A critical appraisal of a combined stomatal-photosynthesis model for C3 plants

Gas-exchange measurements on Eucalyptus grandis leaves and data extracted from the literature were used to test a semi-empirical model of stomatal conductance for CO₂ g_Sc=g_o+a₁A/(c_s-I) (1+D_s/D_o)] where A is the assimilation rate; D_s and c_s are the humidity deficit and the CO₂ concentration at the leaf surface, respectively; g₀ is the conductance as A → 0 when leaf irradiance → 0; and D₀ and a₁ are empirical coefficients. This model is a modified version of g_sc=a₁A h_s/c_s first proposed by Ball, Woodrow & Berry (1987, in Progress in Photosynthesis Research, Martinus Mijhoff, Publ., pp. 221–224), in which h_s is relative humidity. Inclusion of the CO₂ compensation point, τ, improved the behaviour of the model at low values of c_s, while a hyperbolic function of D_s for humidity response correctly accounted for the observed hyperbolic and linear variation of g_sc and c_i/c_s as a function of D_s, where C_i is the intercellular CO₂ concentration. In contrast, use of relative humidity as the humidity variable led to predictions of a linear decrease in g_sc and a hyperbolic variation in c_i/c_s as a function of D_s, contrary to data from E. grandis leaves. The revised model also successfully described the response of stomata to variations in A, D_s and c_s for published responses of the leaves of several other species. Coupling of the revised stomatal model with a biochemical model for photosynthesis of C₃ plants synthesizes many of the observed responses of leaves to light, humidity deficit, leaf temperature and CO₂ concentration. Best results are obtained for well-watered plants.