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Population- and family-level variation of brittlebush (Encelia farinosa, Asteraceae) pubescence: its relation to drought and implications for selection in variable environments¹

²Department of Biological Science, California State University, Fullerton, California 92834 USA; ³Department of Biology, University of Utah, Salt Lake City, Utah 84112 USA

Because leaf pubescence of the desert shrub Encelia farinosa increases in response to drought and influences photosynthesis and transpiration, we hypothesized that differences in water availability across the range of this species may result in genetic differentiation for pubescence and associated productivity traits. We examined maternal family variation of pubescence-moderated light absorption (absorptance) in three populations of E. farinosa. Absorptance was always greatest for plants from the high-rainfall environment and lowest for those from the driest site, but the rate of absorptance change in response to drought was similar among all populations. Similar patterns were found when we compared families within populations—all genotypes had similar initial leaf absorptances, differentiated very early in seasonal growth, then had concordant changes in absorptance thereafter. However, family-level variance was greatest for plants from the driest site, a region with highly heterogeneous precipitation patterns, whereas low variance was found for plants from the wettest, least heterogeneous site. The concordance of leaf absorptance changes, within and among populations, may be due to integration with other drought-related traits; however, the differences in absorptance values within and among populations suggest that variation of leaf pubescence results from selection associated with geographical and local patterns of water availability.

Key Words: absorptance • adaptation • Asteraceae • brittlebush • desert • drought acclimation • Encelia farinosa • leaf pubescence • maternal family variance • spatiotemporal heterogeneity

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