PARENT SESSION
Oral Session #4: Plant Ecology: Gas Exchange. Presiding: J. Pereira.
Monday, August 6, 2001. 8:00 AM to 12:00 PM. Hall of Ideas P&Q.

Desert dogma revisited: coupling stomatal conductance and photosynthesis in a desert shrub.

Ogle, Kiona¹, Reynolds, James¹, ¹

ABSTRACT- The success of the desert shrub Larrea tridentata has been largely attributed to its photosynthetic temperature acclimation ability and its stomatal control of photosynthesis under drought stress. A paucity of data on these relationships in the wild led us to develop a simple photosynthesis (A) model that explicitly couples internal CO₂ (Ci) and stomatal conductance (G). Vapor pressure deficit (VPD) and plant predawn water potential (PD) affect instantaneous G, which cannot exceed the potential maximum (Gmax). Ci is a linear function of G and the intercept is the minimum achievable Ci (Cmin), which varies with temperature (T) and light intensity. The slope depends on atmospheric CO₂, Cmin, and Gmax. After accounting for the effect of growing T on Gmax, diurnal simulations of A and G closely matched field data (predicted vs. observed r² = 0.75). To investigate a range of potential stomatal responses, we compared our G submodel to four G models from the literature. Our model was superior at predicting A and G primarily because it captured the large seasonal and diurnal fluctuations in G and Ci, while the other models frequently over-estimated their magnitude and under-estimated their variability. Application of our model over a wide number of environmental situations suggests that the dogma attributed to Larrea's success is supported with regard to variations in VPD and PD, but not for T acclimation mechanisms and CO₂ demand.

KEY WORDS: photosynthesis model, conductance model, Larrea tridentata