DIURNAL GAS-EXCHANGE CHARACTERISTICS OF 2 STEM PHOTOSYNTHESIZING LEGUMES IN RELATION TO THE CLIMATE AT 2 CONTRASTING SITES IN THE CALIFORNIA DESERT

The diurnal and seasonal patterns of climate, shoot water potential, s tem photosynthesis (Pn), stem conductance, and stem intercellular CO2 were measured for two legume shrubs in the southern California desert at two elevations at four seasons of the year. One species (Senna arma ta), is restricted to the Mojave desert and was studied at 950 m eleva tion, while the other species (Caesalpinia virgata) is endemic to the Sonoran desert and was studied at 180 m elevation. The Sonoran desert site was characterized by higher temperatures, higher vapor pressure d eficits, and more consistent solar radiation than the Mojave desert si te. During the summer, the differences between the microclimates of th e two sites were maximal. Both species have high predawn and midday sh oot water potentials compared with other desert species, most likely b ecause they have vertical stem orientation and low stem conductance. S tem Pn was positive all year, but Pn(max) decreased for C. virgata dur ing the summer. Stem temperature, and its impact on vapor pressure def icit, was the most important regulator of stem photosynthesis. Althoug h there were large changes in stem Pn between winter and summer, there was little change in intercellular CO2 among seasons inferring no cha nge in water use efficiency. Stem Pn most likely provides most of the carbon gain for both species because leaves are small and ephemeral, a nd stems are present and actively gaining carbon all year.