Drought alters the expression of mating system traits in two species of Clarkia

Variation in mating system traits can have important consequences for plant populations by affecting reproductive assurance, the expression of inbreeding depression, and the colonization of and persistence in new or altered habitats. Environmental stressors, such as drought, have been hypothesized to induce higher rates of self-fertilization, yet this hypothesis has rarely been tested. Here we measure the response of two sister species of self-compatible annual herbs from contrasting habitats, Clarkia breweri and C. concinna, to an experimentally imposed greenhouse drought treatment. We find that the species differ in their baseline per-flower autogamy rates and the degree of spatial and temporal separation of male and female function within their flowers. Both species show a reduction in temporal separation of anthesis and stigma receptivity with the drought treatment. However, the species from the more mesic habitat, C. concinna, increases its low autogamy rate under drought conditions, whereas the species from the more xeric habitat, C. breweri, decreases its high autogamy rate under drought conditions. Neither species showed a response to drought in flower size or anther-stigma distance. Our results demonstrate that the induction of selfing under environmental stress cannot be assumed and that, in this case, the developmental timing of flower maturation is more plastic than floral morphology.