1. Introduction
The global demand for food is rising annually due to the rapid growth of the world population. Water shortage is considered one of most the serious factors affecting food production and human health, particularly in arid and semi-arid areas [
1,
2]. Water shortage is also increasing in various parts of the world, which limits agricultural production [
3]. Decreasing water resources globally is associated with climate change, mishandling of water resources, and declining rainfall; these factors have unfavorable impacts on plant growth and production [
4,
5]. Water shortage, known also as a drought stress, for a short time can severely impact growth and development as well as reduce yield quantity and quality [
6,
7]. Several reports stated that water stress can cause dehydration of plant cells, reduction in nutrient absorption, disruption of plant hormone production, damage cell membrane permeability of plants, as well as decrease photosynthesis rate and carbon dioxide assimilation due to stomata closure [
8,
9]. Khan et al. [
10] confirmed that water stress is accountable for the yield reduction in food crops along with imbalanced fertilization, weeds, soil salinity, and nutrient deficiency.
Plants have some approaches to alleviate the negative impacts of drought stress by inducing physiological, biochemical, and morphological alterations [
7,
9]. Among these changes is the accumulation of organic osmolytes, which is of substantial prominence as it conserves osmotic pressure in the plant cell and prevents water loss under drought stress conditions [
9,
11]. This accumulation contributes to stabilizing the structure of different biomolecules. Improving water use efficiency (WUE) and organic osmolytes accumulation in crop plants under water stress conditions is key to enhance plant growth and production [
7,
11].
In this regard, several agronomic strategies have been suggested to improve WUE and yield production of water stressed plants, for example, application of nanofertilizers [
12], utilization of microorganisms [
7], soil addition of organic fertilizers and biochar [
13,
14], use of tolerant rootstocks and cultivars [
8,
9], anti-transparent substances application [
15], implementation of material with high water retention [
16,
17], and application of efficient irrigation methods, particularly in soils characterized by low water hold capacity [
12].
Application of certain nutrients, either foliar or soil addition, improves plant tolerance against abiotic stresses such as water stress, salinity, and other biotic stresses [
18,
19,
20]. Application of zinc (Zn) and silicon (Si) decreases water stress [
18,
20,
21,
22]. Under water stress, both elements play vital roles in the improvement of organic osmolyte accumulation, water balance in plants, rate of photosynthesis, stomata movements, production of endogenous phytohormones, nutrient uptake, accumulation of primary and secondary metabolites which eventually promote the plant growth of plants, biomass accumulation, as well as yield quantity and quality [
20,
23]. At the same time, many scientific reports confirmed that these elements (Si and Zn) significantly reduced the activity level of reactive oxygen species (ROS) and improved the total antioxidant activity and activity levels of antioxidant enzymes under environmental stresses such as drought stress [
18,
19,
20,
21,
22,
23].
Nanotechnology has gained great attention in recent decades due to its increased number of applications. Nanoparticles are characterized by solubility, surface area, and reactivity compared to bulk material [
24,
25]. Furthermore, nanoparticles have acquired a promising status to improve the adverse impacts of environmental stressors, such as drought stress, salinity, and nutrient deficiency, to attain the objective of sustainable agriculture [
26]. Due to their influence on environmental stress tolerance and the nutritional value of crops, studies associated with nanoparticle implementation are increasing. Many nanoparticles have been examined for their protective impact against environmental stresses and other biotic stresses [
26,
27,
28].
Furthermore, numerous studies have been carried out on different plants’ adaptations to drought stress; however, little information is available about drought stress-reducing factors.
Several scientists confirmed that macro and microelement application in nanoform (nano-elements), such as potassium, boron, zinc, and silicon, significantly improved crop yield and plant tolerance against abiotic stresses [
17,
18,
19,
20]. The tiny size of nano-elements is responsible for higher uptake by plants than conventional ones under stress conditions [
26], which consequently improve the physiological and biochemical processes of the stressed plants [
18,
19].
Furthermore, application of water retention agents, (i.e., zeolite, bentonite, and perlite), either bulk or nanoform, to soil in order to enhance soil physical properties is considered an important method to reduce drought stress [
18,
28,
29,
30]. In previous studies, findings confirmed that soil application of water retention agents, such as zeolite, improved moisture and water storage of soil as well as enhanced water use efficiency and yield production for different plants [
31,
32]. Hence, application of this material is considered to be an applicable and low-cost method for plants to alleviate environmental stresses such as drought [
32]. Some positive effects of zeolite on agro-physiological and biochemical measurements of plants, i.e., plant fresh weight, plant dry weight, nutrient content, photosynthesis rate, leaf chlorophyll content, relative water content, plant hormone production, antioxidant compounds, yield quantity and quality, under environmental stress conditions [
30,
31,
32,
33]. Therefore, the objective of this study is to use kaolin, bentonite, perlite, -zeolite nanoparticles (N-zeolite), silicon nanoparticles (N-silicon), and zinc nanoparticles (N-zinc) to reduce the impacts of drought stress on coriander plants. In addition, it also assesses the impacts of these treatments on morphological, physiological, and biochemical traits, as well as the quantity of fruit, seeds, and oil in water-stressed plants.
3. Discussion
Coriander plants are susceptible to drought stress, and irrigation is considered the main source of water in tropical and subtropical regions affecting growth performance, seed quality, and oil yield [
32]. The result of the current study showed that the plant growth parameters (plant height, fresh weight, dry weight, leaf area, and root length) markedly reduced in untreated plants under drought stress conditions (
Figure 1). Recent reports have presented the adverse effects of drought on different economic crops such as cucumber [
7,
8], tomato [
9,
12], cauliflower [
9], maize [
14], and wheat [
10,
13,
15]. These reductions in plant growth performances under drought stress conditions might be associated with declined cell division and elongation due to damage of turgidity, decreased photosynthesis, and reduced energy input [
34,
35]. Conversely, application of N-silicon, N-zinc, and N-zeolite improved the studied growth traits, namely plant height, fresh weight, dry weight, leaf area, and root length, particularly under drought stress (
Figure 1). This could be linked to the enhancement of water use efficiency, nutrient accumulation, photosynthesis rate, and phytohormones (IAA and GA3) as exhibited in this study ((
Figure 5), which provides a better condition for plant growth and development [
22,
27]. Furthermore, Pearson’s correlation analysis confirmed that fresh weight, dry weight, and leaf area connected positively with chlorophyll content, photosynthesis rate, stomatal conductance, water use efficiency, IAA, and GA3 (
Figure 6).
The chlorophyll content and chlorophyll fluorescence are vital indicators for the photosynthesis activity of a plant [
36,
37]. Chlorophyll fluorescence analysis is considered one of the most important techniques used to evaluate the effect of biotic and abiotic stresses on plants which is one of the results of light absorption by plants. The PSII is a sensitive factor of the photosynthesis system concerning water deficit stress [
36].
As shown in
Table 2, leaf chlorophyll content, chlorophyll fluorescence (Fv/Fm), photosystem II efficiency (ϕPSII), and other photosynthetic measurements, including photosynthetic rate, intercellular CO
2 concentration, and water use efficiency of untreated plants (
Table 4 and
Figure 3) significantly reduced under drought stress conditions. The reduction in chlorophyll content in untreated drought-stressed plants could be associated with increasing the activity level of the chlorophyllase enzyme and/or the inhibition of photosynthetic pigment formation [
38]; this consequently reduced leaf photosynthesis rate, stomatal conductance (SC), water use efficiency, and photosystem II efficiency. In addition, Pearson’s correlation analysis shows a positive relationship between chlorophyll content and leaf photosynthesis rate (
Figure 6). Numerous studies have indicated that a reduction in photosynthesis rates is not only predominantly due to the decline in leaf chlorophyll content but is also associated with decreasing stomatal conductance of leaves, which decreases the supply of carbon dioxide into the intercellular spaces [
39,
40]. Contrariwise, the application of silicon, zinc, and zeolite evidently reduced leaf chlorophyll degradation and increased SC and Fv/Fm and photosynthetic apparatus of drought-stressed plants [
41,
42,
43,
44,
45]. These findings could be related to the ability of the previous nano-treatments to alleviate the negative impacts of drought by enhancing hydraulic conductivity, conserving higher transpiratory and photosynthesis rates, photosynthetic pigment concentrations, and reducing oxidative damages [
46,
47]. Furthermore, the improvement in total carbohydrate content in drought-stressed plants supplied with N-silicon, N-zinc, and N-zeolite could be associated with elevating the photosynthesis rate and CO
2 assimilation, as shown in
Figure 2D [
20,
41,
48,
49,
50]. Pearson’s correlation analysis showed that total carbohydrate content correlated positively with photosynthesis rate, intercellular CO
2 concentration, and water use efficiency (
Figure 6).
Water-deficient stress markedly influenced phytohormone concentration, especially IAA, GA3, and ABA (
Table 2). Our findings showed that N-silicon, N-zinc, and N-zeolite caused an improvement in the levels of IAA and GA3 and a reduction in the ABA level in the leaves of coriander plant under drought stress conditions (
Table 2 and
Figure 5). These findings were in harmony with results reported by Othman et al. [
50] and Umair Hassan et al. [
51], who found that application of silicon, zinc, and zeolite nanoparticles upregulated the concentration of IAA and GA3 and downregulated ABA concentration in leaves of different plants exposed to drought stress. This result could be attributed to the improved water and nutrient uptake, especially Zn. Improving the accumulation of Zn in plant tissues (
Table 6) plays an important role in biosynthesizing tryptophan. This amino acid is considered a fundamental compound for IAA formation within the plant [
51]. Regarding leaf GA3 content, the improvement of GA3 is mostly linked to IAA upregulated in plants [
52]. Furthermore, the current study has proven that there is a strong correlation between leaf IAA content and GA3 content (
Figure 6). Some investigators also confirmed that IAA activation promoted GA3 biosynthesis [
51].
The results of the present study displayed that water shortage clearly upgraded the malondialdehyde level (MDA) in the leaves of untreated plants compared with plants treated with N-silicon, N-zinc, and N-zeolite (
Table 2). MDA is considered one of the final products of polyunsaturated fatty acid decomposition in plant cell membranes [
53]; therefore, oxidative injury of lipids in cell membranes of plants is determined by high MDA levels in plant tissues [
53]. Subsequently, augmented lipid peroxidation and hydrogen peroxide levels increase oxidative pressure due to increasing reactive oxygen species (ROS) and interruption of the enzymatic defense in plants grown under water limitation conditions [
53,
54]. Our results are in harmony with the findings of some researchers who stated that malondialdehyde significantly increased under drought stress in various plants [
55]. The current study shows that the application of N-silicon, N-zinc, and N-zeolite on coriander pants alleviated the harmful impacts of drought stress by removing damage caused by oxidative stress and protecting the plant cell through their capability to reduce water and nutrient loss from plants as well as improve the water-hold capacity of soil through zeolite nanoparticle (N-zeolite) application [
41,
42,
43,
45,
50].
Under drought stress conditions, plants produce and accumulate efficient antioxidant compounds to reduce ROS activity [
56]. Those compounds, namely carotenoids, total phenols, flavonoids, ascorbic acid, and thiamine, also play an important role as non-enzymatic free radical scavengers by deactivating singlet oxygen or/and reducing metal ions to safeguard the plant cells exposed to oxidative damages [
7,
8,
9,
11,
18,
19,
50,
51,
52]. The obtained results show an increase in the level of antioxidant compounds (carotenoids, total phenols, flavonoids, ascorbic acid, and thiamine) in leaves of coriander plants treated with N-silicon, N-zinc, and N-zeolite than untreated plants in order to hamper their harmful impacts (
Figure 3). In agreement, Othman et al. [
50] and Maghsoudi et al. [
37] declared that the application of N-silicon, N-zinc, and N-zeolite increase non-enzymatic antioxidants and reduce MDA levels. Numerous scientists revealed that elevated antioxidant enzymes (SOD, CAT, and POD) are associated with increased levels of MDA and H
2O
2 in drought-stressed plants [
7,
8,
11,
50]. A correlation study confirmed that MDA correlated negatively with antioxidant compounds [carotenoids (Caro), total phenols content (TPC), flavonoids (TFC), ascorbic acid (AsA), and thiamine (TAM)], as presented in
Figure 6.
Under drought stress conditions, plants elevate suitable solutes, such as proline, in their cells for assisting water absorption, reducing cell destruction, and improving the osmotic potential of plant cells [
57]. The accumulation of proline is a general indicator of drought stress tolerance and permits osmotic modification that results in cell dehydration prevention and water retention [
58]. In this study, drought-stressed plants treated with N-silicon, N-zinc, and N-zeolite showed higher proline concentrations than untreated plants. Proline accumulation under environmental stress conditions in different crops has been associated with tolerance to stressors, and the proline level is greater in tolerant plants than in susceptible plants [
59].
On the contrary, applying the silicon, zinc, and zeolite nanoparticles led to a reduction in the activity level of CAT and MDA contents. These results can be explained by the involvement of silicon, zinc, and zeolite nanoparticles to reduce water and nutrient losses, hence a decline in the oxidative damage of plant cells [
50,
51]. Comparable results were observed in salt-stressed potato plants exposed to silicon, zinc, and zeolite nanoparticles with reduced ABA and antioxidant enzymes (CAT and POD) [
18,
19].
Likewise, water limitations considerably reduced the accumulation of nutrients (N, P, K, Ca, Mg, Zn, and Fe) in leaf tissues of control plants compared with treated plants with N-silicon, N-zinc, and N-zeolite, as shown in
Table 3. The highest accumulation of endogenous nutrients was recorded in the leaf tissue of plants supplied with N-zeolite followed by N-silicon and N-zinc (
Table 6). This enhancement in the aforementioned nutrients in leaf tissues might be due to the increasing root length and hydraulic conductivity of drought-stressed plants (
Figure 1F) supplied with N-zeolite, N-silicon, and N-zinc, and thus, upgraded the nutrient uptake by the plant [
53,
54].
In terms of coriander productivity, the attained results indicated that the application of N-silicon, N-zinc, and N-zeolite increased seed and oil yield of coriander under drought stress conditions, particularly the weight of 1000 seeds as well as quantity and quality of essential oil, as shown in
Table 4,
Table 5, and
Figure 4. These results were in line with the findings reported by Ghamarnia1 and Daichin [
33] and Afshari et al. [
60] who revealed that the application of N-silicon, N-zinc, and N-zeolite improved the seed and oil yield of coriander plants. The improvement in the quantity of seed and essential oil yield of drought-stressed plants treated with nano-treatments (silicon, zinc, and zeolite) could be related to the enhancement of nutrient uptake, photosynthesis rate, PSII, Fv/Fm, and water use efficiency, which consequently increased fruit yield and weight of 1000 seeds as well as improved the accumulation of secondary metabolites and essential oil in seeds [
61]. In contrast, the EOs yield of untreated drought-stressed plants was significantly reduced due to a high decrement in seed yield. A correlation study showed that EO yield correlated positively with weight per 100 seeds, fruit yield, photosynthesis rate, PSII, Fv/Fm, and WUE (
Figure 6).
Furthermore, the composition of essential oil in treated drought-stressed plants differed from untreated plants. For instance, the application of N-silicon, N-zinc, and N-zeolite elevated the percentage of linalool, β-Pinene, limonene, p-cymene, α-pinne, and camphor and reduced nerol, camphene, sabinene, myrcene, and borneol percentage compared with the control (
Table 5 and
Figure 4). A change in the amount and composition of essential oil is influenced by genetic and environmental stressors, particularly high temperature, salinity, and drought stress. The severity of environmental stressors can also impact on the main compounds of essential oils [
61,
62]. Meanwhile, some scientific investigators reported that the coriander plants exposed to moderate drought stress reached optimum quantity and quality of essential oil more than plants grown in severe drought stress [
60]. In this study, the application of zielote, SA, and Zn may have a stimulatory impact on the expression of the regulatory enzyme (limonene synthase) of the EO biosynthetic pathway, which possibly increases the EOs yield and the concentration of major compounds in the seed of coriander plants [
60,
61,
62].
In general, the application of zeolite, silicon, and zinc nanoparticles in drought-stressed coriander plants stimulates plant tolerance against drought stress by alleviating the adverse impacts of drought by enhancing water use efficiency and the photosynthetic rate, decreasing the oxidative damages, regulating phytohormones. In addition, these nano-treatments also improve nutrient accumulation and the activity of enzymatic and non-enzymatic antioxidants, which eventually augment the plant growth performance, quantity of seeds, and EOs for coriander plants.