DROUGHT AND THE EVOLUTION OF FLOWERING TIME IN DESERT ANNUALS

Abstract

Drought is often thought to stimulate flowering in desert, and sometimes in mesic, annuals. I review experimental studies of the effect of drought on flowering time in both desert and mesic annuals. No convincing experimental evidence presently exists that drought stimulates flowering in any annual plant; some experimental results suggest the opposite. The design and analysis of flowering time studies are also reviewed; most extant studies have serious flaws. Thus, a convincing demonstration of drought‐stimulated flowering will require carefully designed and analyzed experiments.In light of these results, I examine several ways in which drought may affect the ecology and evolution of flowering time in desert annuals, and suggest directions for research. Several mechanisms probably contribute to phenotypic variation in flowering time and size, including water and nutrient limitation, competition, and variation in seed size and germination time. Phenotypic effects of seed traits suggest that seed and flowering time traits may not evolve independently of one another. Water stress during reproduction can influence seed traits; such maternal effects can influence the outcome of selection both on seed traits and on flowering time. The multivariate character of flowering time evolution suggests that genetic and phenotypic correlations among these traits may present important constraints on the evolution of flowering time.