ISBN 978-0-12-425060-4. [Google Scholar]
  19. Haswell, E.S.; Verslues, P.E. The ongoing search for the molecular basis of plant osmosensing. J. General Physiol. 2015, 145, 389–394. [Google Scholar] [CrossRef] [PubMed]
  20. Paudel, G.; Bilova, T.; Schmidt, R.; Greifenhagen, U.; Berger, R.; Tarakhovskaya, E.; Stöckhardt, S.; Balcke, G.U.; Humbeck, K.; Brandt, W.; et al. Changes in Arabidopsis thaliana advanced glycated proteome induced by the polyethylene glycol-related osmotic stress. J. Exp. Bot. 2016, 67, 6283–6295. [Google Scholar] [CrossRef] [PubMed]
  21. Dos Santos, C.L.; de Borja Reis, A.F.; Mazzafera, P.; Favarin, J.L. Determination of the Water Potential Threshold at Which Rice Growth Is Impacted. Plants 2018, 7, 48. [Google Scholar] [CrossRef] [PubMed]
  22. Btittelli, M. Measuring Soil Water Potential for Water Management in Agriculture: A Review. Sustainability 2010, 2, 1226–1251. [Google Scholar] [CrossRef][Green Version]
  23. Kikuta, S.B.; Richter, H. Leaf discs or press saps? A comparison of techniques for the determination of osmotic potentials in freeze-thawed leaf material. J. Exp. Bot. 1992, 43, 1039–1044. [Google Scholar] [CrossRef]
  24. Voronin, P.Y.; Rakhmankulova, Z.F.; Shuyskaya, E.V.; Maevskaya, S.N.; Nikolaeva, M.K.; Maksimov, A.P.; Maximov, T.C.; Myasoedov, N.A.; Balnokin, Y.V.; Rymar, V.P.; et al. New method for quantitative determination of water potential of mesophyll cells’ apoplast in substomatal cavity of the leaf. Russ. J. Plant Physiol. 2017, 64, 452–456. [Google Scholar] [CrossRef]
  25. Razmkhah, H. Comparing Threshold Level Methods in Development of Stream Flow Drought Severity-Duration-Frequency Curves. Water Resour. Manag. 2017, 31, 4045–4061. [Google Scholar] [CrossRef]
  26. Grover, A.; Kapoor, A.; Lakshmi, O.S.; Agarwal, S.; Sahi, C.; Katiyar-Agarwal, S.; Agarwal, M.; Dubey, H. Understanding molecular alphabets of the plant abiotic stress responses. Plant Mol. Biol. 2001, 80, 206–216. [Google Scholar]
  27. Duque, A.S.; de Almeida, A.M.; da Silva, A.B.; da Silva, J.M.; Farinha, A.P.; Santos, D.; Fevereiro, P.; Araujo, S.S. Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive. In Abiotic Stress—Plant Responses and Applications in Agriculture; InTech: London, UK, 2013; pp. 49–101. [Google Scholar] [CrossRef]
  28. Zhang, Q. Strategies for developing Green Super Rice. Proc. Natl. Acad. Sci. USA 2007, 104, 16402–16409. [Google Scholar] [CrossRef] [PubMed][Green Version]
  29. Carboni, M.; Zeleny, D.; Acosta, A.T.R. Measuring ecological specialization along a natural stress gradient using a set of complementary niche breadth indices. J. Veg. Sci. 2016, 27, 892–903. [Google Scholar] [CrossRef]
  30. Levitt, J. Responses of Plants to Environmental Stresses; Academic Press: New York, NY, USA, 1972. [Google Scholar] [CrossRef]
  31. Joao, Z.; Ziany, B.; Silva, I.; Josiane, R.; Valdinei, S. Cotton response to water deficits at different growth stages. Rev. Caatinga 2017, 30, 980–990. [Google Scholar] [CrossRef]
  32. Antunes, C.; Pereira, A.J.; Fernandes, P.; Ramos, M.; Ascensão, L.; Correia, O.; Maguas, C. Understanding plant drought resistance in a Mediterranean coastal sand dune ecosystem: Differences between native and exotic invasive species. J. Plant Ecol. 2018, 11, 26–38. [Google Scholar] [CrossRef]
  33. Muller, B.; Pantin, F.; Génard, M.; Turc, O.; Freixes, S.; Piques, M.; Gibon, Y. Water deficits uncouple growth from photosynthesis, increase C content, and modify the relationships between C and growth in sink organs. J. Exp. Bot. 2011, 62, 1715–1729. [Google Scholar] [CrossRef] [PubMed][Green Version]
  34. Li, L.J.; Gu, W.R.; Meng, Y.; Wang, Y.L.; Mu, J.Y.; Li, J.; Wei, S. Physiological and biochemical mechanism of spermidine improving drought resistance in maize seedlings under drought stress. Ying Yong Sheng Tai Xue Bao 2018, 29, 554–564. [Google Scholar] [CrossRef] [PubMed]
  35. Frolov, A.; Bilova, T.; Paudel, G.; Berger, R.; Balcke, G.U.; Birkemeyer, C.; Wessjohann, L.A. Early responses of mature Arabidopsis thaliana plants to reduced water potential in the agar-based polyethylene glycol infusion drought model. J. Plant Physiol. 2017, 208, 70–83. [Google Scholar] [CrossRef] [PubMed]
  36. Lipiec, J.; Doussan, C.; Nosalewicz, A.; Kondracka, K. Effect of drought and heat stresses on plant growth and yield: A review. Int. Agrophys. 2013, 27, 463–477. [Google Scholar] [CrossRef]
  37. Bhargava, S.; Sawant, K. Drought stress adaptation: Metabolic adjustment and regulation of gene expression. Plant Breed. 2013, 132, 21–32. [Google Scholar] [CrossRef]
  38. Tatrai, Z.A.; Sanoubar, R.; Pluhar, Z.; Mancarella, S.; Orsini, F.; Gianquinto, G. Morphological and Physiological Plant Responses to Drought Stress in Thymus citriodorus. Int. J. Agron. 2016, 10, 1–8. [Google Scholar] [CrossRef]
  39. Lamaoui, M.; Jemo, M.; Datla, R.; Bekkaoui, F. Heat and drought stresses in crops and approaches for their mitigation. Front. Chem. 2018, 6, 26. [Google Scholar] [CrossRef] [PubMed]
  40. Todaka, D.; Zhao, Y.; Yoshida, T.; Kudo, M.; Kidokoro, S.; Mizoi, J.; Kodaira, K.-S.; Takebayashi, Y.; Kojima, M.; Sakakibara, H. Temporal and spatial changes in gene expression, metabolite accumulation and phytohormone content in rice seedlings grown under drought stress conditions. Plant J. 2017, 90, 61–78. [Google Scholar] [CrossRef] [PubMed]
  41. Vinocur, B.; Altman, A. Recent advances in engineering plant tolerance to abiotic stress: Achievements and limitations. Curr. Opin. Biotechnol. 2005, 16, 123–132. [Google Scholar] [CrossRef] [PubMed]
  42. Ford, K.L.; Cassin, A.; Bacic, A. Quantitative proteomic analysis of wheat cultivars with differing drought stress tolerance. Front. Plant Sci. 2011, 2, 44. [Google Scholar] [CrossRef] [PubMed]
  43. Gernot, B.; Alireza, A.; Kaul, H.-P. Management of crop water under drought: A review. Agron. Sustain. Dev. 2015, 35. [Google Scholar] [CrossRef]
  44. Thompson, A.J.; Thorne, E.T.; Burbidge, A.; Jackson, A.C.; Sharp, R.E.; Taylor, I.B. Complementation of notabilis, an abscisic acid-deficient mutant of tomato: Importance of sequence context and utility of partial complementation. Plant Cell Environ. 2004, 27, 459–471. [Google Scholar] [CrossRef]
  45. Vrablova, M.; Vrabl, D.; Hronkova, M.; Kubasek, J.; Santrucek, J. Stomatal function, density and pattern, and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1-1 mutants. Plant Biol. 2017, 19, 689–701. [Google Scholar] [CrossRef] [PubMed]
  46. Seminario, A.; Song, L.; Zulet, A.; Nguyen, H.T.; González, E.M.; Larrainzar, E. Drought stress causes a reduction in the biosynthesis of ascorbic acid in soybean plants. Front. Plant Sci. 2017, 8, 1042. [Google Scholar] [CrossRef] [PubMed]
  47. Ahmad, N.; Malagoli, M.; Wirtz, M.; Hell, R. Drought stress in maize causes differential acclimation responses of glutathione and sulfur metabolism in leaves and roots. BMC Plant Biol. 2016, 16, 247. [Google Scholar] [CrossRef] [PubMed]
  48. Staudinger, C.; Mehmeti-Tershani, V.; Gil-Quintana, E.; Gonzalez, E.M.; Hofhansl, F.; Bachmann, G.; Wienkoop, S. Evidence for a rhizobia-induced drought stress response strategy in Medicago truncatula. J. Proteom. 2016, 136, 202–213. [Google Scholar] [CrossRef] [PubMed]
  49. Koyama, R.; Itoh, H.; Kimura, S.; Morioka, A.; Uno, Y. Augmentation of Antioxidant Constituents by Drought Stress to Roots in Leafy Vegetables. HortTechnology 2012, 22, 121–125. [Google Scholar] [CrossRef]
  50. Ito, Y.; Katsura, K.; Maruyama, K.; Taji, T.; Kobayashi, M.; Seki, M.; Shinozaki, K.; Yamaguchi-Shinozaki, K. Functional analysis of rice DREB1/CBF-type transcription factors involved in cold-responsive gene expression in transgenic rice. Plant Cell Physiol. 2006, 47, 141–153. [Google Scholar] [CrossRef] [PubMed]
  51. Zhou, S.; Duursma, R.A.; Medlyn, B.E.; Kelly, J.W.G.; Prentice, I.C. How should we model plant responses to drought? An analysis of stomatal and non-stomatal responses to water stress. Agric. Forest Meteorol. 2013, 182, 204–214. [Google Scholar] [CrossRef]
  52. Ji, H.; Liu, L.; Li, K.; Xie, Q.; Wang, Z.; Zhao, X.; Li, X. PEG-mediated osmotic stress induces premature differentiation of the root apical meristem and outgrowth of lateral roots in wheat. J. Exp. Bot. 2014, 65, 4863–4872. [Google Scholar] [CrossRef] [PubMed][Green Version]
  53. Hellal, F.A.; El-Shabrawi, H.M.; Abd El-Hady, M.; Khatab, I.A.; El-Sayed, S.A.A.; Abdelly, C. Influence of PEG induced drought stress on molecular and biochemical constituents and seedling growth of Egyptian barley cultivars. J. Genet. Eng. Biotechnol. 2018, 16, 203–212. [Google Scholar] [CrossRef]
  54. Amist, N.; Singh, N.B. PEG imposed water deficit and physiological alterations in hydroponic cabbage. Iran. J. Plant Physiol. 2016, 8, 1653–1658. [Google Scholar]
  55. Hohl, M.; Schopfer, P. Water Relations of Growing Maize Coleoptiles: Comparison between Mannitol and Polyethylene Glycol 6000 as External Osmotica for Adjusting Turgor Pressure. Plant Physiol. 1991, 95, 716–722. [Google Scholar] [CrossRef] [PubMed]
  56. Munns, R. Comparative physiology of salt and water stress. Plant Cell Environ. 2002, 25, 239–250. [Google Scholar] [CrossRef] [PubMed][Green Version]
  57. Verslues, P.E.; Ober, E.S.; Sharp, R.E. Root growth and oxygen relations at low water potentials. Impact of oxygen availability in polyethylene glycol solutions. Plant Physiol. 1998, 116, 1403–1412. [Google Scholar] [CrossRef] [PubMed]
  58. Hassan, N.S.; Shaaban, L.D.; Hashem, E.S.A.; Seleem, E.E. In vitro selection for water stress tolerant callus line of Helianthus annus L. cv. Myak. Int. J. Agric. Biol. 2004, 6, 13–18. [Google Scholar]
  59. Zhong, Y.-P.; Li, Z.; Bai, D.-F.; Qi, X.-J.; Chen, J.-Y.; Wei, C.-G.; Lin, M.-M.; Fang, J.-B. In Vitro Variation of Drought Tolerance in Five Actinidia Species. J. Am. Soc. Hortic. Sci. 2018, 143, 226–234. [Google Scholar] [CrossRef]
  60. Chutia, J.; Borah, S.P. Water Stress Effects on Leaf Growth and Chlorophyll Content but Not the Grain Yield in Traditional Rice (Oryza sativa Linn.) Genotypes of Assam, India II. Protein and Proline Status in Seedlings under PEG Induced Water Stress. Am. J. Plant Sci. 2012, 3, 971. [Google Scholar] [CrossRef]
  61. Meher; Shivakrishna, P.; Reddy, K.A.; Rao, D.M. Effect of PEG-6000 imposed drought sress on RNA content, relative water content (RWC), and chlorophyll content in peanut leaves and roots. Saudi J. Biol. Sci. 2018, 25, 285–289. [Google Scholar] [CrossRef] [PubMed]
  62. Liu, J.; Yang, H.; Gosling, S.N.; Kummu, M.; Flörke, M.; Pfister, S.; Hanasaki, N.; Wada, Y.; Zhang, X.; Zheng, C.; et al. Water scarcity assessments in the past, present, and future: REVIEW ON WATER SCARCITY ASSESSMENT. Earth’s Future 2017, 5, 545–559. [Google Scholar] [CrossRef] [PubMed]
  63. Bressan, R.A.; Hasegawa, P.M.; Handa, A.K. Resistance of cultured higher plant cells to polyethylene glycol-induced water stress. Plant Sci. Lett. 1981, 21, 23–30. [Google Scholar] [CrossRef]
  64. Rao, S.; Ftz, J. In vitro selection and characterization of polyethylene glycol (PEG) tolerant callus lines and regeneration of plantlets from the selected callus lines in sugarcane (Saccharum officinarum L.). Physiol. Mol. Biol. Plants 2013, 19, 261–268. [Google Scholar] [CrossRef] [PubMed]
  65. Senaratna, T.; Touchell, D.; Bunn, E.; Dixon, K. Acetyl salicylic acid (Aspirin) and salicylic acid induce multiple stress tolerance in bean and tomato plants. Plant Growth Regul. 2000, 30, 157–161. [Google Scholar] [CrossRef]
  66. Vanden Bossche, H.; Marichal, P.; Gorrens, J.; Geerts, H.; Janssen, P.A.J. Mode of action studies: basis for the search of new antifungal drugs. Ann. N Y Acad. Sci. 1988, 544, 191–207. [Google Scholar] [CrossRef] [PubMed]
  67. Thielert, W. A unique product: The story of the imidacloprid stress shield. Pflanzenschutz Nachrichten-Bayer 2007, 59, 73. [Google Scholar]
  68. Geissler, T.; Wessjohann, L.A. Whole-Plant Microtiter Plate Assay for Drought Stress Tolerance-Inducing Effects. J. Plant Growth Regul. 2011, 30, 504–511. [Google Scholar] [CrossRef]
  69. Rissel, D.; Heym, P.P.; Thor, K.; Brandt, W.; Wessjohann, L.A.; Peiter, E. No silver bullet-Canonical Poly (ADP-Ribose) Polymerases (PARPs) are no universal factors of abiotic and biotic stress resistance of Arabidopsis thaliana. Front. Plant Sci. 2017, 8, 59. [Google Scholar] [CrossRef] [PubMed]
  70. Funar-Timofei, S.; Borota, A.; Crisan, L. Combined molecular docking and QSAR study of fused heterocyclic herbicide inhibitors of D1 protein in photosystem II of plants. Mol. Divers. 2018, 21, 437–454. [Google Scholar] [CrossRef] [PubMed]
  71. Sardare, M.D.; Admane, S.V. A review on plant without soil. Int. J. Res. Eng. Technol. 2013, 2, 299–304. [Google Scholar] [CrossRef]
  72. Bilova, T.; Lukasheva, E.; Brauch, D.; Greifenhagen, U.; Paudel, G.; Tarakhovskaya, E.; Frolova, N.; Mittasch, J.; Balcke, G.U.; Tissier, A.; et al. A Snapshot of the Plant Glycated Proteome: Structural, Functional, and Mechanistic Aspects. J. Biol. Chem. 2016, 291, 7621–7636. [Google Scholar] [CrossRef] [PubMed]
  73. Simon, C.; Brad, H.; Maclin, D.; Bo, X.; Asmini, A.; Sam, H.; Lucy, A.; Vanessa, C.; Monique, S.; Sigfredo, F.; et al. Protocol: Optimising hydroponic growth systems for nutritional and physiological analysis of Arabidopsis thaliana and other plants. Plant Methods 2013, 9, 4. [Google Scholar] [CrossRef]
  74. Jacomini, E.; Bertani, A.; Mapelli, S. Accumulation of polyethylene glycol 6000 and its effects on water content and carbohydrate level in water-stressed tomato plants. Can. J. Bot. 1988, 66, 970–973. [Google Scholar] [CrossRef]
  75. Blum, A. Osmotic adjustment is a prime drought stress adaptive engine in support of plant production: Osmotic adjustment and plant production. Plant Cell Environ. 2017, 40, 4–10. [Google Scholar] [CrossRef] [PubMed]
  76. Van der Weele, C.M.; Spollen, W.G.; Sharp, R.E.; Baskin, T.I. Growth of Arabidopsis thaliana seedlings under water deficit studied by control of water potential in nutrient-agar media. J. Exp. Bot. 2000, 51, 1555–1562. [Google Scholar] [CrossRef] [PubMed]
  77. Smith, R.H.; Bhaskaran, S.; Miller, F.R. Screening for drought tolerance in Sorghum using cell culture. In Vitro Cell Dev. Biol. 1985, 21, 541. [Google Scholar] [CrossRef]
  78. Chen, T.; Fluhr, R. Singlet Oxygen Plays an Essential Role in the Root’s Response to Osmotic Stress. Plant Physiol. 2018, 177, 1717–1727. [Google Scholar] [CrossRef] [PubMed]
  79. He, Y.; Wu, J.; Lv, B.; Li, J.; Gao, Z.; Xu, W.; Baluška, F.; Shi, W.; Shaw, P.C.; Zhang, J. Involvement of 14-3-3 protein GRF9 in root growth and response under polyethylene glycol-induced water stress. J. Exp. Bot. 2015, 66, 2271–2281. [Google Scholar] [CrossRef] [PubMed][Green Version]
  80. Yang, C.J.; Zhang, X.K.; Zou, C.S.; Cheng, Y.; Zhen, P.Y.; Li, G.Y. Effects of drought simulated by PEG-6000 on germination and seedling growth of rapeseed (Brassica napus L.). Chin.J.Oil Crop Sci. 1998, 29, 425–430. [Google Scholar] [CrossRef]
  81. Channaoui, S.; Kahkahi, R.E.; Charafi, J.; Mazouz, H.; Fechtali, M.E.; Nabloussi, A. Germination and Seedling Growth of a Set of Rapeseed (Brassica napus) Varieties under Drought Stress Conditions. Int. J. Environ. Agric. Biotechnol. 2017, 2. [Google Scholar] [CrossRef]
  82. Hamayun, M.; Khan, S.A.; Shinwari, Z.K.; Khan, A.L.; Ahmad, N.; Lee, I.-J. Effect of polyethylene glycol induced drought stress on physio-hormonal attributes of soybean. Abstractsofpapers 2010, 42, 977–986. [Google Scholar]
  83. Tardieu, F. Drought perception by plants do cells of droughted plants experience water stress? Plant Growth Regul. 1996, 20, 93–104. [Google Scholar] [CrossRef]
  84. Raghavendra, A.S.; Gonugunta, V.K.; Christmann, A.; Grill, E. ABA perception and signalling. Trends Plant Sci. 2010, 15, 395–401. [Google Scholar] [CrossRef] [PubMed]
  85. Govender, M.; Govender, P.; Weiersbye, I.; Witkowski, E.; Ahmed, F. Review of commonly used remote sensing and ground-based technologies to measure plant water stress. Water SA 2009, 35. [Google Scholar] [CrossRef]
  86. Donovan, L.; Linton, M.; Richards, J. Predawn plant water potential does not necessarily equilibrate with soil water potential under well-watered conditions. Oecologia 2001, 129, 328–335. [Google Scholar] [CrossRef] [PubMed]
  87. Soltys-Kalina, D.; Plich, J.; Strzelczyk-Zyta, D.; Sliwka, J.; Marczewski, W. The effect of drought stress on the leaf relative water content and tuber yield of a half-sib family of ‘Katahdin’-derived potato cultivars. Breed. Sci. 2016, 66, 328–331. [Google Scholar] [CrossRef] [PubMed][Green Version]
  88. De Silva, M.A.; Jifon, J.L.; da Silva, J.A.G.; Sharma, V. Use of physiological parameters as fast tools to screen for drought tolerance in sugarcane. Braz. J. Plant Physiol. 2007, 19, 193–201. [Google Scholar] [CrossRef]
  89. Novak, V.A.; Osmolovskaya, N. Phytomonitoring in plant physiology: Organization, arrangement, and possibilities. J. Plant Physiol. 1997, 44, 121–128. [Google Scholar]
  90. Sobeih, W.Y. Long-distance signals regulating stomatal conductance and leaf growth in tomato (Lycopersicon esculentum) plants subjected to partial root-zone drying. J. Exp. Bot. 2004, 55, 2353–2363. [Google Scholar] [CrossRef] [PubMed][Green Version]
  91. Damour, G.; Simonneau, T.; Cochard, H.; Urban, L. An overview of models of stomatal conductance at the leaf level: Models of stomatal conductance. Plant Cell Environ. 2010, 33, 1419–1438. [Google Scholar] [CrossRef] [PubMed]
  92. Burkhardt, J.; Kaiser, H.; Goldbach, H.; Kappen, L. Measurements of electrical leaf surface conductance reveal re-condensation of transpired water vapour on leaf surfaces. Plant Cell Environ. 1999, 22, 189–196. [Google Scholar] [CrossRef][Green Version]
  93. Monteith, J.; Campbell, G.; Potter, E. Theory and performance of a dynamic diffusion porometer. Agric. Forest Meteorol. 1988, 44, 27–38. [Google Scholar] [CrossRef]
  94. Dbira, S.; Al Hassan, M.; Gramazio, P.; Ferchichi, A.; Vicente, O.; Prohens, J.; Boscaiu, M. Variable Levels of Tolerance to Water Stress (Drought) and Associated Biochemical Markers in Tunisian Barley Landraces. Molecules 2018, 23, 613. [Google Scholar] [CrossRef] [PubMed]
  95. Fathi, A.; Tari, D.B. Effect of Drought Stress and its Mechanism in Plants. Int. J. Life Sci. 2016, 10, 1–6. [Google Scholar] [CrossRef]
  96. Guo, P.; Baum, M.; Grando, S.; Ceccarelli, S.; Bai, G.; Li, R.; von Korff, M.; Varshney, R.K.; Graner, A.; Valkoun, J. Differentially expressed genes between drought-tolerant and drought-sensitive barley genotypes in response to drought stress during the reproductive stage. J. Exp. Bot. 2009, 60, 3531–3544. [Google Scholar] [CrossRef] [PubMed]
  97. López-Jurado, J.; Balao, F.; Mateos-Naranjo, E. Deciphering the ecophysiological traits involved during water stress acclimation and recovery of the threatened wild carnation, Dianthus inoxianus. Plant Physiol. Biochem. 2016, 109, 397–405. [Google Scholar] [CrossRef] [PubMed]
  98. Chen, Y.-E.; Liu, W.-J.; Su, Y.-Q.; Cui, J.-M.; Zhang, Z.-W.; Yuan, M.; Zhang, H.-Y.; Yuan, S. Different response of photosystem II to short and long-term drought stress in Arabidopsis thaliana. Physiol. Plant. 2016, 158, 225–235. [Google Scholar] [CrossRef] [PubMed]
  99. Klughammer, C.; Schreiber, U. Complementary PS II quantum yields calculated from simple fluorescence parameters measured by PAM fluorometry and the Saturation Pulse method. PAM Appl. Notes 2008, 1, 27–35. [Google Scholar]
  100. Ruhle, T.; Reiter, B.; Leister, D. Chlorophyll Fluorescence Video Imaging: A Versatile Tool for Identifying Factors Related to Photosynthesis. Front. Plant Sci. 2018, 9, 55. [Google Scholar] [CrossRef] [PubMed]
  101. Krause, G.H. Photoinhibition of photosynthesis. An evaluation of damaging and protective mechanisms. Physiol. Plant. 1988, 74, 566–574. [Google Scholar] [CrossRef]
  102. La Rocca, N.; Pupillo, P.; Puppi, G.; Rascio, N. Erythronium dens-canis L. (Liliaceae): An unusual case of change of leaf mottling. Plant Physiol. Biochem. 2014, 74, 108–117. [Google Scholar] [CrossRef] [PubMed]
  103. Jefferies, R.A. Drought and chlorophyll fluorescence in field-grown potato (Solanum tuberosum). Physiol. Plant. 1994, 90, 93–97. [Google Scholar] [CrossRef]
  104. Wingler, A.; Marès, M.; Pourtau, N. Spatial Patterns and Metabolic Regulation of Photosynthetic Parameters during Leaf Senescence. New Phytol. 2004, 161, 781–789. [Google Scholar] [CrossRef]
  105. Kauser, R.; Athar, H.U.R.; Ashraf, M.; Roubina, K.; Habib, A. Chlorophyll fluorescence: A potential indicator for rapid assessment of water stress tolerance in Canola (Brassica napus L.). Pak. J. Bot. 2006, 38, 1501–1509. [Google Scholar]
  106. Shinozaki, K.; Yamaguchi-Shinozaki, K. Gene networks involved in drought stress response and tolerance. J. Exp. Bot. 2006, 58, 221–227. [Google Scholar] [CrossRef] [PubMed][Green Version]
  107. Daszkowska-Golec, A.; Szarejko, I. Open or Close the Gate—Stomata Action under the Control of Phytohormones in Drought Stress Conditions. Front. Plant Sci. 2013, 4. [Google Scholar] [CrossRef] [PubMed]
  108. Seki, M.; Umezawa, T.; Urano, K.; Shinozaki, K. Regulatory metabolic networks in drought stress responses. Curr. Opin. Plant Biol. 2007, 10, 296–302. [Google Scholar] [CrossRef] [PubMed]
  109. Wilkinson, S.; Davies, W.J. Drought, ozone, ABA and ethylene: New insights from cell to plant to community. Plant Cell Environ. 2010, 33, 510–525. [Google Scholar] [CrossRef] [PubMed]
  110. Munemasa, S.; Oda, K.; Watanabe-Sugimoto, M.; Nakamura, Y.; Shimoishi, Y.; Murata, Y. The coronatine-insensitive 1 Mutation Reveals the Hormonal Signaling Interaction between Abscisic Acid and Methyl Jasmonate in Arabidopsis Guard Cells. Specific Impairment of Ion Channel Activation and Second Messenger Production. Plant Physiol. 2007, 143, 1398–1407. [Google Scholar] [CrossRef] [PubMed][Green Version]
  111. Dalal, M.; Tayal, D.; Chinnusamy, V.; Bansal, K.C. Abiotic stress and ABA-inducible Group 4 LEA from Brassica napus plays a key role in salt and drought tolerance. J. Biotechnol. 2009, 139, 137–145. [Google Scholar] [CrossRef] [PubMed]
  112. Desikan, R.; Last, K.; Harrett-Williams, R.; Tagliavia, C.; Harter, K.; Hooley, R.; Hancock, J.T.; Neill, S.J. Ethylene-induced stomatal closure in Arabidopsis occurs via AtrbohF-mediated hydrogen peroxide synthesis. Plant J. 2006, 47, 907–916. [Google Scholar] [CrossRef] [PubMed]
  113. Fraire-Velazquez, S.; Emmanuel, V. Abiotic Stress in Plants and Metabolic Responses. In Abiotic Stress—Plant Responses and Applications in Agriculture; InTech: London, UK, 2013; ISBN 978-953-51-1024-8. [Google Scholar][Green Version]
  114. Krasensky, J.; Jonak, C. Drought, salt, and temperature stress-induced metabolic rearrangements and regulatory networks. J. Exp. Bot. 2012, 63, 1593–1608. [Google Scholar] [CrossRef] [PubMed][Green Version]
  115. Valadez-Bustos, M.G.; Aguado-Santacruz, GA.; Tiessen-Favier, A.; Robledo-Paz, A.; Munoz-Orozco, A.; Rascon-Cruz, Q.; Santacruz-Varela, A. A reliable method for spectrophotometric determination of glycine betaine in cell suspension and other systems. Anal. Biochem. 2016, 498, 47–52. [Google Scholar] [CrossRef] [PubMed]
  116. Alimohammadi, M.; Lahiani, M.H.; Khodakovskaya, M.V. Genetic reduction of inositol triphosphate increases tolerance of tomato plants to oxidative stress. Planta 2015, 242, 123–135. [Google Scholar] [CrossRef] [PubMed]
  117. Birkemeyer, C.; Osmolovskaya, N.; Kuchaeva, L.; Tarakhovskaya, E. Distribution of natural ingredients suggests a complex network of metabolic transport between source and sink tissues in the brown alga Fucus vesiculosus. Planta 2018. [Google Scholar] [CrossRef] [PubMed]
  118. Tarakhovskaya, E.; Lemesheva, V.; Bilova, T.; Birkemeyer, C. Early Embryogenesis of Brown Alga Fucus vesiculosus L. is Characterized by Significant Changes in Carbon and Energy Metabolism. Molecules 2017, 22, 1509. [Google Scholar] [CrossRef] [PubMed]
  119. Milkovska-Stamenova, S.; Schmidt, R.; Frolov, A.; Birkemeyer, C. GC-MS Method for the Quantitation of Carbohydrate Intermediates in Glycation Systems. J. Agric. Food Chem. 2015, 63, 5911–5919. [Google Scholar] [CrossRef] [PubMed]
  120. Ghasemzadeh, A.; Jaafar, H.Z.; Rahmat, A. Synthesis of phenolics and flavonoids in ginger (Zingiber officinale Roscoe) and their effects on photosynthesis rate. Int. J. Mol. Sci. 2010, 11, 4539–4555. [Google Scholar] [CrossRef] [PubMed]
  121. Ma, D.; Sun, D.; Wang, C.; Li, Y.; Guo, T. Expression of flavonoid biosynthesis genes and accumulation of flavonoid in wheat leaves in response to drought stress. Plant Physiol. Biochem. 2014, 80, 60–66. [Google Scholar] [CrossRef] [PubMed]
  122. Bechtold, U.; Rabbani, N.; Mullineaux, P.M.; Thornalley, P.J. Quantitative measurement of specific biomarkers for protein oxidation, nitration and glycation in Arabidopsis leaves. Plant J. 2009, 59, 661–671. [Google Scholar] [CrossRef] [PubMed][Green Version]
  123. Bilova, T.; Paudel, G.; Shilyaev, N.; Schmidt, R.; Brauch, D.; Tarakhovskaya, E.; Milrud, S.; Smolikova, G.; Tissier, A.; Vogt, T.; et al. Global proteomic analysis of advanced glycation end products in the Arabidopsis proteome provides evidence for age-related glycation hot spots. J. Biol. Chem. 2017, 292, 15758–15776. [Google Scholar] [CrossRef] [PubMed]
  124. Frolov, A.; Schmidt, R.; Spiller, S.; Greifenhagen, U.; Hoffmann, R. Arginine-derived advanced glycation end products generated in peptide-glucose mixtures during boiling. J. Agric. Food Chem. 2014, 62, 3626–3635. [Google Scholar] [CrossRef] [PubMed]
  125. Greifenhagen, U.; Nguyen, V.D.; Moschner, J.; Giannis, A.; Frolov, A.; Hoffmann, R. Sensitive and site-specific identification of carboxymethylated and carboxyethylated peptides in tryptic digests of proteins and human plasma. J. Proteome Res. 2015, 14, 768–777. [Google Scholar] [CrossRef] [PubMed]
  126. Greifenhagen, U.; Frolov, A.; Hoffmann, R. Oxidative degradation of N(ε)-fructosylamine-substituted peptides in heated aqueous systems. Amino Acids 2015, 47, 1065–1076. [Google Scholar] [CrossRef] [PubMed]
  127. Schmidt, R.; Böhme, D.; Singer, D.; Frolov, A. Specific tandem mass spectrometric detection of AGE-modified arginine residues in peptides. J. Mass Spectrom. 2015, 50, 613–624. [Google Scholar] [CrossRef] [PubMed]
  128. Soboleva, A.; Vikhnina, M.; Grishina, T.; Frolov, A. Probing Protein Glycation by Chromatography and Mass Spectrometry: Analysis of Glycation Adducts. Int. J. Mol. Sci. 2017, 18, 2557. [Google Scholar] [CrossRef] [PubMed]
  129. Fedorova, M.; Frolov, A.; Hoffmann, R. Fragmentation behavior of Amadori-peptides obtained by non-enzymatic glycosylation of lysine residues with ADP-ribose in tandem mass spectrometry. J. Mass Spectrom. 2010, 45, 664–669. [Google Scholar] [CrossRef] [PubMed]
  130. Jacobson, E.L.; Cervantes-Laurean, D.; Jacobson, M.K. Glycation of proteins by ADP-ribose. Mol. Cell. Biochem. 1994, 138, 207–212. [Google Scholar] [CrossRef] [PubMed]
  131. Ashraf, M.; Foolad, M.R. Roles of glycine betaine and proline in improving plant abiotic stress resistance. Environ. Exp. Bot. 2007, 59, 206–216. [Google Scholar] [CrossRef]
  132. Giri, J. Glycinebetaine and abiotic stress tolerance in plants. Plant Signal Behav 2011, 6, 1746–1751. [Google Scholar] [CrossRef] [PubMed][Green Version]
  133. Templer, S.E.; Ammon, A.; Pscheidt, D.; Ciobotea, O.; Schuy, C.; McCollum, C.; Sonnewald, U.; Hanemann, A.; Förster, J.; Ordon, F.; et al. Metabolite profiling of barley flag leaves under drought and combined heat and drought stress reveals metabolic QTLs for metabolites associated with antioxidant defense. J. Exp. Bot. 2017, 68, 1697–1713. [Google Scholar] [CrossRef] [PubMed]
  134. Doczi, R.; Csanaki, C.Z.; Banfalvi, Z. Expression and promoter activity of the desiccation-specific Solanum tuberosum gene, StDS2. Plant Cell Environ. 2002, 25, 1197–1203. [Google Scholar] [CrossRef][Green Version]
  135. Muvunyi, B.P.; Yan, Q.; Wu, F.; Min, X.; Yan, Z.Z.; Kanzana, G.; Wang, Y.; Zhang, J. Mining Late Embryogenesis Abundant (LEA) Family Genes in Cleistogenes songorica, a Xerophyte Perennial Desert Plant. Int. J. Mol. Sci. 2018, 19, 3430. [Google Scholar] [CrossRef] [PubMed]
  136. Park, C.J.; Seo, Y.S. Heat Shock Proteins: A Review of the Molecular Chaperones for Plant Immunity. Plant Pathol. J. 2015, 31, 323–333. [Google Scholar] [CrossRef] [PubMed][Green Version]
  137. Wang, W.; Vinocur, B.; Shoseyov, O.; Altman, A. Role of plant heat-shock proteins and molecular chaperones in the abiotic stress response. Trends Plant Sci. 2004, 9, 244–252. [Google Scholar] [CrossRef] [PubMed]
  138. Yang, Y.; Dong, C.; Yang, S.; Li, X.; Sun, X.; Yang, Y. Physiological and Proteomic Adaptation of the Alpine Grass Stipa purpurea to a Drought Gradient. PLoS ONE 2015, 10, e0117475. [Google Scholar] [CrossRef] [PubMed]
  139. Xiang, J.; Chen, X.; Hu, W.; Xiang, Y.; Yan, M.; Wang, J. Overexpressing heat-shock protein OsHSP50.2 improves drought tolerance in rice. Plant Cell Rep. 2018, 37, 1585–1595. [Google Scholar] [CrossRef] [PubMed]
  140. Kovacs, D.; Kalmar, E.; Torok, Z.; Tompa, P. Chaperone Activity of ERD10 and ERD14, Two Disordered Stress-Related Plant Proteins. Plant Physiol. 2008, 147, 381–390. [Google Scholar] [CrossRef] [PubMed][Green Version]
  141. Dure, L., III; Greenway, S.C.; Galau, G.A. Developmental biochemistry of cottonseed embryogenesis and germination: Changing messenger ribonucleic acid populations as shown by in vitro and in vivo protein synthesis. Biochemistry 1981, 20, 4162–4168. [Google Scholar] [CrossRef] [PubMed]
  142. Hatanaka, R.; Hagiwara-Komoda, Y.; Furuki, T.; Kanamori, Y.; Fujita, M.; Cornette, R.; Sakurai, M.; Okuda, T.; Kikawada, T. An abundant LEA protein in the anhydrobiotic midge, PvLEA4, acts as a molecular shield by limiting growth of aggregating protein particles. Insect Biochem. Mol. Biol. 2013, 43, 1055–1067. [Google Scholar] [CrossRef] [PubMed][Green Version]
  143. Kovacs, D.; Agoston, B.; Tompa, P. Disordered plant LEA proteins as molecular chaperones. Plant Signal. Behav. 2008, 3, 710–713. [Google Scholar] [CrossRef] [PubMed][Green Version]
  144. Magwanga, R.O.; Lu, P.; Kirungu, J.N.; Lu, H.; Wang, X.; Cai, X.; Zhou, Z.; Zhang, Z.; Salih, H.; Wang, K.; et al. Characterization of the late embryogenesis abundant (LEA) proteins family and their role in drought stress tolerance in upland cotton. BMC Genet. 2018, 19, 1–6. [Google Scholar] [CrossRef] [PubMed]
  145. Cruz de Carvalho, M.H. Drought stress and reactive oxygen species: Production, scavenging and signaling. Plant Signal. Behav. 2008, 3, 156–165. [Google Scholar] [CrossRef] [PubMed]
  146. Møller, I.M.; Jensen, P.E.; Hansson, A. Oxidative modifications to cellular components in plants. Annu. Rev. Plant Biol. 2007, 58, 459–481. [Google Scholar] [CrossRef] [PubMed]
  147. Ma, F.; Chen, X.-B.; Sang, M.; Wang, P.; Zhang, J.-P.; Li, L.-B.; Kuang, T.-Y. Singlet oxygen formation and chlorophyll. Photosynth. Res. 2009, 100, 19–28. [Google Scholar] [CrossRef] [PubMed]
  148. Møller, I.M. Plant mitochondria and oxidative stress: Electron transport, NADPH turnover, and metabolism of reactive oxygen species. Annu. Rev. Plant Biol. 2001, 52, 561–591. [Google Scholar] [CrossRef] [PubMed]
  149. Foyer, C.H. Reactive oxygen species, oxidative signaling and the regulation of photosynthesis. Environ. Exp. Bot. 2018, 154, 134–142. [Google Scholar] [CrossRef] [PubMed]
  150. Pospisil, P. Molecular mechanisms of production and scavenging of reactive oxygen species by photosystem II. Biochim. Biophys. Acta 2012, 1817, 218–231. [Google Scholar] [CrossRef] [PubMed]
  151. Zulfugarov, I.S.; Tovuu, A.; Eu, Y.-J.; Dogsom, B.; Poudyal, R.S.; Nath, K.; Hall, M.; Banerjee, M.; Yoon, U.C.; Moon, Y.-H.; et al. Production of superoxide from Photosystem II in a rice (Oryza sativa L.) mutant lacking PsbS. BMC Plant Biol. 2014, 14, 242. [Google Scholar] [CrossRef] [PubMed]
  152. Velasquez, M.; Santander, I.P.; Contreras, D.R.; Yanez, J.; Zaror, C.; Salazar, R.A.; Pérez-Moya, M.; Mansilla, H.D. Oxidative degradation of sulfathiazole by Fenton and photo-Fenton reactions. J. Environ. Sci. Health Part A 2014, 49, 661–670. [Google Scholar] [CrossRef] [PubMed]
  153. Noctor, G.; Veljovic-Jovanovic, S.; Driscoll, S.; Novitskaya, L.; Foyer, C.H. Drought and oxidative load in the leaves of C3 plants: A predominant role for photorespiration? Ann Bot. 2002, 89, 841–850. [Google Scholar] [CrossRef] [PubMed]
  154. Atkin, O.K.; Macherel, D. The crucial role of plant mitochondria in orchestrating drought tolerance. Ann. Bot. 2009, 103, 581–597. [Google Scholar] [CrossRef] [PubMed]
  155. Moore, K.; Roberts, L.J. Measurement of Lipid Peroxidation. Free Radic. Res. 1998, 28, 659–671. [Google Scholar] [CrossRef] [PubMed]
  156. Del Rio, D.; Stewart, A.J.; Pellegrini, N. A review of recent studies on malondialdehyde as toxic molecule and biological marker of oxidative stress. Nutr. Metab. Cardiovasc. Dis. 2005, 15, 316–328. [Google Scholar] [CrossRef] [PubMed]
  157. Davey, M.W.; Stals, E.; Panis, B.; Keulemans, J.; Swennen, R.L. High-throughput determination of malondialdehyde in plant tissues. Anal. Biochem. 2005, 347, 201–207. [Google Scholar] [CrossRef] [PubMed]
  158. Rhee, S.G.; Chang, T.-S.; Jeong, W.; Kang, D. Methods for detection and measurement of hydrogen peroxide inside and outside of cells. Mol. Cells 2010, 29, 539–549. [Google Scholar] [CrossRef] [PubMed]
  159. Foyer, C.H.; Noctor, G. Ascorbate and glutathione: The heart of the redox hub. Plant Physiol. 2011, 155, 2–18. [Google Scholar] [CrossRef] [PubMed]
  160. Smirnoff, N. BOTANICAL BRIEFING: The Function and Metabolism of Ascorbic Acid in Plants. Ann. Bot. 1996, 78, 661–669. [Google Scholar] [CrossRef][Green Version]
  161. Noctor, G.; Foyer, C.H. Ascorbate and Glutathione: Keeping Active Oxygen under Control. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1998, 49, 249–279. [Google Scholar] [CrossRef] [PubMed]
  162. Meyer, A.J. The integration of glutathione homeostasis and redox signaling. J. Plant Physiol. 2008, 165, 1390–1403. [Google Scholar] [CrossRef] [PubMed]
  163. Kang, G.Z.; Li, G.Z.; Liu, G.Q.; Xu, W.; Peng, X.Q.; Wang, C.Y.; Zhu, Y.J.; Guo, T.C. Exogenous salicylic acid enhances wheat drought tolerance by influence on the expression of genes related to ascorbate-glutathione cycle. Biol. Plant. 2013, 57, 718–724. [Google Scholar] [CrossRef]
  164. Moon, H.-D.; Lee, M.-S.; Kim, S.-H.; Jeong, W.-J.; Choi, D.-W. Identification of a drought responsive gene encoding a nuclear protein involved in drought and freezing stress tolerance in Arabidopsis. Biol. Plant 2016, 60, 105–112. [Google Scholar] [CrossRef]
  165. Huang, T.; Jander, G. Abscisic acid-regulated protein degradation causes osmotic stress-induced accumulation of branched-chain amino acids in Arabidopsis thaliana. Planta 2017, 246, 737–747. [Google Scholar] [CrossRef] [PubMed]
  166. Soleymani, A. Light response of barley (Hordeum vulgare L.) and corn (Zea mays L.) as affected by drought stress, plant genotype and N fertilization. Biocatal. Agric. Biotechnol. 2017, 11, 1–8. [Google Scholar] [CrossRef]
  167. Niu, G.-L.; Gou, W.; Han, X.-L.; Qin, C.; Zhang, L.-X.; Abomohra, A.E.-F.; Ashraf, M. Cloning and Functional Analysis of Phosphoethanolamine Methyltransferase Promoter from Maize (Zea mays L.). Int. J. Mol. Sci. 2018, 19, 191. [Google Scholar] [CrossRef] [PubMed]
  168. Lu, X.; Zhang, X.; Duan, H.; Lian, C.; Liu, C.; Yin, W.; Xia, X. Three stress-responsive NAC transcription factors from Populus euphratica differentially regulate salt and drought tolerance in transgenic plants. Physiol. Plant. 2017, 162, 73–97. [Google Scholar] [CrossRef] [PubMed]
  169. Bundig, C.; Vu, T.H.; Meise, P.; Seddig, S.; Schum, A.; Winkelmann, T. Variability in Osmotic Stress Tolerance of Starch Potato Genotypes (Solanum tuberosum L.) as Revealed by an In Vitro Screening: Role of Proline, Osmotic Adjustment and Drought Response in Pot Trials. J. Agron. Crop Sci. 2016, 203, 206–218. [Google Scholar] [CrossRef]
  170. Bothe, A.; Westermeier, P.; Wosnitza, A.; Willner, E.; Schum, A.; Dehmer, K.J.; Hartmann, S. Drought tolerance in perennial ryegrass (Lolium perenne L.) as assessed by two contrasting phenotyping systems. J. Agron. Crop Sci. 2018, 204, 375–389. [Google Scholar] [CrossRef]
  171. Landi, S.; Nurcato, R.; De Lillo, A.; Lentini, M.; Grillo, S.; Esposito, S. Glucose-6-phosphate dehydrogenase plays a central role in the response of tomato (Solanum lycopersicum) plants to short and long-term drought. Plant Physiol. Biochem. 2016, 105, 79–89. [Google Scholar] [CrossRef] [PubMed]
  172. Defez, R.; Andreozzi, A.; Dickinson, M.; Charlton, A.; Tadini, L.; Pesaresi, P.; Bianco, C. Improved Drought Stress Response in Alfalfa Plants Nodulated by an IAA Over-producing Rhizobium Strain. Front. Microbiol. 2017, 8. [Google Scholar] [CrossRef] [PubMed][Green Version]
  173. Vasques, A.R.; Pinto, G.; Dias, M.C.; Correia, C.M.; Moutinho-Pereira, J.M.; Vallejo, V.R.; Santos, C.; Keizer, J.J. Physiological response to drought in seedlings of Pistacia lentiscus (mastic tree). New For. 2016, 47, 119–130. [Google Scholar] [CrossRef]
  174. Tatli, O.; Ozdemir, B.S.; Doganay, G.D. Time-dependent leaf proteome alterations of Brachypodium distachyon in response to drought stress. Plant Mol. Biol. 2017, 94, 609–623. [Google Scholar] [CrossRef] [PubMed]
  175. Diaz-Vivancos, P.; Faize, L.; Nicolás, E.; Clemente-Moreno, M.J.; Bru-Martinez, R.; Burgos, L.; Hernández, J.A. Transformation of plum plants with a cytosolic ascorbate peroxidase transgene leads to enhanced water stress tolerance. Ann. Bot. 2016, 117, 1121–1131. [Google Scholar] [CrossRef] [PubMed][Green Version]
  176. Li, X.; Yang, Y.; Yang, S.; Sun, X.; Yin, X.; Zhao, Y.; Yang, Y. Comparative proteomics analyses of intraspecific differences in the response of Stipa purpurea to drought. Plant Divers. 2016, 38, 101–117. [Google Scholar] [CrossRef] [PubMed]
  177. Rampazzo, P.; Marcos, F.; Cipriano, M.; Marchiori, P.; Freitas, S.; Machado, E.; Nascimento, L.; Brocchi, M.; Ribeiro, R. Rhizobacteria improve sugarcane growth and photosynthesis under well-watered conditions. Ann. Appl. Biol. 2018, 172, 309–320. [Google Scholar] [CrossRef]
  178. Tavakol, E.; Jakli, B.; Cakmak, I.; Dittert, K.; Karlovsky, P.; Pfohl, K.; Senbayram, M. Optimized potassium nutrition improves plant-water-relations of barley under PEG-induced osmotic stress. Plant Soil 2018, 430, 23–35. [Google Scholar] [CrossRef]
  179. Xiong, X.; Chang, L.; Khalid, M.; Zhang, J.; Huang, D. Alleviation of Drought Stress by Nitrogen Application in Brassica campestris ssp. Chinensis L. Agronomy 2018, 8, 66. [Google Scholar] [CrossRef]
  180. Pang, Y.; Chen, K.; Wang, X.; Xu, J.; Ali, J.; Li, Z. Recurrent selection breeding by dominant male sterility for multiple abiotic stresses tolerant rice cultivars. Euphytica 2017, 213, 268. [Google Scholar] [CrossRef]
  181. Redox imbalance contributed differently to membrane damage of cucumber leaves under water stress and Fusarium infection. Plant Sci. 2018, 274, 171–180. [CrossRef] [PubMed]
  182. Argyrokastritisa, I.G.; Papastylianoub, P.T.; Alexandrisa, S. Leaf Water Potential and Crop Water Stress Index variation for full and deficit irrigated cotton in Mediterranean conditions. Agric. Agric. Sci. Procedia 2015, 4, 463–470. [Google Scholar] [CrossRef]
  183. Pieczynski, M.; Marczewski, W.; Hennig, J.; Dolata, J.; Bielewicz, D.; Piontek, P.; Wyrzykowska, A.; Krusiewicz, D.; Strzelczyk-Zyta, D.; Konopka-Postupolska, D.; et al. Down-regulation of CBP80 gene expression as a strategy to engineer a drought-tolerant potato. Plant Biotechnol. J. 2013, 11, 459–469. [Google Scholar] [CrossRef] [PubMed]
  184. Socias, X.; Correia, M.J.; Chaves, M.; Medrano, H. The role of abscisic acid and water relations in drought responses of subterranean clover. J. Exp. Bot. 1997, 48, 1281–1288. [Google Scholar] [CrossRef][Green Version]
  185. Bano, A.; Ullah, F.; Nosheen, A. Role of abscisic acid and drought stress on the activitiesof antioxidant enzymes in wheat. Plant Soil Environ. 2012, 58, 181–185. [Google Scholar] [CrossRef]
  186. Ullah, N.; Yüce, M.; Gökçe, Z.N.Ö.; Budak, H. Comparative metabolite profiling of drought stress in roots and leaves of seven Triticeae species. BMC Genom. 2017, 18, 969. [Google Scholar] [CrossRef] [PubMed][Green Version]
  187. Weng, M.; Cui, L.; Liu, F.; Zhang, M.; Shan, L.; Yang, S.; Deng, X. Effects of drought stress on antioxidant enzymes in seedlings of different wheat genotypes. Pak. J. Bot. 2015, 47, 49–56. [Google Scholar]
Figure 1. The main drought resistance strategies employed by plants to counter water deficit periods (drought escape, drought avoidance, and drought tolerance) and the main steps of the plant response to dehydration.
Figure 1. The main drought resistance strategies employed by plants to counter water deficit periods (drought escape, drought avoidance, and drought tolerance) and the main steps of the plant response to dehydration.
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Figure 2. Experimental drought models based on osmotic stress and established by supplementation of growth medium with polyethylene glycol (PEG): (A) Lemna minor model, established with aqueous growth medium supplemented with PEG6000 ([68]); (B) Brassica napus model, established with aerated aqueous culture supplemented with PEG8000; and (C) agar-based PEG infusion Arabidopsis thaliana model, established by overlaying solidified agar medium with PEG8000 solution for five days.
Figure 2. Experimental drought models based on osmotic stress and established by supplementation of growth medium with polyethylene glycol (PEG): (A) Lemna minor model, established with aqueous growth medium supplemented with PEG6000 ([68]); (B) Brassica napus model, established with aerated aqueous culture supplemented with PEG8000; and (C) agar-based PEG infusion Arabidopsis thaliana model, established by overlaying solidified agar medium with PEG8000 solution for five days.
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Figure 3. (A) The main pathways of reactive oxygen species (ROS) generation in plants and (B) the major pathways of plant enzymatic antioxidant defense. SOD, superoxide dismutase; CAT, catalase; APx, ascorbate peroxidase; MDHA, monodehydroascorbate; MDHAR, monodehydroascorbate reductase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GSH, reduced glutathione; GSSG, oxidized glutathione; GR, glutathione reductase.
Figure 3. (A) The main pathways of reactive oxygen species (ROS) generation in plants and (B) the major pathways of plant enzymatic antioxidant defense. SOD, superoxide dismutase; CAT, catalase; APx, ascorbate peroxidase; MDHA, monodehydroascorbate; MDHAR, monodehydroascorbate reductase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GSH, reduced glutathione; GSSG, oxidized glutathione; GR, glutathione reductase.
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