Decoupling of stomatal conductance from net assimilation at high temperature as a mechanism to increase transpiration

Abstract

O_LIPhotosynthetic assimilation (Anet) and stomatal conductance (gs) are usually strongly coupled, but this relationship is decreased or even lost at high temperatures (Tair). The contributions of environmental drivers (Tair, vapour pressure deficit (VPD), and soil moisture) in interaction with the physiological mechanisms behind this process are still unclear. C_LIO_LIWe exposed saplings of three temperate and tropical species to rising Tair (20 to 40{degrees}C) at low (1.2 to 1.9 kPa) and increasing VPD (1.1 to 5.6 kPa), and at stable Tair (35{degrees}C) to increasing VPD (1.4 to 4.3 kPa) under well-watered or chronic soil drought conditions ([≤]10 %). Anet, gs, and transpiration (E) in the light and the dark and leaf thermoregulation were tracked throughout the experiment. C_LIO_LIWhen VPD remained low, gs continued to increase while Anet decreased at Tair > 35{degrees}C, leading to stomatal decoupling. In contrast, under rising VPD, trees maintained the coupling between Anet and gs at high Tair. C_LIO_LIWhile a decoupling of Anet and gs only occurred when VPD was low, Anet and E decoupled under both VPD regimes at high Tair. C_LIO_LIOur results indicate that, since gs and VPD collectively drive E, stomatal decoupling is needed to increase E when VPD is not sufficiently high. C_LI