Modelling stomatal responses to high temperatures under a range of different vapor pressure deficits

Low stomatal conductance (gs) has been identified as an indicator of several plant stresses, including heat stress. Models for gs that are coupled to photosynthesis such as the BWB-Leuning-Yin model rely among others on the leaf-to-air vapor pressure deficit (VPDleaf) as well as photosynthesis as an input. These models assume stomatal closure upon a decrease in photosynthesis when all other factors are constant. However, at high leaf temperatures, low levels of carbon fixation along with high gs have been reported. This is known as a decoupling between photosynthesis and gs, and might be related to high gs resulting in higher transpiration and therefore more intense leaf cooling to mitigate heat stress. Furthermore, it is known that a high VPDleaf can lead to stomatal closure, however, the effect of VPDleaf as a driver of gs at high temperatures remains unknown. To investigate the relation between photosynthesis and gs as well as the effects of VPDleaf on gs at high leaf temperatures, we conducted an experiment in a climate controlled growth cabinet. Light response curves were measured on tomato leaves at leaf temperatures of 30, 33, 35, 38 and 42°C in combination with VPDleaf of 0.9, 1.2, 1.5 and 2 kPa. Data were used to fit the Farquhar type of photosynthesis model including a BWB-Leuning-Yin model for gs. Preliminary results suggest that at high temperatures, photosynthesis was decoupled from gs, while gs rose to excessively high values. Our results also indicate that at high temperatures stomata dynamics are slowed down including that stomata become less responsive to changes in VPDleaf. We further discuss the need to include these relations at high temperatures in models to improve the prediction quality of stomatal conductance under temperature extremes.