Measurement And Modelling Of Crop Stomatal Conductance And Water Use Efficiency Under Drought Stress And Elevated CO₂ Concentration

Recently,CO2 concentration keeps elevated,and the frequent occurrence of extreme drought and high temperature weather is the main feature of global climate change.Both elevated CO2 concentration(e[CO2])and drought stress will affect the plants in terms of growth and physiology.In addition,appropriate nitrogen(N)supply contributes to well plant growth.Therefore,investigating the effects of e[CO2]combined drought stress under different N supply on crop physiology and growth,understanding the physiological regulatory mechanism of crops in response to climate change can contribute to provide a solid theoretical basis for formulating reasonable water-saving irrigation strategies and improving efficient water use of crops under e[CO2]and water resource constraints in the future.In this study,two pot-experiments in two separate phytotrons combined with stomatal conductance models were conducted in order to investigate the mechanism about the effects of e[CO2]and drought stress on growth,physiology,and water and nitrogen use efficiency of barley and tomato plants,and the applicability and accuracy of the Ball-Berry model for simulating and predicting stomatal conductance(gs)and water use efficiency(WUE)under different soil water status were discussed.In experiment I,two levels of N supply(N1,0.5 g N pot-1 and N2,1.0 g N pot-1)at ambient CO2 concentration(a[CO2])and e[CO2]were set,and barley and tomato seedlings were well-watered or subjected to progressive soil drying,in order to explore the responses of leaf water relationship and nitrogen use efficiency(NUE)of both crops to e[CO2]combined with N supply levels and progressive drought stress.In experiment Ⅱ,three irrigation regimes,including full irrigation(FI),deficit irrigation(DI),and alternate partial root-zone irrigation(PRD)were applied to tomato plants at two levels of N supply(LN,1.0 g N pot-1 and HN,2.0 g N pot-1)under a[CO2]and e[CO2],and the applicability and accuracy of the Ball-Berry model for simulating stomatal conductance and water use efficiency were tested based on the leaf gas exchange data.The main results are as follows:(1)The response characteristics of N supply levels to gas exchange,leaf water relationship,dry matter accumulation,and water and nitrogen use efficiency of barley and tomato plants in response to soil progressive drying under e[CO2]were clarified.e[CO2]increased photosynthesis and WUE while decreased specific leaf area(SLA)in both species;whereas N supply differentially influenced WUE in barley and tomato plants.The gs of barley plants was more sensitive to leaf ABA variation than tomato plants during progressive soil drying,while CO2 environment did not affect the response in both species.Compared to a[CO2],e[CO2]reduced plant transpiration rate(Tplant)in barley but not in tomato.In addition,e[CO2]increased the leaf C:N ratio([C:N]leaf)in plants by enhancing leaf C concentration([C]leaf)in barley and by dilution of leaf N concentration([N]leaf)in tomato,respectively,but N2 substantially decreased[C:N]leaf.Collectively,appropriate N supply may modulate the acclimation of plants to e[CO2]and soil water deficits.(2)The applicability and accuracy of the Ball-Berry model for simulating stomatal conductance and water use efficiency of tomato leaves under e[CO2]and different degrees of soil drought were studied.The decrease of the slope(m)in Ball-Berry model due to soil water deficit reflected the insensitivity to photosynthesis,CO2 concentration([CO2]),and relative humidity when suffering from drought stress,while the increase of m was found at e[CO2],suggesting that the simulation of gs at different[CO2]in the Ball-Berry model can be independent of photosynthesis.The Ball-Berry model performed high accuracy and broad adaptability for the prediction of gs in tomato plants at different soil water status when under a[CO2]and e[CO2],while at a[CO2],the higher sensitivity of the model to soil water regimes affected the predicted range and accuracy of WUE in tomato plants,and this limitation has been ameliorated at e[CO2].In addition,in view of the sensitivity of the Ball-Berry model to soil water status,the introduction of water regime function to the Ball-Berry model may improve its prediction range and prediction accuracy,which is of great significance for improving the applicability and simulation accuracy of the Ball-Berry model.