Perennial desert plants experience a broad range of temperatures over the course of their active growing season. Changing the optimal temperature for photosynthesis (thermal acclimation) is one adaptive mechanism for tolerating such fluctuations, but not all species are known to have this capability. Previous studies found that thermal acclimation is absent in the genus Encelia, although comparisons among species in this genus indicate that the capacity for evolutionary change in thermal optimum exists. We investigated thermal-acclimation potential in four populations of Encelia farinosa which span a broad temperature gradient. In each population there was some evidence of thermal acclimation, but the degree of acclimation was small in comparison to the temperature range in which the plants were grown. The thermal optimum for all populations was fairly consistent for plants raised in high temperature, but thermal optimum for plants raised at lower temperatures was correlated with average minimum spring temperatures of the populationís habitat. These results suggest that some local adaptation for optimal photosynthetic temperature exists in this species, but that thermal acclimation may be limited as a mechanism for tolerating large seasonal temperature changes.

Key words: desert, Encelia, photosynthesis, thermal acclimation