Physiological differentiation within an Encelia farinosa population along a short topographic gradient in the Sonoran Desert

Encelia farinosa growing along a 135-m slope in the Sonoran Desert of Arizona experienced differences in their seasonal moisture regimes. During the spring and summer the slope was characterized by progressively increasing moisture stress from the bottom (wash site) to the top (ridge site). The moisture differences were reflected at the community level by the presence of more cacti at the ridge site and more grasses at the wash site, and at the organismic level by more negative plant water potentials and less negative δ¹³C values (higher water-use efficiencies) in plants of E. farinosa at the ridge site. Gene flow along the gradient was high enough to overcome genetic differentiation by genetic drift alone. Despite high levels of gene flow along the topographic gradient, there was evidence of genetic differentiation at the genotype level and physiological level. Differential genotype selection was supported through observations that wash plants exhibited deficiencies of heterozygotes at two out of five variable loci, compared to ridge plants. In a series of greenhouse studies, physiological differentiaiton was observed through lower stomatal conductances and more conservative use of limited soil moisture supplies in ridge plants, compared to plants from the more mesic wash site. The responses of photosynthesis rate and stomatal conductance to decreasing pre-dawn water potentials were similar between wash and ridge plants. Results indicate that physiological differentiation in this population of E. farinosa occurs with respect to patterns of water use, rather than patterns of tolerance to water stress. -from Authors