Ecophysiological studies of Sonoran Desert plants : I. Diurnal photosynthesis patterns of Ambrosia deltoidea and Olneya tesota.

The gas exchange of two Sonoran Desert plants was measured near optimum soil water conditions occurring in the summer and winter. Photosynthesis and stomatal conductance rates of a drought-deciduous shrub, Ambrosia deltoidea, and an evergreen non-riparian tree, Olneya tesota, are mainly affected by plant water potential. During such periods the diurnal gas exchange patterns are characterized by maximum rates of photosynthesis and stomatal conductance occurring early in the day, which decrease progressively thereafter. The effect of plant water potential on gas exchange is both direct and indirect. Decreasing plant water potential indirectly affects 14CO2 assimilation by closure of stomata, and the effect is similar in both species. However, the direct effects of decreasing plant water potential are dissimilar in the two species. Plants of the shrub species have a higher potential maximum photosynthesis, but are more sensitive to plant water stress than are plants of the tree species. Both species respond to favorable growth conditions in the summer and winter, and have the potential for rapid carbon input into the Sonoran Desert ecosystem.