Ecophysiological studies of Sonoran Desert plants. VII. Photosynthetic gas exchange of winter ephemerals from sun and shade environments.

A comparison of photosynthetic adaptations of desert ephemerals from sun and shade environments has shown differences in the magnitude and timing of maximum CO2 assimilation. Photosynthetic rates of species from the sun environment were significantly higher, and maximum observed levels occurred both earlier in the day and earlier in the growing season. For plants of both sites the CO2 assimilation rates were relatively low when the soil water content was highest in the winter months. Anatomical modifications of specific leaf weight occurred for species of both environments, but were not correlated with improved water use efficiency. Throughout the growing season the stomatal conductance of most species was higher than 0.30 cm/s and effects a maximization of photosynthesis when water is available, at the expense of water use efficiency.