Constraints on reproductive success due to timing of flowering in Cassia fistula L.

摘要

Natural variation in reproductive success of trees of Cassia fistula flowering at different times over the flowering period was investigated in a dry thorny forest of Mudumalai Wildlife Sanctuary, southern India. Pollen number deposited on the stigma, fruit set percentage, insect visitation rates and number of seeds per fruit were observed for the tagged trees that flowered at different times within the flowering period of the species. These parameters were also observed for trees selected at low and high density of flowering individuals, at peak flowering to know if density of flowering individuals may influence reproductive success. The trees flowering in synchrony with other individuals in the population showed higher reproductive success thanindividuals that flowered out of synchrony. The pollen deposited on the stigma, insect visitation rates, number of fruits set per flower and seed number per fruit was found high during mid flowering season. These variations were attributed to the variations in insect visitation rates for the flowers that lead to variation in fruit-flower ratios and seed set ratios. Path coefficient analysis revealed that insect visitation contributes greatly to the number of seeds per fruit by enhancing the pollen deposited on the stigma. It is argued that the individuals benefit by synchronizing their flowering schedules either through reducing the heterospecific pollen loads or by increasing the pollinator visitation rates. It was also observed that the flowering individuals at different densities did not differ with respect to visitation rates, pollen number deposited on the stigma, fruit and seed set ratios and also the number of seeds per fruit. The lack of such difference could be because of the isolation distance may not be critical for bees to pollinate the flowers. It is argued that stabilizing selection may be influencing for flowering time of Cassia fistula.