Combined impacts of irradiance and dehydration on leaf hydraulic conductance: insights into vulnerability and stomatal control

The leaf is a hydraulic bottleneck, accounting for a large part of plant resistance. Thus, the leaf hydraulic conductance (Kleaf) is of key importance in determining stomatal conductance (gs) and rates of gas exchange. Previous studies showed that Kleaf is dynamic with leaf water status and irradiance. For four species, we tested the combined impacts of these factors on Kleaf and on gs. We determined responses of Kleaf and gs to declining leaf water potential (?leaf) under low and high irradiance (<6 and >900 mu mol photons m-2 s-1 photosynthetically active radiation, respectively). We hypothesized greater Kleaf vulnerability under high irradiance. We also hypothesized that Kleaf and gs would be similar in their responses to either light or dehydration: similar light-responses of Kleaf and gs would stabilize ?leaf across irradiances for leaves transpiring at a given vapour pressure deficit, and similar dehydration responses would arise from the control of stomata by ?leaf or a correlated signal. For all four species, the Kleaf light response declined from full hydration to turgor loss point. The Kleaf and gs differed strongly in their light- and dehydration responses, supporting optimization of hydraulic transport across irradiances, and semi-independent, flexible regulation of liquid and vapour phase water transport with leaf water status.