Seasonal change in response of stomatal conductance to vapor pressure deficit and three phytohormones in three tree species

Stomatal behavior under global climate change is a central topic of plant ecophysiological research. Vapor pressure deficit (VPD) and phytohormones can affect stomata of leaves which can affect gas exchange characteristics of plant. The role of VPD in regulating leaf gas exchange of three tree species was investigated in Jinan, China. Experiments were performed in June, August, and October. Levels of three phytohormones (GA3, IAA, ABA) in the leaves of the three trees were determined by high-performance liquid chromatography in three seasons. The responses of stomatal conductance (gs) to an increasing VPD in the leaves of the three trees had peak curves under different seasons, which differed from the prevailing response pattern of gs to VPD in most literature. The peak curve could be fitted with a Log-Normal Model (R2 = 0.838–0.995). The VPD/RH values of the corresponding maximum of gs (gs-max-VPD/RH) could be calculated by fitted models. The gs-max-RH could be affected by environmental conditions, because of positive correlation between gs-max-RH and the mean monthly temperature in 2010 (R2 > 0.81). Two typical stomatal models (the Leuning model and the optimal stomatal behavior model) were used to estimate gs values, but they poorly predicted gs in the three trees. The concentration of ABA was positively correlated to sensitivity in response of stomatal conductance to VPD in the leaves of the tree species during the different seasons.