Resource distribution and reproductive efficacy of dimorphic stamens of Senna occidentalis (L.) Link: a case study on the division-of-labour hypothesis

Heteranthery, the presence of different sets of stamens in the same flower, reduces the conflict between effective pollinators and flowers by ensuring adequate resources for the pollinators without affecting reproductive success. Here, we tested the division-of-labour hypothesis in Senna occidentalis, a buzz-pollinated, self-compatible, heterantherous species. The fecundity of pollen grains from long and short stamens was evaluated using hand cross-pollination experiments and by comparing the quantity, size, and quality of pollen in different sets of stamens. The visitation rate and behaviour of floral visitors were also assessed, while their preferences for different sets of stamens were checked using stamen exclusion experiments. The long-pollinating stamens produced a greater number and volume of pollen grains compared to the short-feeding stamens. The pollen grains from long stamens exhibited better germination rates with higher production of fruit and seeds than short stamen pollens in cross-pollination experiments. Nine floral visitors foraged on the flowers, and the large carpenter bees (Xylocopa sp.) were the most dominant and effective pollinators. The long stamens deposited pollen on the lateral and ventral thorax of large-sized pollinators, while the short stamens placed pollen on the ventral and abdomen portions. Therefore, the short stamens might produce pollen grains primarily to feed their pollinators, while the long stamens might generate pollen for pollination and successful fertilization. Further, breeding system experiments indicated a significant pollen limitation and dependency on insect pollination for the increased reproductive outputs (fruit and seed sets). Overall, our findings could indicate a differential distribution of pollen grains in long and short stamens. The differences in the quality and quantity of pollen grains produced by dimorphic stamens of S. occidentalis could fit the division-of-labour hypothesis.