Putative phylogenetic constraint of plasticity for physiological and morphological traits in the genus Encelia (Asteraceae) appears to have resulted in strong selection for plasticity of leaf pubescence within at least one species, Encelia farinosa. Leaf pubescence is a crucial component of the energy balance, water relations and photosynthetic physiology of this drought-deciduous desert shrub, but pubescence development is also irreversible and thus prevents adaptive responses under certain conditions such as mid-drought rainfall. Carbon-gain and water-use tradeoffs associated with these developmental responses play a complex role in adaptation of this species across a broad environmental matrix. I will discuss how a combination of population genetic and ecophysiological studies has been used to elucidate these complex relationships and relate them to the environmental heterogeneity of this species’ range. Of these analyses, one (carbon isotope ratio) provides an integrated measurement of multiple physiological responses (photosynthesis and water-use). Due to the unique but fairly well understood interplay of phenotypic limitations and plasticity in this species, Encelia farinosa provides a model system for the study of phenotypic integration and adaptation in plants.

Key words: drought, Encelia, leaf pubescence, photosynthesis, plasticity, water-use