Trade off between quantity and size of pollen grains in the heterandrous flowers of Senna pendula (Fabaceae)

ABSTRACT Heteranthery, the presence of different types of stamens in a flower, may reduce the conflict between pollinators and plants by ensuring the resource for the pollinator without drastically affecting the availability of viable male gametes for fertilization, according to the division-of-labor hypothesis. We investigated whether the poricidal anthers of Senna pendula, a buzz-pollinated heterantherous species, present morphological and physiological differences among pollen grains from the three sets of stamens. We compared quantity, ornamentation, size and fecundity of pollen from long, medium and short stamens. The short feeding stamens produced larger but fewer pollen grains than the long pollinating stamens, which produced smaller pollen grains but in higher quantity. The total pollen volume of pollinating and feeding stamens per flower, however, was the same. The medium stamen produced less-fertile small pollen grains and seems to play no specific role in bee feeding and pollination. Our results indicate differential allocation of pollen for pollinating and feeding stamens mediated by heteranthery. The differences in volume versus quantity of pollen grains fit the division-of-labor hypothesis well for heterantherous pollen-only flowers with poricidal anthers.

摘要 雄蕊异型（heteranthery）指一朵花中存在不同类型的雄蕊，根据劳动分工假说（division-of-labor hypothesis），该特征可通过为传粉者提供食物资源，同时又不会大幅影响可用于受精的有活力雄配子的可用性，从而缓解传粉者与植物间的冲突。本研究针对振粉传粉（buzz-pollinated）的雄蕊异型物种弯叶决明（Senna pendula）的孔裂花药（poricidal anthers），探究其三组雄蕊所产生的花粉粒是否存在形态及生理差异。我们对比了长、中、短三类雄蕊所产生的花粉的数量、纹饰、大小及育性。相较于产生更多数量、更小体积花粉的长传粉雄蕊，短取食雄蕊所产生的花粉体积更大但数量更少。不过，单朵花的传粉雄蕊与取食雄蕊的花粉总容积并无差异。中等雄蕊所产生的花粉体积较小且育性较低，似乎在蜜蜂取食与传粉过程中未发挥特定作用。本研究结果表明，雄蕊异型介导了传粉雄蕊与取食雄蕊间的花粉差异化分配策略。对于具孔裂花药、仅以花粉为传粉报酬的雄蕊异型花而言，花粉在容积与数量上的差异与劳动分工假说高度契合。