Effects Of Elevated Atmospheric [CO₂] And Temperature On Water Use Efficiency Of Maize

Since the industrial revolution,atmospheric CO₂ concentrations have risen substantially,contributing to a gradual increase in global temperatures.Climate change has had a profound impact on agricultural ecosystems,with rising atmospheric CO₂concentrations and temperatures promoting changes in crop photosynthesis,transpiration,and water use efficiency（WUE）,which in turn have implications for agricultural water resource allocation and water management.Most current research on the effects of climate change on crop growth and water use has generally involved large-scale studies,such as model assessments or field trials.However,few studies have examined the effects of climate change on the carbon–water relationships in crop at microscopic scales,such as the stomata and leaf anatomy structure.Changes in atmospheric CO₂ concentration and temperature will change stomata and leaf anatomy of crops,which will affect photosynthesis and transpiration,and eventually lead to changes in crop growth,water consumption and WUE.Therefore,the effects of increasing CO₂ concentration and temperature on WUE are not only closely related to photosynthesis and transpiration,but also to stomata and leaf anatomy structure.Here,we examined the effects of temperature regimes（day/night:25/19°C,31/25°C,and 37/31°C）on maize stomata,leaf anatomy structure,gas exchange,leaf WUE and plant WUE under the CO₂ concentration of 400μmol·mol^-1（C400）and 800μmol·mol^-1（C800）.We also examined the responses of the net photosynthetic rates in maize（P_n）,growth parameters,and WUE to changes in the stomata and leaf anatomy structure traits.The main findings of the study are as follows:（1）At both C400 and C800 CO₂ concentrations,the P_n peaked at 31/25°C,were 25.95μmol·m^-2·s^-1（C400）and 26.45μmol·m^-2·s^-1（C800）,whereas both P_n and maximum photochemical efficiency（F_v/F_m）were significantly reduced at 37/31°C.These findings indicate that 31/25°C were the optimum temperature regime for maize photosynthesis,whereas maize physiology and growth were both inhibited by high temperatures at 37/31°C.In addition,elevated temperature promoted an increase in the stomatal density of maize,and a positive correlation between P_n and stomatal density on both adaxial/abaxial surfaces was detected at 25/19°C and 31/25°C,thereby indicating that P_n is limited by stomatal factors under 25/19°C and 31/25°C.However,the value of correlation coefficients between P_n and stomatal density decreased at 37/31°C,implying the stomatal traits were no longer the main factor limiting P_n.（2）The response of P_n to temperature was not affected by elevated CO₂ under 25/19°C and 31/25°C,while the CO₂enrichment increased P_n by 16.4%（P<0.05）under 37/31°C,suggesting that elevated CO₂concentration might have improved the tolerance of maize to thermal stress.（3）Elevated temperature promoted an increase in the density of vascular bundles in maize leaves,and the transpiration rate（T_r）was positively correlated with the vascular bundle density（R²=0.86）,thus indicating that maize plants would increase the density of vascular bundles to reduce the resistance to water transport in leaves.This in turn would contribute to increasing T_r and decrease leaf temperatures to prevent the adverse effects of heat stress on leaf.（4）Although we detected a significant reduction in the WUE of maize leaves in response to increasing temperatures,the elevated CO₂ was found to mitigate the adverse effects of high temperature on leaf WUE.However,the influence mechanism of elevated CO₂ on leaf WUE was different at 25/19°C,31/25°C and 37/31°C.At 25/19°C,the main factor contributing to an increase in leaf WUE was a reduction in T_r,whereas at 37/31°C,it was mainly attributable to an enrichment CO₂ had mitigated the adverse effects of high temperatures on photosynthesis and retarded the reduction in P_n.（5）Elevated CO₂ significantly increased leaf WUE but did not affect plant WUE.In addition,we found that the correlation between leaf WUE and plant WUE was not consistent,whereas the correlation between stomatal density and plant WUE tended to be more consistent.Therefore,it can be considered to use stomatal parameters to characterize plant WUE.In this study,the combination of stomatal and leaf structure characteristics with gas exchange processes not only helps to accurately understand the mechanism of elevated CO₂concentration and temperature on WUE,but also provides a large amount of data for agricultural water resources utilization under climate change conditions.