Stomatal response to humidity: implications for transpiration

Abstract

Abstract.Transpiration rates from apple leaves are analysed in terms of the ratio of latent heat flux (λE) to leaf net radiation (Q1) and the climatological resistance (ri). Increases in stomatal resistance with increasing leaf to air vapour pressure gradient (D), described by an empirical model, are incorporated in the analysis. This humidity effect causes the proportion of energy dissipated as latent heat to fall asQ1increases, so that leaf transpiration rates in high energy environments are likely to be similar to those in lower energy environments. Boundary layer resistance (ra) exerts an increasingly important effect on transpiration rates asQ1increases. At constantQ1stomatal closure in response to increasingDresults in very small changes in leaf temperature (T1) across a wide range of ambient vapour pressure deficits (δe);rais then the major factor determiningT1. The implications of these results are discussed.