The Competition between Liquid and Vapor Transport in Transpiring Leaves1[W][OPEN]

In leaves, the transpirational flux of water exits the veins as liquid and travels toward the stomata in both the vapor and liquid phases before exiting the leaf as vapor. Yet, whether most of the evaporation occurs from the vascular bundles (perivascular), from the photosynthetic mesophyll cells, or within the vicinity of the stomatal pore (peristomatal) remains in dispute. Here, a onedimensional model of the competition between liquid and vapor transport is developed from the perspective of nonisothermal coupledheat andwatermolecule transport in a compositemediumof airspace and cells.Ananalytical solution to themodel is found in terms of the energy and transpirational fluxes from the leaf surfaces and the absorbed solar energy load, leading tomathematical expressions for the proportions of evaporation accounted for by the vascular,mesophyll, and epidermal regions. Thedistribution of evaporation in a given leaf is predicted to be variable, changing with the local environment, and to range from dominantly perivascular to dominantly peristomatal depending on internal leaf architecture, with mesophyll evaporation a subordinate component. Usingmature red oak (Quercus rubra) trees,we show that themodel can be solved for a specific instance of a transpiring leaf by combining gas-exchange data, anatomical measurements, and hydraulic experiments. We also investigate the effect of radiation load on the control of transpiration, the potential for condensation on the inside of an epidermis, and the impact of vapor transport on the hydraulic efficiency of leaf tissue outside the xylem.