Ecophysiological studies of Sonoran Desert plants : III. The daily course of photosynthesis for Acacia greggii and Cercidium microphyllum.

The gas exchange and water relations of two Sonoran Desert plants are compared during contrasting periods of water and heat stress. Photosynthesis of Acacia greggii, a winter deciduous shrub, and Cercidium microphyllum, a chlorophyllous stemmed tree, show a moderate correlation with dawn plant water potential. For both species a relationship between stomatal conductance and dawn plant water potential was not apparent, although A. greggii demonstrated a greater overall stomatal conductance. This affected a greater daytime decrease in plant water potential at all levels of water stress and suggests A. greggii is less sensitive to water stress. Our results suggest the lower limit for gross photosynthesis occurs when dawn plant water potentials are less than -44 and -31 bars for the shrub and tree species, respectively. During periods of extreme water and heat stress the photosynthetic capacity of both species is regulated more by mesophyll than stomatal conductance. However, partial stomatal closure causes plant water potential to increase during the day and exceed dawn values. During periods of minimal water and heat stress the daily course of photosynthesis parallels the change in stomatal conductance and irradiance. Maximum gross photosynthesis rates are nearly three-fold higher than the rates observed during periods of stress, with those of A. greggii generally greater than the rates observed in plants of C. microphyllum.