Dynamics of Plant Water Uptake under Modified Environment

Abstract

The environmental control of crop physiology increases relative sensitivity of crop water movement within the soil plant atmosphere continuum (SPAC), so as the dynamics of crop water usage under modified climate. The variable environmental conditions determine the relative direction of change. Elevated CO2 exposure depressed the water movement of transpiration flux through reduced stomatal conductance and facilitated partial stomatal closure. However, the net impact may not be ensued the significant reduction in crop water usage at the end of crop season due to modified leaf area dynamics, but with obvious rise in crop water use efficiency (WUE). Thermal stresses are often combined with drought events depressed crop WUE beyond a threshold sourced from adverse impact on total dry matter production under elevated temperature condition. The pathogenic role of O3 exposure induced stomatal sluggishness and reduction in dry matter accumulation (or yield) are sourced from reduced photosynthetic assimilation and aberration in stomatal conductance and thereby reduction in crop WUE under well-watered condition. However, the protective roles of other co-existing abiotic stress factors are subjected to more explanatory research. However, the genetic resilience toward such climate change induced abiotic stress factors and supportive field management options will provide our future facets of sustainable crop production with higher WUE crop under variable environmental conditions.

1. Introduction

Efficient crop water use is a vital physiological strategy to combat water insufficiency in surrounding environment and restoring sufficient water to endure ecosystem services toward sustainable crop production [1]. With the expansion of modern civilization, increased radiative forcing from increased anthropogenic greenhouse gas (GHG) emission poses an additional threat to our existing crop production system and food security with increased frequency of different weather aberrations from the recorded changes in atmospheric composition and warming, frequent exposure of biotic–abiotic stress factors and environmental pollution [2, 3, 4]. Globally, 1.5–4.0°C rise in atmospheric temperatures is predicted for the next century [5] and is coupled with rising atmospheric CO₂ concentration @ 2.4 μmol mol⁻¹ year⁻¹ (https://research.noaa.gov/article/ArtMID/587/ArticleID/2636/Rise-of-carbon-dioxide-unabated). Multimodal projections predicted the rise to continue by 794–1142 μmol mol⁻¹ by 2100 without any desist scenarios against 270 μmol mol⁻¹ in preindustrial era. Water is one of the most precious resources, and its utilization pattern became the crucial issue for attending both environmental and agricultural sustainability under projected climate change scenarios in near future. The rise in water-holding capacity of air (7% °C⁻¹) and surface evaporation could result the intensification of earth hydrological cycle [6, 7]. Climate change research studies often accentuated the obvious climate change impact that could exacerbate the net impact on seasonal crop water use and regional crop production [8, 9]. However, reduction of global radiation from global dimming often interacts with the unprecedented ongoing atmospheric warming [10]. Seasonal crop water use and final yield components of different agricultural crops are often significantly and differently impacted by such climate variations [11, 12]. Over past few decades, a noticeable warming of the climate were evident, which has been consistently linked to changes in a number of hydrological cycle and hydrological system components, including: altered precipitation patterns, intensities, and extremes; rising atmospheric water vapor; rising evaporation; and altered soil moisture and runoff [13, 14, 15]. The observed patterns of such environmental changes have not been uniform across the globe. Crop, climate, water, and soil connections are intricate, involving several biological, physiological, physical, and chemical processes (Figure 1).

Figure 1.

Drivers of plant water uptake under modified environmental conditions.

Projected increase in dry spells will make the issue of crop yields constrained by water worse [15]. Water shortages are caused by limited water accessibility in addition to poor water quantity and availability. Realistic estimation of crop water requirement determines the extent of successful irrigation scheduling, planning, and design of water management under any particular set of climatological circumstances of a region [ 6, 16]. Under limited resource availability, the availability and effectiveness of resource usage became the dominant cofactors of maintaining crop productivity. Therefore, a clear understanding on the ecophysiological basis for improved water resource management became increasingly important in order to achieve higher system productivity and modeling crop growth under modified environmental conditions. In this section, we outline the paradigm of crop water dynamics under modified environment affecting the effective resource capture through both aerial (solar radiation, atmospheric CO₂) and soil (moisture and nutrients) medium in order to sustain crop growth and agricultural production system worldwide.

2. Physiology of crop water use under modified environment

Root water uptake regulates the plant growth and development and has indirect control on soil moisture availability and other biophysical processes, including nutrient cycling and transport within soil environment. Mass flow and diffusion porometry contributed to the combined nutrient–water flow and supportive plant uptake phenomenon in viable textured soils, depending on differences in water potential, availability of transpiring leaf surface, and differential relative permeability of water within soil–plant–atmosphere continuum (SPAC) system [17, 18]. Due to the shared mechanism of transpiration and photosynthetic assimilation process controlling water flow through SPAC, plant growth process is defined as mass energy transport and energy transformation via photosynthesis into biological entity. Seasonal water availability often determined the degree of alteration across different crop growth stages. The C₄ plants have more efficient water utilization system from their inherent photosynthetic pathway (higher carboxylation efficiency) and C₄ pump strength over C₃ crop species [19]. Potassium also plays an integral role in maintaining plant–water relations through modification of plant physiological and biochemical processes viz. enzyme activation, maintaining cell osmotic potential, neutralization of soluble and insoluble molecular anions, stabilization of cell pH, and modification of metabolic/biomolecule synthesis process synthesis [20]. External application of K fertilization increases cell turgor pressure and optimizes cell solute concentration, particularly under abiotic stress environment [21]. In the absence of any environmental stress factors, root water uptake is often eloquent with evapotranspiration (ET) across different crop growth stage-specific computation of “consumptive use.”

2.1 Rising atmospheric CO₂ concentration

Reduction in crop water use is quite evident under rising atmospheric CO₂ levels [8]. Changes in stomatal density for elevated CO₂-grown plants [22, 23] are often argued against the decreased stomatal conductance through partially closed stomata under short-term exposure toward elevated CO₂ environment. Under sufficient irradiation availability, absence of water-deficit, and temperature stress, the response of stomatal density is inversely related to atmospheric CO₂ concentration [22, 24]. Structural plasticity of plant traits often enable plants to respond favorability toward rising CO₂ levels, particularly under suboptimal range [25]. However, several reviews on short-term elevated CO₂ exposure impact studies on differential crop species confirmed that stomatal density barely decreased by an average of 5% [26] or did not alter significantly [27]. Limited photosynthetic activities under suboptimal level atmospheric CO₂ exposure compel plant to expand the stomatal pore area for enhanced CO₂ uptake, not evident for exposure toward elevated atmospheric CO₂ concentrations [28]. In contrast, experimental outcomes confirmed the variable sensitivity (C₃ plants) to respond elevated CO₂ level (up to 700 μmol mol⁻¹) with down-regulation of leaf photosynthesis, partially mediated by sufficient substrate availability and reduced photorespiration without any prominent “CO₂ ceiling effect” for current CO₂ levels, so as for stomatal density and stomatal index [24]. Moreover, the plant leaves’ altered stomatal densities are sought to be genetic adaptation, having gene-regulated impact on stomatal distribution, development, and functioning for differential expression toward long-term increased atmospheric CO₂ concentration [29].

The regulation of stomata by guard cells often resulted widespread decline in leaf-level stomatal conductance under elevated atmospheric CO₂ exposure that contributed the most for lowering seasonal crop water use to a greater extent, irrespective of changes in photosynthetic assimilation for both C₃ and C₄ plants [30, 31]. The effective threshold for such reduction is crop-specific and may vary between 10 and 20 s cm⁻¹. Nevertheless, the C₃ crop species actually had a greater drop in transpiration due to increasing CO₂ than the C₄ crop species [32]. Stimulated ABA biosynthesis [33], hydrogen peroxide and nitric oxide signaling [34], modulation of anion channel and decrease in K⁺/Ca²⁺ transport, and organic solute (sucrose, malate, etc.) concentration in guard cells ensued partial closure of stomatal aperture reducing leaf-level transpiration under elevated CO₂ environment [35, 36]. The reduction in leaf stomatal conductance is typically regulated by soil water potential and vapor pressure deficit [8, 37] and generally contributed to reduced bimolecular availability and nutrient concentration (e.g. protein, iron, zinc) under elevated CO₂ environment [38], but not always resulted in reduced transpiration flux [39]. Reduced stomatal conductance enhanced leaf temperature buildup that facilitated more transpiration from leaf to air vapor pressure deficit under elevated CO₂ environment [40, 41]. The canopy temperature of chickpea varied between 0.86 and 1.3°C in semiarid environment of North India, sourced from reduced leaf-level transpiration, but with no significant changes in cumulative crop water use [42]. Kim et al. [22] reported rise in 1°C daytime leaf temperature from reduced leaf-level transpiration that ameliorated the negative impact of water deficit stress and conserve soil moisture under elevated CO₂ environment [43].

The net reduction in crop water use under elevated CO₂ exposure was mostly attributed to reduced canopy conductance followed by plant canopy transpiration [26, 44]. However, elevated CO₂ induced rise in leaf area index (LAI) often determines the extent of reduction in leaf-level transpiration rate toward the determination of net elevated CO₂ impact on cumulative crop water use at plant canopy level. Lowering of canopy conductance with expanded leaf area under elevated CO₂ exposure offsets each other impact depending on canopy size, with no significant change in crop water use [45]. The observed reduction in leaf-level transpiration increases the leaf-level intrinsic water use efficiency (WUE) under elevated CO₂ exposure [46, 47]. However, the enhanced leaf internal resistance to CO₂ diffusion induced by the substantial reduction in stomatal conductance under elevated CO₂ gradually countered the stimulation of rubisco carboxylation rates in C₃/C₄ grassland species [48], thus indirectly hampering photosynthetic assimilation process [49]. Enhanced belowground allocation (particularly toward) root anchored the crop with finer root proliferation increases plants capacity to extract soil water under elevated CO₂ environment [50, 51]. The cumulative impact of amplified LAI counteracted the leaf-level transpiration, often failed to ensure soil water savings for larger plant canopies with no significant change in cumulative crop water use under elevated CO₂ environment [8, 52]. The reported observations on changing magnitude in crop water use efficiency and plant water dynamics under elevated CO₂ exposure are summarized in Table 1.

Sl. no.	Elevated/ambient CO2 (ppm)	Facility used	Crop (variety)	Dynamics of crop water use (%)	Changes in water use efficiency (%)	Reference citations
1.	700/350	Open top chambers (OTC)	Winter wheat (Triticum aestivum L. cv. MV16)	−32 to −44 (canopy conductance)	+10 to +37.4	[53]
2.	717/363	Glass house	Maize (Zea mays)	- 29 (transpiration)	+ 27	[54]
3.	700/350	Open-top chambers	Cherry (Prunus avium)	no change in crop water use	+56 to +103	[55]
4.	570/370	Free-air CO2 enrichment	Spring wheat (T. aestivum L., cv. Yecora Rojo)	−1.2 to −4 (evapotranspiration)	+ 20	[56]
5.	570/370	Free-air CO2 enrichment	Sorghum (Sorghum bicolor)	−10; (evapotranspiration)	+9 to +16	[57]
6.	700/350	Soil–plant–atmosphere research	Soybean [Glycine max (L.) Merr.]	−9; (evapotranspiration)	+50 to +60	[58]
7.	700/350	Controlled growth chambers	Wheat (T. aestivum L.)	+16.7 (root water uptake)	+62 to +128	[59]
8.	712/358	Open top chambers	Winter wheat (T. aestivum L. cv. Kenong9204)	+52.9 (root water uptake)	+ 9.9	[60]
9.	720/360	Sunlit-controlled environment chambers	Maize (Z. mays cv. Saturn Yellow)	−28.6 (transpiration)	+37.84	[35]
			Grain sorghum (S. bicolor cv. DeKalb 28E)	−27.38 (transpiration)	+40.27
10.	528–615/ 368–454	Free Air CO2 enrichment	poplar plantation (Populus alba L., Populus nigra L., Populus x euramericana Dode Guiner)	—	+73 to +77	[61]
11.	700/380	Controlled environmental growth chambers	Rye (Secale cereale L. Winter -cv Musketeer, Spring- cv SR 4A)	−25 to −35 (transpiration)	+40 to +85	[62]
12.	1000/700/380	Environmental chambers	Potato (Solanum tuberosum L.)	−19 to −40 (transpiration)	+43 to +62	[63]
13.	741/380	Open-top chambers	Soybean (G. max, cv. JiHuang13)	−18.1 (canopy conductance)	+26.2 to +55.4	[64]
14.	590/380	Greenhouse	Tomato (Solanum lycopersicum L.; ST 22 and ST 52)	−15 (stomatal conductance)	+14 to +22	[65]
15.	580/380	Open-top chambers	(G. max (L.) Merr.; Zhonghuang 35)	No change in canopy conductance	+ 115.6	[66]
16.	700/550	Open-top chambers	Cocoa (Theobroma cacao L.)	+25 to +27 (leaf potential)/ -15.9 to −23.8 (transpiration)	+10.3 to +33.3	[67]
17.	867/421	Gas exchange chambers	Pinus halepensis (Aleppo pine)	−39.1 (canopy conductance)/ -22.3 midday leaf water potential	+80	[68]
18.	550/400	Free-air CO2 enrichment	Field pea (Pisum sativum L. cv BPBA Twilight)	+28 (Relative leaf water content)	+37	[69]
19.	700/400	Climate chamber and weighing lysimeters	Maize (Z. mays cv. Zheng Dan 958)	+15.4 to +22.7 (root water uptake)/ -28.1 (transpiration)	+30.5	[50]
20.	580/384	Open-top chambers	kabuli chickpea (Cicer arietinum L.; Pusa 1105)	+21 to +46; (leaf water potential), no change in consumptive use	+30	[8]

Table 1.

Dynamics of crop water use under elevated CO2 environment.

2.2 Thermal stress and drought events

Exposure to high temperature (thermal/heat stress) is often coupled with drought (water-deficit stress) that triggered significant reduction of crop productivity/grain yield [70, 71]. Modeling studies estimated global wheat production was projected to decline by 6% °C⁻¹ rise in temperature with scattered beneficial effect across cooler regions [2, 72]. The negative impact of heat stress is more pronounced for belowground allocation in roots, nutrient uptake (particularly Zn; [73]), and cell metabolism (enzyme inactivation; [74]), rather than other normal ecophysiological processes like photosynthesis and stomatal conductance [75]. Heat stress reduces plant water content, significantly impairing the structure and function of membranes [76]. Hence, three DHNs with apparent molecular masses of 21, 23, and 27 kDa played the protective role under thermal stress exposure, and their expression was unaffected by the changes in leaf water relations [77]. Reduction of root water content [78], root water uptake [79], and root hydraulic conductivity [80, 81] are likely lead to stomatal closure, under extended heat stress or severe elevated soil temperature stress exposure only. With respect to the efficiency of photosynthetic pathway, plant N status, and pre-heat stress growth temperature exposure, elevated CO₂ influences photosynthetic resistance to acute heat stress [82]. Nonetheless, decrease in leaf-level transpiration and root water uptake was apparent from narrowing down of vapor pressure difference between leaf surface and surrounding atmosphere under low-temperature exposure (~ 0–15°C; [83]). Rise in viscosity and decline in water absorption rate hinder root water uptake under low-temperature environment [84].

Periodic moisture availability in SPAC system determines the extent and variability of crop response toward warming process in soil environment [49]. Hence, alteration of gene expression through rapid investiture of protein molecules sensitive to dehydration process is effective to alleviate the negative impact thermal stress in standing crop species [85, 86]. Plant hormones like abscisic acid (ABA), indole-3-acetic acid (IAA), and ethylene are involved in the responses of crops to elevated temperature by balancing transpiration through affecting stomatal conductance [87]. The association between crop ET and CO₂ levels is subjective by the rising temperature range [58]. Under sufficient water availability, the positive effect of restricted stomatal aperture under elevated CO₂ environment toward limited crop water use got nullified with higher evaporation and transpiration loss under elevated air temperature exposure [88].

Typically, higher temperatures and increased atmospheric evaporative demand co-occur naturally toward the advent of seasonal drought occurrences [89]. When compared to high-temperature exposure, drought had a greater impact on plant water relations viz. relative water content and relative leaf area [90]. Opening of leaf stomata facilitates transpiration cooling as plants’ feedback activated under lower soil moisture tensions (3 to 10 bars; [91]) under heat stress exposure [92]; if coupled with water-deficit stress stomatal closure is inevitable to reduce water loss with subsequent rise in leaf temperature [93], increased electrolyte leakage, leaf wilting, and altered oxidative metabolism with the symptoms of oxidative injury, i.e. increased lipid peroxidation and reduction in antioxidant enzymes activity [94, 95] but increased phosphorus uptake and accumulation [96]. Plants experienced water-deficit stress either sourced from limited water supply to the roots or excess water loss through transpiration cooling [94], thus affecting crop growth, nutrient uptake [97], water relations, photosynthetic assimilation, and its relative partitioning toward final yield [98]. Under combined heat-drought stress exposure, water saving acclimation became evident either by increase in water use efficiency or reduction in available leaf area [99]. C₄ plants often exhibited more tolerance to combined drought and heat stress than C₃ crop species [100]. Moreover, ABA signaling under limited moisture availability may also facilitate stomatal closure while coupled with heat stress and often hinder transpiration cooling toward attaining higher leaf temperature, often lethal for plant survival [101].

2.3 Elevated ozone exposure

The majority of crop species are sensitive to elevated ozone (O₃) exposure. The net magnitude of crop response is cultivar-specific and often determined by internal antioxidative defense systems [102, 103]. Atmospheric O₃ enters in to the plant system via stomatal pores as powerful oxidant and triggers cellular reaction with biomolecules to generate reactive oxygen species (ROS) and adversely affect cell functioning and reduction in net productivity/crop yield [104, 105]. Stomatal closure is a common adaptive strategy for plants to prevent O₃ uptake [106]. The reduction in hydraulic conductance in root and leaflet was contrasted with unchanged shoot hydraulic conductance [107] with reduced stomatal conductance [108] and increased canopy temperature [109]. Moreover, atmospheric O₃ exposure depressed leaf cuticle development [110, 111] that has dissension with plant’s internal leaf water conservation strategy for plant survival under intensive drought stress exposure [112, 113]. Elevated O₃ exposure also disrupts ion of K⁺ channels in guard cells that inhibited stomatal opening in leaf epidermis in order to reduce the capacity of photosynthetic assimilation in plant system [114]. Under ample soil water supply, the O₃ led disruption on stomatal control may resulted into reduced relative leaf water content and leaf water potential, so as water use efficiency both at leaf and canopy level during early morning time, but reversed under mid-day sun due to stomatal sluggishness [107, 109]. Elevated O₃ induced stomatal sluggishness is effective physiognomic trait for assessing plant susceptibility toward O₃ pollution with unrestrained crop water use, often combined concurrent drought stress under variable light environment [115]. Such aberrant stomatal responses determine the inherent capacity of plants to endure drought stress [116]. Stomatal sluggishness as modified after elevated O₃ exposure endures modifies plant signaling system that extend the time for stomatal opening (reducing atmospheric CO₂ capture) and slower the rate of partial stomatal closure (facilitating water loss through transpiration) even during night time [117, 118]. Lower stomatal conductance and early leaf shedding of damaged leaves counterbalance for net canopy-level water loss, even though stomatal sluggishness is expected to exacerbate water loss [119]. Moreover, elevated temperature exposure restricts plant O₃ uptake [120]. Simultaneous exposure of water deficit/drought stress played protective role against similar negative impact ozone on stomatal conductance [121].

2.4 Modified radiation environment

Increased air pollution reduces light quality of solar radiation with proportionate increase in diffuse light fraction. The impact of higher diffuse is still very less studied. Increased diffuse radiation enhances primary production often counterweighed by enhanced leaf transpiration loss [122]. Under diffuse light, canopy photosynthesis surpassed the rise in leaf transpiration, i.e. increased water use efficiency, and often modified the community structure in the long run and [123]. Under increased UV exposure, gene-regulated response of stomatal control is regulated by the salicylic and abscisic acid hormones with observed delay in leaf senescence process [124].

3. Crop water use efficiency under modified climate

Plant growth and water loss have opposing role in crop production. In order to maintain higher dry matter production, plant facilitates rapid gas exchange in the expense of water loss through transpiration [125]. Physiologically, water use efficiency (WUE) is commonly defined as amount of biomass (total dry matter or yield) produced per unit of crop water usage (mostly evapotranspiration). Increased CO₂ and temperature as a result of climate change have an impact on evapotranspiration (ET) and crop water use efficiency (WUE) [31, 126]. In contrast, the hydrological definition of WUE refers to proportion of water volume used that productively includes total contribution from rainfall, irrigation, and soil moisture storage that represents the coupling of water and carbon cycles which could act as a useful trait for analyzing the impact of climate change on vegetation [127]. The hydrological concept of WUE follows the interest of soil water conservation engineering. Our interest on assessing WUE under modified climate is confined to physiological definition of WUE restricted for agronomy or crop physiological studies rather than hydrological definition. Rise in WUE may refer to significant increase in dry matter production per unit water usage or dry matter production with significant decline in crop growth specific net water usage integrated over any specific time period. Changes in photosynthetic assimilation rate play the pivotal role in determining the degree of changes in crop WUE under modified climate [46].

Statistical trend analysis studies confirmed that WUE has increased over time because the grain yields have increased, while water use has remained relatively constant [128]. It is anticipated that the crop WUE would rise as a result of the minor decrease in water consumption and significantly greater yields under rising CO₂ levels. There are enormous number of reports showing that higher CO₂ levels lower stomatal conductance which is sustained over time. The increase in aboveground biomass and grain yield was found larger than the total consumption of water in growth chamber conditions leading to enhanced WUE under elevated CO₂ [59]. In their experiment, wheat crop exposed to higher CO₂ under high soil moisture conditions consumed more water throughout the growth season, with the exception of the dough stage. Under high soil N and well-watered circumstances, a small drop in ET of wheat was noticed [129]. Under conditions of high CO₂, an increase in soil moisture can momentarily relieve plants of their water stress [8, 130]. Though reports have also shown no statistically significant trend of soil moisture since the commencement of the CO₂ enrichment experiment [131]. The impacts of high CO₂ on soil water availability may be mitigated by elevated temperature [40] and increased transpiration due to higher leaf area [8]. In the same line, Saha et al. [132] reported increased water use in pigeon pea under elevated CO₂ due to higher leaf area index.

Increase in temperature might increase evaporation from soil leading to rapid depletion of soil water, while increases in plant growth under increased CO₂ may hasten the exhaustion of other finite soil resources [60]. The lowest soil moisture levels were found when seasonal temperatures were at their highest, and warming had the largest influence on soil moisture [133]. With increasing CO₂ to a level of 550 μmol mol⁻¹, cotton ET assessed using three different methods, soil water balance [134], sap flow gauges [135], and energy balance [136], did not alter appreciably. In a well-watered treatment, seasonal ET fell by 4.5 and 5.8%, but increased under water-deficit stress by an average of 4.8 and 0.9% over the course of the 2 years [137]. It has been discovered that eggplants growing in high CO₂ environments may utilize water more effectively when there is a soil water scarcity [130]. According to Schapendonk et al. [138], total transpiration on a canopy basis was the same for both ambient and elevated CO₂ concentrations, even though the average WUE increased at elevated (doubled) CO₂. This higher WUE was fully offset by a higher leaf area index. Overall, temperature and soil moisture had a greater impact (both positive and negative) on species and community-level responses than did high CO₂ levels [139]. Crop simulation studies reported that wheat and maize yields would rise 38% and 12%, respectively, if atmospheric CO₂ concentrations reach approximately 600 ppm, and WUE will improve 40 and 25%, respectively, compared to crops grown without CO₂ fertilization [140].

However, C₃ plants respond differently to rising CO₂ levels than C₄ plants. There is a positive impact due to the functioning of the carboxylation pathway in C₃ plants under elevated CO₂, while the effect of elevated CO₂ is negligible in C₄ plants under ideal soil water conditions [141]. The benefits of elevated CO₂ in C₄ plants are only observed under water-deficit stress conditions due to partial closure of stomata which reduces transpiration and their capacity to assimilate carbon even when stomata remain closed [142]. Analysis of free-air CO₂ enrichment (FACE) experimentations over last 27 years reported a 10% decrease in ET both in C₃ and C₄ plants with 19% increase in yields for C₃ crops, while no change in yield was observed in C₄ crops under optimal water supply [143]. Under water-deficit stress, the WUE was found to increase at a greater degree compared to well-watered conditions in C₃ crops like wheat [60], barley [144], and even in C₄ crops with elevated CO₂ [143]. Therefore, elevated CO₂ may help to overcome the impacts of water-deficit stress.

Under elevated temperature, rise in leaf stomatal conductance below a particular temperature threshold increases leaf-level WUE increases with greater due to greater photosynthetic assimilation than in transpiration [127]. Increase in temperature beyond optimum will cause reduction in rate of photosynthesis, biomass accumulation leading to reduced WUE [145]. Further temperature rise decreased WUE once the threshold was reached because of the domination of higher rates of ET. The temperature threshold depends on plant species [146]. Crop ET was significantly lower under combined elevated CO₂ and temperature exposure for soybean crop, with significantly higher WUE [39]. Similarly doubling CO₂ caused a 9% drop in ET at a temperature combination of 28/18°C, while there was minimal effect at 40/30 and 44/34°C under controlled environmental chambers [58]. The combined impact of rise in temperature and CO₂ under future climate change scenarios may be beneficial for growing crops until the temperature crosses the optimum thresholds under diverse agro-climatic conditions.

Elevated O₃ exposure modifies leaf microenvironment through modification of canopy energy fluxes [106, 109]. Reduction in latent heat dissipation from stomatal sluggishness often modulated canopy thermal environment [115]. The combined contribution reduced ecosystem services from decline in leaf-level transpiration/canopy evapotranspiration [ 107, 117] and reduction in dry matter accumulation [108] are often attributed to the linear reduction in WUE even up to 50% under elevated O₃ exposure [109]. Environmental pollution enforced aerosol forcing that resulted decrease of global solar radiation is reported to be 1.3% per decade during 1960 to 2000 [147]. It has been also reported that diffuse fraction solar radiation is increasing worldwide [10]. The decrease in global solar radiation may be due to rise in atmospheric aerosols caused by urbanization, industrialization, fossil fuel burning, and other anthropogenic activities. Aerosols absorb and/or scatter incoming solar radiation. Reduction in solar radiation due to global dimming with increased diffuse radiation in places with high radiation will either have no impact on yield or will increase the yield leading to increased WUE [12]. Another reason for increased WUE under reduced solar radiation may be due to reduced evaporative demand resulting in lower crop ET [148]. In the places with low solar radiation, global dimming may cause significant yield reductions [10]. Moreover, increased ultraviolet-B radiation exposure is reported to decrease stomatal density and stomatal conductance, thereby increasing leaf-level water use efficiency [149].

4. Conclusion

Relative dynamics of photosynthetic assimilations engulfs its active control on plant water uptake under modified environmental conditions. The nature and degree of exposure with co-existing abiotic stress factors often modify the degree of response. It is quite evident that most of the plant species will have more efficient crop water utilization system in future elevated CO₂ world. The sole or concurrent exposure of other abiotic stress factors (heat, drought, ozone exposure, etc.) needs to be studied more intensively that will determine the net magnitude of local, region, and global feedback system within the planetary boundary layer. However, future research initiatives for achieving higher crop WUE may source from either selection of efficient genotypes with higher assimilation rates exposed under abiotic stress environment or shifting management practices toward facilitating root zone soil moisture conservation, facilitating plants adaptive capacity for attending higher yield whenever exposed under modified surrounding environmental condition.