Response Of Photosynthetic Characteristics Of Maize Leaves To Drought And Simulation Of Stomatal Conductance

In the context of climate change,the frequency and intensity of drought will further increase,and drought will become one of the most important factors that limit plant production and affect the global material circulation and energy flow.Understanding how crops respond to drought and the basic response mechanisms will help implement crop management measures during climate change and ensure the food security.Accurate simulation of stomatal behavior under drought conditions is important for predicting the carbon cycle and water cycle between plants and the atmosphere under climate change.However,in many existing studies,when the stomatal conductance models were applied,the effects of water stress were not considered and the application of the stomatal conductance models was ignored,which weakened the ability to simulate the response of vegetation to drought.Based on the continuous drought manipulation experiment of summer maize carried out in 2014 and the drought and rehydration manipulation experiment of spring maize carried out in 2016,the response characteristics and response mechanisms of leaf photosynthetic characteristics of different maize varieties to drought were analyzed,and the physiological and ecological factors affecting the photosynthetic characteristics of the spring leaves during drought were revealed.The applicability of four common stomatal conductance models for different maize varieties under wide range of soil moisture conditions was evaluated,and the applicable range of soil moisture of each stomatal conductance models for different maize varieties was clarified.The main conclusions are as follows:（1）Drought stress resulted in a decrease in photosynthetic rate,stomatal conductance and transpiration rate of summer maize leaves,and stomatal conductance was the most sensitive to water loss.In the early stage of water stress,summer maize would protect the function of photosynthesis system II through heat dissipation.However,under the conditions of long-term water stress and aggravation with water stress,the chlorophyll fluorescence parameters of summer maize were affected,which revealed that the photosynthetic system II was damaged.During this process,the main factors affecting the photosynthesis of summer maize leaves changed from stomatal limitation to non-stomatal limitation.At the same time,the strategy of water use of summer maize turned from maintain the high level of CO₂ assimilation by the consumption of water to limit the assimilation of CO₂ and maintain the leaf water.（2）When simulating the stomatal conductance of summer maize leaves under wide range of soil moisture conditions,the USO model performed best,followed by the BBL model and the BWB model,and the Jarvis model performed worst.The effects of soil moisture made a difference in the relative performance among the models.By introducing a soil water response function,the performance of the Jarvis,BWB,and USO models improved;however,the performance of the BBL model was negative.After the introduction of the soil water response function,the modified USO model performed best,followed by the modified Jarvis model and the modified BWB model,the modified BBL model performed worst.Overall,the USO models had the best performance under the wide range of soil moisture conditions,regardless of whether the soil water response function was introduced or not.（3）It was observed that the Jarvis,BWB,BBL and USO models were applicable within different ranges of soil relative water content（i.e.,55%—65%,56%—67%,37%—79%and37%—95%,respectively）for summer maize based on the 95%confidence limits.Moreover,the Jarvis and BWB models could be applied under the current wide range of soil moisture conditions（37%—95%）by introducing the soil water response function.（4）Affected by the continuous decline of soil moisture,the photosynthetic rate,transpiration rate and stomatal conductance of spring maize were all reduced.The ecological factors affecting the photosynthetic rate and transpiration rate of spring maize under drought conditions were consistent,and the order of the effects was the same.The effects of air relative humidity was the greatest,followed by the atmospheric CO₂ concentration,the soil relative water content,and the air temperature.The atmospheric CO₂ concentration and air relative humidity were the most important ecological limiting factors for the photosynthetic rate and transpiration rate,respectively.The soil relative water content and air temperature were the main ecological decision factors for the photosynthetic rate and transpiration rate of spring maize,respectively.The physiological factors affecting the photosynthetic rate and transpiration rate were not completely consistent.The direct effects of physiological factors on photosynthetic rate of spring maize showed that the stomatal conductance had the greatest effect,followed by the intercellular CO₂ concentration and the leaf water content.However,the direct effects of physiological factors on transpiration rate of spring maize showed that the stomatal conductance had the greatest impact,followed by the vapor pressure deficit and the leaf water content.The leaf water content was the main physiological decision factor for both the photosynthetic rate and transpiration rate,and the stomatal conductance was the most important physiological limiting factors.（5）When simulating the stomatal conductance of spring maize leaves under progressive drought conditions,the BBL model performed best,followed by the USO model and the BWB model,and the Jarvis model performed worst.After the introduction of the soil water response function,the modified USO model performed best,followed by the modified BBL model and the modified BWB models,and the modified Jarvis model performed worst.The simulation accuracy of the BWB and USO models was improved by the introduction of the soil water response function.However the performance of the Jarvis and BBL models decreased after the introduction of the soil water response function.（6）According to the relationship between stomatal conductance of spring maize and soil relative water content and in terms of the 95%confidence intervals,the Jarvis,BBL,and USO models were applicable when SRWC was from 33%to 83%,whereas the BWB model was applicable when SRWC was from 33%to 76%.After the introduction of the soil water response function,the modified BWB model was applicable for the current experimental soil moisture range.（7）Due to the differences in soil moisture conditions and the differences in biological characteristics of different maize varieties and the interaction with environmental factors,the coefficients of the BWB,BBL and USO models established based on the relationship between stomatal conductance and photosynthetic rate were not the same in different varieties of maize,and the performance were also not the same.However,the soil water response function had the same effects on the performance of the three stomatal conductance models under wide range of soil moisture conditions of different varieties of maize,which improved the performance of the BWB and USO model,but reduced the performance of the BBL model.The BWB model increased the most.