Intermittent mass blooming, midnight anthesis and rockbee pollination in Pterocarpus santalinus (Fabaceae)

Pterocarpus santalinus blooms massively in the dry season. Flowering is not continuous during the season and mass bloom recurs only on certain days with 2–5 days interval or sometimes there may be little flowering in between two days of massive bloom. A tree may have five such occasions of mass bloom. At the inflorescence level such intermittent flowering occurs also. The flowers open at the mid of night and are bright yellow in colour. They are typically papilionaceous flag blossoms and thus zygomorphic. They are homogamous and are visited by the rockbee Apis dorsata at moonlit nights or otherwise at dawn and up to 0730 h. The flowers permit several effective visits by the rockbee and as long as the pressure of the bee continues the stamens and stigma project from the keel and permit sternotribic pollination. Controlled pollinations show fruit formation after autogamy (25%), geitonogamy (60%) and xenogamy (85%). Staggered anther dehiscence minimises pollen discounting. Flower bud, flower and fruit abortions occur on a large scale. Natural fruiting is very low compared to the mass bloom. Compatibility to geitonogamy and autogamy may function as a reproductive assurance mechanism. The natural habitat of P. santalinus has a hot dry climate, and the nocturnal flowering and foraging of the rockbee at that time during moonlit nights appear to be an adaptation to avoid the adverse effects of daytime high temperatures.