Intersexual flower differences in an andromonoecious species: small pollen-rich staminate flowers under resource limitation.

Andromonoecy, the presence of perfect and staminate flowers in the same individual, has evolved repeatedly in angiosperms. The staminate flowers are generally smaller than the perfect flowers in species that produce staminate flowers plastically when resources are limited. The smaller staminate flowers are expected to be less attractive to pollinators and have reduced size-matching with pollinators than perfect flowers. We hypothesized that these potential disadvantages of staminate versus perfect flowers facilitate the evolution of sex-specific floral morphology, such as allometric relationship between flower size and male reproductive organ. We compared six floral morphology traits, pollen production, pollinator visits and pollen removal from anthers between staminate and perfect flowers in several natural Commelina communis populations. Nectarless and zygomorphic C. communis flowers have polymorphic stamens with attracting, feeding and pollinating anthers and were visited by diverse pollinators. Staminate flowers were significantly smaller than perfect flowers, despite a large overlap in size between sexes. The lengths of pollinating stamens did not differ between staminate and perfect flowers, and staminate flowers produced significantly more pollen. We observed significantly more pollinator visits to perfect flowers than to staminate flowers. By contrast, pollen removal from pollinating stamens was significantly higher in staminate flowers than in perfect flowers. There is sexual dimorphism in flower morphology in C. communis. Staminate flowers with smaller attraction organs, similar pollinating stamens and higher pollen production assure higher pollen donor success relative to perfect flowers. Our results suggest that the morphological changes in staminate flowers enhance pollination success, even with limited resources.

Covering and shading by neighbouring plants diminish pollinator visits to and reproductive success of a forest edge specialist dwarf species.

The pollination and reproductive success of flowering plants can be negatively influenced in various ways by neighbouring heterospecific plants, such as resource competition and reproductive interference. We hypothesized that covering together with shading by neighbouring plants may reduce pollinator visits to and reproductive success of plants by reducing floral attractiveness and pollinator activity and by interrupting flower access, respectively. To test this hypothesis, we examined whether shaded and covered flowers suffered from pollinator limitation and low reproductive success in a population of the dwarf herb Lithospermum zollingeri, which co-exists with woody and herbaceous plants in anthropogenically maintained forest edge meadows. Here, shaded and covered flowers were defined as those beneath the shade of the woods and those whose front portion was covered by any vegetative part of neighbouring plants, respectively. The shaded and covered flowers were visited by significantly fewer pollinators than sunlit and open flowers in the field. However, three major pollinator species responded differently to shading and covering. Significant pollen limitation reduced seed set in covered flowers, and shaded flowers produced fewer seeds. Pollen removal from the anthers was not influenced by shading or covering. Our study demonstrates the negative effects of covering on pollinator visits and seed production. It also elucidates the negative effects of shading on reproductive success in L. zollingeri, which depends on managed semi-natural conditions. Land management abandonment, which has increased shaded and covered conditions in artificial forest edge meadows and open forest floors, might promote a rapid reduction in the populations of such dwarf plants.