The Optimization of Seed Yield across the Flowering Season of Gentiana leucomelaena (Gentianaceae), an Herbaceous Tibetan Annual

Abstract

The seed yield of herbaceous annuals may be resource- or pollinator-limited. The extent to which either or both of these limitations negatively affect seed yield is hypothesized to vary with the timing of flowering because of seasonal changes in pollinator and resource availabilities. We tested this hypothesis by examining the relationship between seed yield and pollinator visitation rates, flowering phenology, aboveground plant biomass, and biomass allocation patterns, fruit set, and seed set, seed number per flower, and seed mass for Gentiana leucomelaena, an annual herbaceous dicot that produces four types of flowering shoots as a result of differences in flower color (blue and white) and dichogamy-type (protogyny and protandry). Bivariate plots of reproductive efficiency (total seed mass per shoot) versus vegetative mass were curvilinear and showed that early- and late-flowering shoots were more likely to be limited in seed yield than shoots flowering during the middle of the flowering season. Early-flowering shoots had greater biomass but had lower fruit set and seed set with fewer but larger seeds per fruit. These features were consistent with pollinator (visitation rate) limitations on seed yield. In contrast, late-flowering shoots had smaller biomass but had higher fruit set and seed set with many but smaller seeds per fruit. These features were consistent with resource (plant biomass) limitations on seed yield. Moreover, early-flowering shoots tended to produce white and protogynous flowers and allocated more to leaf growth, whereas late-flowering shoots tended to produce blue and protandrous flowers and allocated more to stem growth in height (perhaps to reduce shading by neighbors). In addition, the peak time of flowering shoot density coincided with the peak of reproductive efficiency. In conclusion, limitations on the seed yield of this species varied with the timing of flowering. G. leucomelaena appears to have evolved a system of changing flower color and dichogamy-type that optimizes seed yield in an alpine meadow habitat.