Ecophysiological studies of sonoran Desert plants : V. Photosynthetic adaptations of Machaeranthera gracilis, a Winter annual.

Photosynthetic responses to light and temperature are compared for two genetically related chromosomal races of Machaeranthera gracilis. The ancestral foothills race occurs in cooler, more mesic environments, while the derived desert race occurs in more arid environments. The desert race exhibited greater rates of net photosynthesis at all levels of irradiance than did plants of the foothills race. This enhancement is due primarily to a greater quantum yield and leaf density. High light pretreatment significantly increased the photosynthetic capacity of the desert race, with little or no effect on the foothills race. Furthermore, the leaf density of the desert race was affected proportionately more than the foothills race by light pretreatment. The desert race also possessed higher stomatal and mesophyll conductances to CO2. Both races exhibited enhanced photosynthetic capacities when grown in a warm thermoperiod (35/25 C), relative to a cool thermoperiod (25/15 C), concomitant with slight increases in leaf density. There was a lack of thermal acclimation for both photosynthesis and respiration of the two races. The greater photosynthetic capactity of the desert race is suggested as an evolutionary modification which could account for invasion into and survival in the sonoran Desert.