High and dry: Drought stress, sex-allocation trade-offs, and selection on flower size in the alpine wildflower Polemonium viscosum (Polemoniaceae)

Sex-allocation trade-offs may maintain variation in secondary sexual charac teristics if such traits vary in their benefits or costs in association wit h different genders. In Polemonium viscosum, large flowers benefit both mal e and female aspects of reproduction. In this study, I explore how resource investment in flower size influences the cost of allocation to male and fe male function. Large Hewers exact a water cost in P. viscosum under dry con ditions. In an extreme drought in 1997, experimentally watered plants had h igher survival and fecundity than controls. By comparing allocation pattern s between plants dying from drought and survivors, I tested whether the dem ographic cost of large flowers increases with allocation to fecundity. Cont rols that died showed a positive relationship between flower size and fruit production, while survivors showed a negative relationship or trade-off. W atered plants showed no such trade-off To test whether drought affects the relationship of corolla size to male function, I used leaf-water potential in 1998 to classify plants as stressed or unstressed. Corolla size showed p ositive correlations to pollen per flower regardless of drought stress. I c onclude that under drought the demographic cost of producing large flowers is gender dependent, such that viability selection favors either small-flow ered plants with female-biased reproduction or larger-flowered plants with male-biased reproduction.