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 CO 2 , g sc = g 0 + a 1 /[(c s −Γ)(1 + D s /D 0 )], where A is the assimilation rate; D s and c s are the humidity deficit and the CO 2 concentration at the leaf surface, respectively; g 0 is the conductance as A → 0 when leaf irradiance → 0; and D 0 and a 1 are empirical coefficients. This model is a modified version of g sc = a 1 A 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 CO 2 compensation point, Γ, 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 CO 2 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 C 3 plants synthesizes many of the observed responses of leaves to light, humidity deficit, leaf temperature and CO 2 concentration. Best results are obtained for well-watered plants