Effect of vapour pressure deficit on gas exchange of field‑grown cotton

Background:Plants respond to changes in vapour pressure deficit(VPD)between the leaf and the atmosphere through changes in stomatal response,which can consequently affect transpiration,photosynthesis,and leaf-level water use efficiencies.With projected warmer air temperatures,changes in rainfall distribution and altered VPD in future climates,it is important to understand the potential effect of VPD on leaf-level physiology of field-grown crops.The aim of this study was to assess the impact of altered VPD on leaf-level physiology of field-grown cotton to improve the current understanding of the plant-by-environment interaction,thereby contributing to validation and improvement of physiological and yield response models.Different VPD environments in the field were generated by planting cotton on three dates within the sowing window(early-season(S1)=5th October 2011;mid-season(S2)=9th November 2011;and late-season(S3)=30th November 2011).VPD was also modified by altering crop irrigations.Results:VPDL accounted for the largest proportion of the explained variation in both stomatal conductance(32%∼39%)and photosynthetic(16%∼29%)responses of cotton.Generally,smaller percentages of variation were attributed to other main factors such as the individual plant(Plant),and accumulated temperature stress hours(ASH;a measure of plant water status over time)and interactive factors,including leaf vapour pressure deficit(VPDL)×Plant and Plant×ASH;however,a proportion of variation was unexplained.In addition,the Asat/E(instantaneous transpiration efficiency,ITE)model developed based on cotton grown in the glasshouse was applied to cotton grown in the field.We found that the modelled Asat/E and field-measured Asat/E were very similar,suggesting that the mechanistic basis for ITE was similar in both environments.Conclusions:This study highlights the importance of accounting for VPD in climate change research,given that stomata are highly responsive to changes in VPD.This experiment provides a basis for physiology and production models,particularly in terms of cotton response to projected climatic environments.