Stomatal response to humidity: implications for transpiration

Abstract. Transpiration rates from apple leaves are analysed in terms of the ratio of latent heat flux (λ E ) to leaf net radiation ( Q ₁) and the climatological resistance ( r_i ). 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 as Q ₁ increases, so that leaf transpiration rates in high energy environments are likely to be similar to those in lower energy environments. Boundary layer resistance ( r _a) exerts an increasingly important effect on transpiration rates as Q ₁ increases. At constant Q ₁ stomatal closure in response to increasing D results in very small changes in leaf temperature ( T ₁) across a wide range of ambient vapour pressure deficits (δ e ); r _a is then the major factor determining T ₁. The implications of these results are discussed.