Elevated CO2 and Water Stress in Combination in Plants: Brothers in Arms or Partners in Crime?

Simple Summary The changing climate scenario envisages elevated CO2 (eCO2) and drought in many parts of the world. Elevated CO2 is known to increase yields in C3 crops like rice and wheat, on the other hand, it does not cause a similar increase in C4 crops like maize and sorghum. Drought is known to reduce crop growth and yield. In this mini-review we discuss the combined effects of both eCO2 and drought which is typical of a climate change scenario. We try and explain how C3 and C4 crops are differentially affected by these two manifestations of climate change. We specifically show how eCO2 in addition to its known beneficial effects can also be effective in ameliorating the effects of drought in crops. We have critically analysed the current literature and have come up with some mechanistic explanations in terms of water relations, hormonal regulation, photosynthesis and growth, nutrient uptake, and enzyme dynamics. We present here how these processes operate across a range, from ecosystem to organismal level and from molecular to the whole plant level. The information presented will help researchers to devise strategies for adaptation in crops in agricultural systems. Abstract The changing dynamics in the climate are the primary and important determinants of agriculture productivity. The effects of this changing climate on overall productivity in agriculture can be understood when we study the effects of individual components contributing to the changing climate on plants and crops. Elevated CO2 (eCO2) and drought due to high variability in rainfall is one of the important manifestations of the changing climate. There is a considerable amount of literature that addresses climate effects on plant systems from molecules to ecosystems. Of particular interest is the effect of increased CO2 on plants in relation to drought and water stress. As it is known that one of the consistent effects of increased CO2 in the atmosphere is increased photosynthesis, especially in C3 plants, it will be interesting to know the effect of drought in relation to elevated CO2. The potential of elevated CO2 ameliorating the effects of water deficit stress is evident from literature, which suggests that these two agents are brothers in arms protecting the plant from stress rather than partners in crime, specifically for water deficit when in isolation. The possible mechanisms by which this occurs will be discussed in this minireview. Interpreting the effects of short-term and long-term exposure of plants to elevated CO2 in the context of ameliorating the negative impacts of drought will show us the possible ways by which there can be effective adaption to crops in the changing climate scenario.


Introduction
Agriculture is one of the dominant drivers of change in the Anthropocene era; at present about 11 per cent, which is about 1.5 billion ha of the total land surface area, is used for the production of crops on about 36 per cent of the land that is suitable for agriculture [1]. Agriculture is affected by the changing climate, and paradoxically, is also contributing to it. The changing dynamics in climate are the primary and important determinants of agriculture productivity. The effects of this changing climate on overall productivity in agriculture can be understood when we study the effects of individual components contributing to the changing climate on plants and crops.
There is a continuing need to feed the growing population, and globally, the human population of 7.2 billion in mid-2013 is expected to increase to almost 8.1 billion in 2025, and to further grow to 9.6 billion by 2050 [2]. Food security in terms of food availability is imperative in such a scenario. Changing climate is a reality, and slowly we are learning to adapt to it; we are also in the process of devising mitigation strategies so that we can put the brakes on the changes that in general are harmful. The competition for natural resources like land, water and energy will keep growing at a pace with which it would be difficult for us to manage unless we have sound strategies in place to adapt to the harmful effects of changing climate and further mitigate the effects of changing climate.
The effects of climate change on particular areas, specifically agriculture, are difficult to predict with a great degree of accuracy, although the overall effects are known and understood. Reports indicate that global average temperatures have increased by about 1 • C since the pre-industrial era, and that anthropogenic warming is adding around 0.2 • C to global average temperatures every decade [3]. The global CO 2 in the atmosphere reached 407 ppm in 2018 [4]. Given the current rate of generation of CO 2 , it can be expected that it will exceed 600 ppm by the end of this century [5]. The levels of greenhouse gas (GHG) are changing rapidly; CO 2 concentration in the atmosphere can directly affect the growth and development of vegetation in general, and it is indirectly affecting plant growth due to seasonality and variability in rainfall it causes.
Elevated CO 2 and drought due to low variability in rainfall are important manifestations of the changing climate. There is a considerable amount of literature that addresses these aspects in terms of effects on plant systems from molecules to ecosystems. Of particular interest is the effect of increased CO 2 on plants in relation to drought and water stress. Increases in the source of carbon can have favourable effects on plants in relation to their growth and development, and this can be more pronounced in the presence of optimum-to-high levels of nutrients in the soil and increased water availability. These effects may be of short duration and can vary according to the photosynthetic metabolism of the plants like in C 3 , C 4 , CAM and C 3 -C 4 intermediate plants.
In addition, there are studies which show that C 3 crops show increased growth and yield under eCO 2 when grown under both wet and dry growing conditions. C 4 crops show increased growth and yield only under dry growing conditions and drought leads to stomatal limitations of C 3 and C 4 crops and is alleviated by eCO 2 .
The forecasts for the coming decades have projected varying changes in precipitation that can result from the increasing frequency of droughts and floods [6]. Drought is one of the important abiotic stresses in the present changing climate scenario and the study of the mechanism by which it affects plant metabolism, growth and development is of paramount importance. In the past decade, global losses in crop production due to drought totalled USD 30 billion [7]. The loss in crop production due to drought in the past ten years has been close to about 30 billion and it is estimated that about 5 billion people will be in water-scarce regions of the world by 2050, emphasising the importance of studying all the facets of drought and plant growth. Interestingly there are studies where crops are grown under field conditions, and the positive impact of elevated atmospheric CO 2 concentrations on productivity was found to be significantly stronger under soil water limitation than under potential growth conditions, as reported in [8] for cotton, [9] for wheat, [10] for alfalfa, and also for temperate pasture species [11]. There are also evident interactive effects of elevated CO 2 and other environmental conditions that are indicative of changing climate like drought, heat, and other stresses that invariably accompany elevated CO 2 conditions in the atmosphere. The importance of understanding this complex relationship is imperative in a high CO 2 atmosphere that is envisaged in future, to counter the effects of changing climate. As it is known that one of the consistent effects of increased CO 2 in the atmosphere is increased photosynthesis, especially in C 3 plants, it will be interesting to know the effect of drought in relation to elevated CO 2 . The possible mechanisms by which this occurs will be discussed in this minireview. Interpreting the effects of short-term and long-term exposure of plants to elevated CO 2 in the context of ameliorating the negative impacts of drought will show us the possible ways by which there can be effective adaption for crops in the changing climate scenario.

Stomatal Dynamics
Elevated CO 2 concentration is known to mitigate the effects of drought stress, and in a study on Populus spp. and Salix spp. by [12] it was found that when these two species were grown in ambient (350 µmol mol −1 ) or elevated (700 µmol mol −1 ) predawn water potential reduced as water stress increased, as against midday water potential which did not show any changes. The changes observed were 0.1 MPa at predawn and 0.2 MPa at midday. Increased elasticity of the cell wall is usually observed when there are altered water relations. These cellular changes allow the tress to maintain higher turgor at lower water potentials and tissue water content. The mitigating effect of higher CO 2 was by increasing ψ p at the same levels ψ w which can result in osmotic adjustment. This mechanism of osmotic adjustment can improve plant metabolism or at least maintain plant metabolism at optimal levels resulting in acclimation to drought. Stomatal dynamics drive the carbon uptake during water deficit stress and when there is accompanying stress like short-term elevated CO 2 , the role of stomatal limitation in the assimilation of carbon may reduce with a reduction in photorespiration and increase in the partitioning of soluble sugars and increase in water use efficiency.
The eCO 2 -mediated regulation of stomatal conduction and transpiration rate is mainly by regulating stomatal aperture as a short-duration response [13][14][15] and other longduration morphological modifications like changes in stomatal density [16,17]. Varied crop-specific responses were also seen [17] in stomal density where eCO 2 increased the density of stomata in maize whereas the same decreased in Amarnath. It is interesting to note here the differences in dicot and monocot response of both the C 4 crops. The general response observed in both the C 4 crops is because under water deficit conditions C 4 crops are better performing under elevated CO 2 as they have a CO 2 concentrating mechanism; this mechanism favours optimum photosynthesis even under lower stomatal conductance, and they can close their stomata and still perform the dark reaction with an optimum amount of CO 2 . On the other hand, the differences between dicot and monocot C 4 plants under elevated CO 2 may be due to the higher degree of suberization in the kranz anatomy specifically in the NADP-ME subtypes which are not seen in the dicots specifically in the NAD-ME subtype [17].
Studies on stomatal density have been indecisive in their outcomes as to what exactly is governing the decrease and increase in the density under stress conditions, although a large body of evidence says that it is one of the key morphological traits that regulates transpirational flux resistance in the leaf and conductance of stomata under eCO 2 . The underlying mechanism is shifting the balance in favour of CO 2 uptake by increasing it under water loss conditions. On the other hand, a recent study has also suggested that stomatal density may be equally or more affected by temperature, specifically the large continental-scale geographical variations with an interplay of precipitation [18].
The mechanism of guard cell sensing of CO 2, especially in enriched conditions and this sensing playing a role in the turgor dynamics of the cells, has gained much acceptance in recent times; the support for this comes from the fact the CO 2 itself is lipophilic and can easily diffuse across membranes and also move through mass flow across aquaporins. The mechanism is explained by the triggering of CO 2 of the efflux channels of K + out which in turn increases the water potential inside the cell, and this results in water moving out and in effect resulting in stomatal closure [19,20].

The ABA Conundrum
Abscisic acid (ABA) is mainly involved in the regulation of many important physiological processes in the plant at the cellular level. ABA synthesis activates many types of countering mechanisms in plants under stress, among which the main mechanism is the stomatal movement. Opening and closing are regulated in such a way that there is minimum loss of water during water deficit conditions [21,22]. The interplay of ABA and eCO 2 has been of interest to researchers as some of the mechanisms by which they regulate stomatal dynamics seems to be the same.
We have seen that eCO 2 can mitigate drought-induced stress in plants through osmotic adjustment, changes in turgor pressure and changes in root shoot ratio, and the mechanism here is higher hydraulic conductance induced maintenance of higher relative water content (RWC). This is in addition to optimum water status being maintained by hydraulic conductance. On the other hand, when there is an interaction of eCO 2 with drought, we are faced with the question as to what exactly is contributing to the stomatal dynamics. Is it the eCO 2 -induced changes in the stomata, or is it the drought-induced ABA production that is instrumental, or is it an action of both these agents in tandem?
The mechanism and the effect become complex when we see that both ABA and eCO 2 induce stomatal closure: in the case of ABA it is reasoned that closure is to prevent excessive loss of water during stress, and in the case of eCO 2 the reason for the induced closure is debated. The complexity further increases when we see that under water stress conditions in eCO 2 there can be a combined action of both ABA and eCO 2 . Further, taking the complexity to the next level is the differential effects of eCO 2 seen in C 3 and C 4 plants where the responses are distinct and conserved within the photosynthetic types [19].
Two Tomato genotypes, one of them being a mutant deficient in ABA, were tested for responses of hydraulic conductance at eCO 2 by [23]; they found that a reduction in the transpiration rate and a concomitant increase in the water use efficiency (WUE) was seen in the wild type and not in the mutant, clearly indicating a role of ABA in this response. On the other hand, both in the mutant and the wild type, increased water use and osmotic adjustment was seen, showing us that plant water consumption which also includes water transpired is not entirely controlled by ABA. This also shows that osmotic adjustment as a response to water stress can have several other metabolic players and can occur even in the absence of ABA. It is possible that the protective role of ABA under stress is regulated by a higher concentration of CO 2 and is manifested in higher WUE and reduced transpiration rate. It is generally thought that the eCO 2 -mediated closure of stomata and the opening of stomata are independent of the ABA pathway; on the other hand, some signalling components of the ABA pathway have been implicated to work in tandem, suggesting that some of the components of the regulatory mechanism are shared [24].
The response triggered by both these agents eCO 2 and water deficit is interconnected, where ABA is shown to modulate and also regulate the effect of eCO 2 . Water deficit stress is known to have a stronger effect on stomatal conductance as compared to eCO 2 and when in combination with water deficit stress causes a larger decrease in the stomatal conductance which could be an additive effect.
We see here that ROS is a necessary intermediate for ABA-mediated stomatal action in both eCO 2 and water deficit, and while ROS is a well-known response under water stress. It is not so in eCO 2 , so the condition of ROS being a necessary intermediate for stomatal dynamics under sole eCO 2 throws up some mechanistic challenges as to how this condition is satisfied, or if there is an alternative mechanism. This question, to an extent, justifies the certain degree of controversy that exists in the convergence of ABA and CO 2 signalling [25].
SLAC1 is a membrane protein that is multispanning and is mainly expressed in the guard cells; it has an important role in the regulation of ion homeostasis in the cell and is also involved in S-type anion currents. It is a ubiquitous protein for effecting stomatal closure under various environmental signals like eCO 2 , water deficit stress, ozone, light regimes and many more. Studies have shown that SLAC1 activity loss due to mutation continues to affect CO 2 responsiveness in stomatal closure and does not affect the same way under ABA, suggesting the presence of an ABA independent signalling network under eCO 2 conditions to cause stomatal closure. This adds to the intrigue in the signalling response, and possible answers can be found when we can characterize the full complement of guard cell signalling sensors [26][27][28][29][30]. The role of guard cell chloroplasts in regulating CO 2 has also been extensively studied; they are not directly involved in the control of stomatal closure as induced by CO 2 , as it is controlled by the conversion of CO 2 to protons by carbonic anhydrases with HCO 3 being the primary signalling molecules bringing about changes in the proton concentration, and as a result, controlling the opening and closure of the stomata [31][32][33][34].
We already know that the lower the concentration of CO 2, the more the opening of stomata, and as it goes higher the stomata start to close; CO 2 -induced closure is mediated by Ca 2+ and protein phosphorylation, and the specific phosphorylation events are set into motion by signal transduction by Calcium-dependent protein kinases (CPKs) and calcineurin-B-like proteins (CBLs), with the secondary messenger being Ca 2+ [35]. Ca 2 + also has an ABA modulated and accelerated response, hence Ca 2+ transporters and proteins may have a twin function connected to both eCO 2 and ABA [36][37][38]. Recent research has shown a role for both eCO 2 and ABA in stomatal closure.
The common pathway or overlap, or sometimes called the convergence point in the mechanism of stomatal closure, involves three different events. The first is the signal perception by SLAC1 of HCO 3 where there is an involvement of several protein kinases, and this signalling activates the SLAC1 anion channel. The signalling is downstream of the Open Stomata 1 and Sucrose non-fermenting related Kinase (1OST1/SnRK1) pathway [15,39,40]. The mechanistic differences in the eCO 2 -mediated stomatal closure and ABA-mediated stomatal closure are shown in Figure 1.

Water Relations
In a study with field experiments and process-based simulations [41], the authors have shown that CO 2 enrichment contributes to decreased water stress and also contributed to higher yields of maize under restricted water conditions. They showed from their studies that elevated CO 2 decreases transpiration without any effect on soil moisture and at the same time it increases evaporation. Modelling has shown that water stress is reduced to an extent of 37 per cent under elevated CO 2 , a simulated increase in stomatal resistance being the reason for this.
Some of the effects of water stress in combination with elevated CO 2 can be understood when we see the effects observed in Free Air CO 2 enrichment (FACE) experiments. In maize elevated CO 2 reduces transpiration and this, in turn, contributed to the increase in soil moisture and evaporation. In a simulated study [41] it was seen that transpiration was reduced by 22 per cent in the first year of the experiment. In another study [42] the authors showed that in a FACE experiment transpiration in maize was reduced significantly under 550 ppm CO 2 concentration. Daily sap flow and vapour pressure deficit (VPD) of maize were investigated [43], and it was seen that whole-plant transpiration was reduced by 50 per cent in drought as compared to wet in ambient CO 2 concentrations, and 37 per cent reduction was observed in elevated CO 2 concentration of 550 ppm. Enrichment of CO 2 did not affect sap flow under drought and a 20 per cent decrease was seen under wet conditions. Maize under elevated CO 2 had a higher transpiration rate which was due to lower sap flow in the preceding period when plant-available soil water was minimum, this shows that reduction in canopy transpiration by elevated CO 2 can delay the effects of water stress and can contribute to increased plant biomass production. protein phosphatases (PP2Cs) and inhibits them. The proton translocating ATPase is inhibited in the process by ABA and this prevents proton entry into the guard cells and regulates its pH. There is a subsequent release of Ca 2+ -independent protein kinases (SnRK2s). SLAC1 ion channel is phosphorylated by the activation of SnRK2 and calcium dependent protein kinases (CPK). The convergence step under both eCO2 and ABA is the activation of Rapid type ion channel aluminium-activated malate transporter 12/quickly activating anion channel 1 (ALMT12/QUAC1) which leads to the turgor dynamics and K + ion efflux and resultant stomal closure. protein phosphatases (PP2Cs) and inhibits them. The proton translocating ATPase is inhibited in the process by ABA and this prevents proton entry into the guard cells and regulates its pH. There is a subsequent release of Ca 2+ -independent protein kinases (SnRK2s). SLAC1 ion channel is phosphorylated by the activation of SnRK2 and calcium dependent protein kinases (CPK). The convergence step under both eCO 2 and ABA is the activation of Rapid type ion channel aluminiumactivated malate transporter 12/quickly activating anion channel 1 (ALMT12/QUAC1) which leads to the turgor dynamics and K + ion efflux and resultant stomal closure.
Another study [44] on the physiological response of two C 3 and C 4 mechanisms syndromes, examined Napier grass (Pennisetum purpureum Schumach × Pennisetum glaucum (L.) R. Br) and hydric common reed grass (Phragmites australis (Cav.) Trin. Ex Steud); under water stress and elevated CO 2 it was seen that there was a general response of increase in photosynthesis, reduced leaf water potential, and increase in transpiration in both the grass species. A contrasting response was seen in the two types of grass to elevated CO 2 and water stress; the difference in the species response was due to the stomatal characteristics as evidenced by the changes in transpiration rate and osmotic adjustment. Water status adjustment by modification of xylem anatomy and hyrodolyic properties is a mechanism found in many plants, and its relationship with the observed effect of elevated CO 2 to increase plant water potential via reduced stomatal conductance and water loss has been studied [45]. One of the known adaptations to water stress by plants is to maintain high water potential and turgor pressure under water-deficient conditions. The authors saw in their study that water deficit significantly decreased xylem vessel diameter, conduit roundness and stem cross-section area, and it was seen that these impacts of water deficit were relieved at elevated CO 2 . In another study [46] where the adverse effects of the drought were studied on soyabean under elevated CO 2 , the authors found that elevated CO 2 increased WUE contributing to countering drought, but they did not find any positive effects on osmotic adjustments.
The effects of Elevated CO 2 individually and in combination with a water deficit in soyabean were studied [47]. In instantaneous water stress treatment, elevated CO 2 reverted the expression of genes related to stress, transport and nutrient deficiency that was induced by water stress; the interaction of drought and elevated CO 2 affected the expression of genes with physiological and transcriptomic analysis showing that elevated CO 2 can mitigate the negative effects of water stress in soyabean roots.

Photosynthesis and Growth
In addition to understanding the acclimation pattern of plants under a combination of water stress and elevated CO 2 , future yield prediction can also be done under the changing climate scenario from precise data on the effects of elevated CO 2 and drought on biomass and soil water conditions. Growth modelling under these conditions has contributed to our knowledge of these effects. Under sufficient water supply, C 3 crops recorded increased yield under elevated CO 2, whereas C 4 crops did not show much change in the yield. A 10-15 percent increase in biomass has been seen in C 3 crops under FACE experiments due to the CO 2 fertilizing effect [48,49]; on the other hand, C 4 crops maize and sorghum did not respond similarly under water sufficient conditions [50,51]. In a study [52], it was seen that adverse effects of heat and drought were alleviated by improved water relations under elevated CO 2 . The authors also saw that the mechanism of photosynthesis reduction under the combination of heat and drought was due to increased drying of soil and a decrease in stomatal conductance.
In C 3 plants, the most prominent effect is the increased photosynthesis due to the higher availability of CO 2 to rubisco and due to reduction in photorespiration [53,54]. The effects of eCO 2 on C 3 plants have been widely studied, and from a recent comprehensive meta-analysis [55] it is seen that under eCO 2 that leaf biomass per unit leaf area (LMA) increases with a slight decrease in leaf N content. The effect on photosynthesis is mainly due to higher concentrations of CO 2 in the vicinity of Rubisco and thus an increased affinity and reduced photorespiration. Some of the other effects were an almost doubling of photosynthetic rate at higher concentrations of CO 2 (1000 ppm range), with a halving of stomatal conductance and transpiration per unit lead area [55]; this was seen when the stomatal density as such was not affected, implying that the reduced transpiration is due to closure of stomata (the mechanism of which is discussed in another section in this review). This increase in photosynthesis accompanied by reduced transpiration leads to higher WUE, which is also a characteristic of C 4 plants under eCO 2 and it is one of the reasons for eCO 2 under water deficit conditions being less detrimental to the plant as against water-deficit stress in isolation. A schematic representation of the interconnected effects of both eCO 2 and water deficit stress in isolation and in combination in both C3 and C4 crops is given in Figure 2. In a study on Macauba palm [56], the authors investigated the effects of elevated CO2 and drought on photosynthesis, and found that at elevated CO2 the plants could recover more from water stress due to increased Rubisco carboxylation rate and electron transport rate, thus preventing reduction in total dry matter production. The authors noted that drought and increased CO2 affected stem length and total dry matter production, and it was seen that at elevated CO2 there was no reduction in stem length and total biomass due to drought.
In coffee, it was seen by authors [57] that at 723 ± 83 ppm concentration of CO2 for a period of seven months there was increased biomass accumulation even under water deficit treatments with reduced rates of photorespiration and oxidative pressure under drought. The plants under drought and elevated CO2 showed high respiratory carbon flux, which is high respiration rates and an energy status that supported increased root growth under drought. These results show a new mitigating method of elevated CO2 for the maintenance of photosynthetic performance under drought. Other studies have shown that in soyabean [58] drought effect on photosynthesis was not alleviated by elevated CO2, the authors found that net photosynthetic rate and chlorophyll b content reduced under drought and elevated CO2. Another study [48] evaluated biomass accumulation in long-term experiments under elevated CO2 and drought, and the authors reported that there was a multiple response pattern and suggested long-term experiments to access the future impact of climate change. One of the ways eCO2 increases the biomass is by increasing Leaf Area Index (LAI) at early developmental stages leading to increased utilization of incident radiation and in turn higher carbon fixation [59,60]. In a study on Macauba palm [56], the authors investigated the effects of elevated CO 2 and drought on photosynthesis, and found that at elevated CO 2 the plants could recover more from water stress due to increased Rubisco carboxylation rate and electron transport rate, thus preventing reduction in total dry matter production. The authors noted that drought and increased CO 2 affected stem length and total dry matter production, and it was seen that at elevated CO 2 there was no reduction in stem length and total biomass due to drought.
In coffee, it was seen by authors [57] that at 723 ± 83 ppm concentration of CO 2 for a period of seven months there was increased biomass accumulation even under water deficit treatments with reduced rates of photorespiration and oxidative pressure under drought. The plants under drought and elevated CO 2 showed high respiratory carbon flux, which is high respiration rates and an energy status that supported increased root growth under drought. These results show a new mitigating method of elevated CO 2 for the maintenance of photosynthetic performance under drought. Other studies have shown that in soyabean [58] drought effect on photosynthesis was not alleviated by elevated CO 2 , the authors found that net photosynthetic rate and chlorophyll b content reduced under drought and elevated CO 2 . Another study [48] evaluated biomass accumulation in longterm experiments under elevated CO 2 and drought, and the authors reported that there was a multiple response pattern and suggested long-term experiments to access the future impact of climate change. One of the ways eCO 2 increases the biomass is by increasing Leaf Area Index (LAI) at early developmental stages leading to increased utilization of incident radiation and in turn higher carbon fixation [59,60].
Among the growth parameters, relative growth rate (RGR), net assimilation rate (NAR), and leaf area ratio (LAR) are the key parameters that have shown consistent increase due to eCO 2 . Under water deficit, we see that all these parameters are negatively affected, and the positive effect of CO 2 is shown to protect the plants from adverse effects of drought in isolation. Under eCO 2, the higher NAR is compensated by reduced LAR which in turn is caused by the decrease in specific leaf area (SLA); this has been consistent with all the plant parts above ground and below ground [55,61,62].
The observed negative response or lack of acclimation of photosynthetic carbon fixation to long-term eCO 2 can be due to enrichment-induced disruption of RuBP and Pi regeneration in the leaves, which can alter photosynthetic rates effectively lowering it down. This long-term effect has not been studied well for reasons mainly that most experiments with eCO 2 are for two crop seasons for 3-5 years which does not simulate the environmentally relevant concentration of CO 2 over a long term, which constitutes the future scenario of climate change.

Malate Maelstrom
Some of the major difference in the response to eCO 2 and drought among plants are due to the operation of different types of photosynthetic pathways in plants, namely the C 3 and C 4 pathways. The C 4 plants are adapted to low concentrations of CO 2 in the atmosphere as they have a special anatomy called Kranz anatomy, which helps them concentrate CO 2 within the cells and reduce the oxygenase activity of RuBPcase thus reducing photorespiration and increasing photosynthesis. Consequently, the C 4 plants are not benefitting from increased CO 2 concentrations in the atmosphere as are C 3 plants. On the other hand, the temperature optimum for C 4 plants is high, and so under eCO 2 and drought conditions this adaptability confers a degree of tolerance to these plants. The main enzymes of the C 4 plant that play an important role in photosynthesis are the decarboxylating enzymes, NADdependent malic enzyme (NAD-ME), and NADP-dependent malic enzyme (NADP-ME). They are of diverse phylogenetic origin, and they are present in mitochondrial isoforms in C 3 plants not playing a role in carbon fixation as they do not have the oxaloacetate decarboxylase (OAD) action [63][64][65][66]. In higher vascular plants the Malic enzymes are seen as a widely distributed isoforms in the plastids and mitochondria; they have multivarious functions ranging from deference, conferring tolerance to abiotic stresses, metabolic control, and stomatal dynamics [65][66][67][68].
In the NAD-ME type plants, the enzyme has the main function of decarboxylation of malate to produce pyruvate, NADH and CO 2 ; this enzyme uses NAD + , does not decarboxylate Oxalo Acetic Acid (OAA) and is present in mitochondria. Drought and C 4 enzymes, specifically the NAD and NADP malic enzyme action dynamics, have been well studied [14,67,69,70]. The possible mechanism can be that the entry of CO 2 is restricted under eCO 2 conditions due to reduction in stomatal density and closure, either due to eCO 2 or drought; the ME in C 4 can contribute to higher accumulation of CO 2 in the bundle sheath cells, thus increasing photosynthesis, and in contrast to the commonly accepted view that C 4 plants do not have a distinct advantage under eCO 2 conditions. There is also evidence that ME has an important role in balancing out malic acid and decreasing the Reactive Oxygen Species (ROS) generation, thereby offering protection against oxidative damage [71]. Considering all these aspects of Malic Enzymes, we did a sequence search, alignment, and homology modelling of four different kinds of MEs to understand if there are possible structural differences in these different enzymes that can explain the functional diversity. It was seen that there were differences in the protein secondary structure (Table 1), and distinct hairpin bend differences in the structure of the enzyme (Figure 3).  The difference in the beta-hairpin motifs across these NAD-ME and NADP-ME can play an important role in the versatility of the structure and the enzyme action. Positional differences in the amino acids in the hairpins can explain substrate binding and affinity differences in these enzymes. Engineering these enzymes by CRISPR CAS genome editing techniques can be a way to change the activities of these enzymes in both C3 and C4 plants, and it can play an important role under both eCO2 and water deficit conditions. The difference in the beta-hairpin motifs across these NAD-ME and NADP-ME can play an important role in the versatility of the structure and the enzyme action. Positional differences in the amino acids in the hairpins can explain substrate binding and affinity differences in these enzymes. Engineering these enzymes by CRISPR CAS genome editing techniques can be a way to change the activities of these enzymes in both C 3 and C 4 plants, and it can play an important role under both eCO 2 and water deficit conditions.

Nutrients in a Nutshell
Nutrition limitation under eCO 2 and water deficit conditions is one of the main reasons for lower dry matter production apart from the other stress-related responses. Adequate nutrient supply and an adaptive mode of transport into the plant system can alleviate these stresses in a big way. The long-term responses and short-term responses to both eCO 2 and water deficit could be very different and even opposite in their manifestation. Critical inquiry into the mechanistic process involved here will throw light on possible ways by which these stresses can be tackled. In a study on the possible adaptive response of semi-dwarf durum wheat cultivars by physiological and molecular mechanisms [72], it was seen that elevated CO 2 and water stress increased d15N, which was cultivar dependent, and the effect diminished as water stress increased. Shifts in N metabolism could reflect in decreased root-to-shoot translocation of N. The authors observed d13C increased under moderate stress irrespective of the CO 2 concentration indicative of higher water-use efficiency. Phosphoenol Pyruvate Carboxylase (PEPC) expression was increased under water stress and elevated CO 2 combination. Carbohydrates, which are the substrates for PEPC, increased under these stresses and this showed the role of PEPC in providing carbon skeleton for amino acid and lipid biosynthesis. It is seen that there is a transcript level coordination in C and N metabolism under a combination of water stress and elevated CO 2 . The dehydrin genes DHN11 and DHN16 showed changes in expression under water stress and elevated CO 2 with genotype-dependent change in transcript levels, this shows that the interactive effects of both elevated CO 2 and water stress varies according to the genotype in wheat.
In systems where there is no limitation for nutrients, and where there is a sufficient amount of nutrients either in the natural ecosystem or managed system, some species do not show any adaptation or acclimation to higher levels of CO 2 [73]. There has been ongoing research on the reasons for this lack of acclimation to elevated CO 2, especially on an ecosystem scale, and the possible reasons are not clearly explained yet; although some explanations have been put forth among which is feedback inhibition in the source-sink relationship caused by increased carbohydrate loading in the leaves which can reduce photosynthesis [74].
The observed decrease in nitrate concentration in plants under eCO 2 especially in the leaves can be explained by the lower rates of transpiration also observed at high CO 2, which causes a reduced soil solute mass flow along the plant's above-ground parts starting from the roots in the soil plant atmosphere continuum [75,76]. The reduction could also be due to a down-regulation of nitrogen assimilation, differentially regulating N transporters which maybe dose-dependent [77,78].
A summary of effects of e CO2 and drought in plants is given in Table 2. A proposed mechanistic explanation for this is also reduced photorespiration which can reduce N assimilation [79], where the authors show that chloroplast vesiculation (cv) gene expression which destabilizes photosynthesis when silenced maintained the N assimilation status in plants under eCO 2 . The key point to note here is that the total N in the plant is increased under eCO 2 because of the biomass increase seen in these conditions; this mechanism as such may not be a part of the mitigating factor when eCO 2 and water deficit are in combination, as the reduction in the biomass induced by water stress is not fully compensated by eCO 2 although the combination is seen to reduce the reduction caused by water deficit stress.
Adequate Phosphorus nutrition is an ameliorative agent under water deficit stress in eCO 2 conditions. The mechanism by which P acts under eCO 2 conditions is by increasing the concentration of soluble sugars and Pi and maintaining relative water content in the leaves. In a study [80] in field peas, the authors found that the stress tolerance index (STI) was higher in plants with adequate P under eCO 2 conditions under water deficit, which exemplifies the interactive role of P and eCO 2 as drought ameliorative combination. The role of P is to stabilize water relations, stomatal conductance, and transpiration rate synergistically with eCO 2 . It is instrumental in decreasing stomatal conductance and concomitantly reduces water loss through transpiration, thus increasing the transpiration efficiency under drought stress conditions. Total soluble sugars increase in leaves due to accumulation of leaf Pi under eCO 2 and offers better disposition of plants to stave off drought-related stress than when in eCO 2 . The positive effects of Pi namely maintenance of high metabolic readiness to tackle low water status are enhanced under eCO 2 . A similar trend was seen in Potassium (K) supplementation under water deficit conditions [81]. The authors found that K deficiency impeded the ameliorative effect of eCO 2 on drought. Nutrient sufficiency is an important prerequisite to realise the beneficial effects of eCO 2 and the drought mitigatory effect of eCO 2 is dependent on nutrient sufficiency.

Future Perspectives
Deficit irrigation to economize water use and to induce acclimation by plant physiological adjustments is an approach that can be advocated to counter the adverse effects of changing climate; our mini-review here shows that this can be an important strategy in future agriculture under elevated CO 2, which effectively decreases the impact of low soil water on photosynthesis and in turn biomass accumulation and yield in crops. Plant water relations are mainly affected by gas exchange and stomatal physiology which in turn is affected by elevated CO 2 and drought and there is a complex manifestation when these stresses act in combination; these are the critical factors when the goal is to evolve climate ready cultivars. To devise strategies for adaptation in crops in agricultural systems, we have to understand and elucidate how these processes operate across a range from ecosystems to organismal level and from cellular and biochemical to the molecular level. Adaptation in agriculture to changing climate is occurring all over the world; the practices should now be based on the findings that eCO 2 under drought and water stress conditions can be effective in alleviating the effects of climate change. There is a consensus and better understanding of effects now that can be put to use for tackling climate-related effects on crop production.
One of the important facets that have come out of this mini-review is that most of the effects observed need to be looked into with a mechanistic perspective, to arrive at correct inferences that can help us move ahead with the goal of evolving climate-ready cultivars.
In many of the studies, the casual association is observed that needs further investigation; we trust that this mini-review will invigorate researchers.