Elevated CO2 and Water Stress in Combination in Plants: Vanguards for Adaptation to Changing Climate

The changing dynamics in climate is the primary and important determinant 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 and drought due to low variability in rainfall is one of the important manifestations of the changing climate. There is considerable amount literature that addresses these aspects in terms of effects on plants 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 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 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 percent which is about 1.5 billion ha of the total land surface area is used for production of crops which is about 36 percent of land which is suitable for agriculture (Alexandratos and Bruinsma 2012). Agriculture on one hand is affected by the changing climate and on the other hand is also contributing to it. The changing dynamics in climate is the primary and important determinant 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, 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 in 2050 (UN 2013). 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 and also in the process devising mitigation strategies so that we can put the brakes on the changes which in general is 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 catch up, 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 changing climate on particular areas specifically like agriculture is 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 (IPCC 2018). The global CO 2 in the atmosphere has reached 407 ppm in 2018 (Friedlingstein et al 2019). 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 (IPCC 2007). The levels of greenhouse gas (GHG) is 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 is one of the important manifestations of the changing climate. There is considerable amount literature that addresses these aspects in terms of effects on plants 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 favorable effects in plants in relation to their growth and development, this can be all the more pronounced in the presence of optimum to high levels of nutrients in the soil and increased water availably. These effects may be of short duration and can vary according the photosynthesis types of the plant like in C3, C4, CAM and C3-C4 intermediate plants. In addition, there are studies which show that C3 crops show increased growth and yield under both wet and dry growing conditions. C4 crops show increased growth and yield only under dry growing conditions and drought leads to stomatal limitations of C3 and C4 crops and is alleviated by eCO 2 .
The forecasts for the coming decades have projected varying changes in precipitation that can result increasing frequency of droughts and floods (Shanker et al 2014). Drought is one of the important abiotic stresses in the present changing climate scenario and the study of the mechanism by which is affects plants is metabolism, growth and development is of paramount importance. In the past decade, global losses in crop production due to drought totaled ~$30 billion (Gupta et al 2020). The loss in crop production due to drought in the past ten years has been close to about 30 billion and with the estimate that about 5 billion people will be in water scare regions of the world by 2050, emphasizes the importance of studying all the facets of drought and plant growth. Interestingly there are studies that crops grown under field conditions, 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 Kimball et al. (1994) for cotton, Pinter et al. (1996) for wheat, De Luis et al. (1999 for alfalfa and also for temperate pasture species (Clark et al., 1999). There is also evident interactive effects of elevated CO 2 and other environmental conditions which are indicative of changing climate like drought, heat and other stresses which invariably accompany elevated CO 2 conditions in the atmosphere. The importance in understanding this complex relationship is imperative in a high CO 2 atmosphere which 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 C3 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 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.

Water relations, transpiration and stomatal conductance
Elevated CO 2 concentration is known to mitigate effects of drought stress, in a study in Populus spp. and Salix spp. by Johnson et al (2002) 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 mid-day. An increased elasticity of the cell wall is usually observed when there is 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 ψp at the same levels ψw 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 drives the carbon uptake during water deficit stress and when there is an accompanying stress like short term elevated CO 2 , the role of stomatal limitation in assimilation of carbon may reduce with reduction in photorespiration and increase in the partitioning of soluble sugars and increase in water use efficiency.
In a study on the possible adaptive response of semi dwarf durum wheat cultivars by physiological and molecular mechanisms (Medina et al 2016) 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 and this could reflect in decreased root to shoot translocation of a decrease in N. The authors observed d13C increased under moderate stress irrespective of the CO 2 concentration indicative of higher water-use efficiency. PEPC expression was increased under water stress and elevated CO 2 combination. Carbohydrates which are the substrates for PEPC increased under theses stresses and this showed the roles of PEPC in providing carbon skeleton for amino acid and lipid biosynthesis. It is seen that a transcript level coordination in C and N metabolism is seen 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 a study with field experiments and process based simulations Kellner et al (2019) 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 effect on soil moisture at the same time it increases evaporation. Modelling has shown that water stress reduced to an extent of -37 percent under elevated CO 2 , a simulated increase in stomatal resistance being the reason for this.
Some of the effects water stress in combination with elevated CO 2 can be understood when see the effects observed in 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 by Kellner at al 2019 it was seen that transpiration was reduced by 22 percent in 2007 (wet and dry) and in 2008 (wet). Hussain et al (2013) 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 was investigated by Manderscheid et al (2016). Whole plant transpiration was reduced by 50 percent in drought as compared to wet in ambient CO 2 concentrations and 37 percent reduction was observed in elevated CO 2 concentration of 550 ppm. Enrichment of CO 2 did not affect sapflow under drought and a 20 percent 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.
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 grasses to elevated CO 2 and water stress, the difference in the species response was due to the stomatal characteristics as evident by the changes in transpiration rate and osmotic adjustment. Water status adjustment by modification of xylem anatomy and hyrdolyic properties is a mechanism found in many plants, its relationship with the observed effect of elevated CO 2 to increase plant water potential via reduced stomatal conductance and water loss was studied by Liu et al (2020). One the known adaptation 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, it was seen that these impacts of water deficit were relieved at elevated CO 2 . In another study by Wang et al (2018) where the adverse effects of drought was studied on soyabean under elevated CO 2 , the authors found that elevated CO 2 increased WUE contributing towards countering drought, they did not find any positive effects on osmotic adjustments.
The effects of Elevated CO 2 individually and in combination with water deficit in Soyabean was studied by Bencke-Malato et al (2019). In instantaneous water stress treatment elevated CO 2 reverted the expression of genes related to stress, transport and nutrient deficiency that were 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, growth and biomass
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 effects of elevated CO 2 and drought on biomass and soil water conditions. Growth modelling under these conditions have contributed to our knowledge on these effects. Under sufficient water supply C3 crops recorded increased yield under elevated CO 2 where as C4 crops did not show much change in the yield. A 10 -15 percent increase in biomass has been seen in C3 crops under FACE experiments due to the CO 2 fertilizing effect (Andresen et al 2018;Weigel and Manderscheid 2012), on the other hand C4 crops maize and sorghum did not respond similarly under water sufficient conditions (Leakey et al 2006;Manderscheid et al 2014). In a study by Diksaityte et al (2019), it was seen that adverse effects of heat and drought was alleviated by improved water relations under elevated CO 2 . The authors also saw that the mechanism of photosynthesis reduction under combination of heat and drought was due to increased drying of soil and decrease in stomatal conductance.
In a study on Macauba palm by Rosa et al (2019) the author investigated the effects of elevated CO 2 and drought on photosynthesis, they found that at elevated CO 2 the plants were capable of recovering 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 drymatter production, 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 Avila et al (2020) that at 723 ± 83 ppm concentration of CO 2 for a period of seven months has increased biomass accumulation even 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 also an energy status that supported increased root growth under drought. These results show a new mitigating method of elevated CO 2 for maintenance of photosynthetic performance under drought. Other studies have shown that in soyabean (Li et al 2020) 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 . In another study Andresen et al (2016) evaluated biomass accumulation in long term experiments under elevated CO 2 and drought and saw that there was a multiple response pattern and the pattern itself was likely to change and they suggested long term experiments to access future impact of climate change.

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 decrease the impact of low soil water on photosynthesis and in turn biomass accumulation and yield in crops. Plant water relations is mainly affected by gas exchange and stomatal physiology which in turn is affected by elevated CO 2 and drought and there are complex manifestation when these stresses act in combination, these are the critical factors when the goal is to evolve climate ready cultivars. In order to device strategies for adaption in crops in agricultural systems we have understand and elucidate how these processes operate across a range from ecosystems to organismal and from cellular, biochemical to molecular level.
Adaptation in agriculture to changing climate is occurring all over the world, the practices should now be based on the findings that drought and water stress conditions can be effective in alleviating the effects of climate change. There is a general consensus and better understanding of effects not that can be put to use for tackling climate related effects on crop production.
One of the important facets that has come out of this mini review is that most of the effects observed needs 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 are observed which need further investigations which we trust this mini review will invigorate in researchers. Birami, B., Nägele, T., Gattmann, M., Preisler, Y., Gast, A., Arneth, A. and Ruehr, N.K., 2020. Hot drought reduces the effects of elevated CO 2 on tree water-use efficiency and carbon metabolism. New Phytologist.
Clark, H., Newton, P.C.D. and Barker, D.J., 1999. Physiological and morphological responses to elevated CO 2 and a soil moisture deficit of temperate pasture species growing in an established plant community. Journal of Experimental Botany, 50(331), pp.233-242.