APPLYING AN EMPIRICAL-MODEL OF STOMATAL CONDUCTANCE TO 3 C-4 GRASSES

An empirical equation for stomatal conductance has been developed. The equation is based on a linear index, which was modified to represent nonlinear independent effects of CO2 flux and water vapor pressure deficit. The equation was applied to data from caucasian bluestem (Bothriochloa caucasia (Trin.) C.E. Hubb.) and two accessions of Eastern gamagrass (Tripsacum dactyloides (L.) L.), measuring responses of leaves of the three grasses to wide ranges of environmental conditions. The equation accurately predicts stomatal conductance in these C-4 grasses, but requires measured photosynthesis as an input variable. Dependence on only environmental inputs was achieved by including the equation as the conductance submodel in a complete leaf gas exchange model, along with a photosynthesis submodel derived from a biochemically based model. This simplified submodel also describes the data well, as does the integrated model. Comparisons of model results and derived parameter values indicate important differences among gas exchange properties of the three grasses. Implementation details of the model are discussed, along with approaches for adapting it for simulating interleaf variability, water stress effects, and patchy stomatal function.