Stamen elongation in sunn hemp appears to allow delayed self-pollination in the absence of pollinators – A case of bet-hedging?

Sunn hemp, Crotolaria juncea L., is an important green manure crop in the tropics. In this species, flowers are hermaphrodite, self-compatible, but herkogamous with the stigma located well above the anthers, and thus appear to be cross-pollinated. Anthers are dimorphic (5 stamens with short and 5 stamens with long filaments) and hidden beneath keel petals. The longevity of flowers was three days. Of the 16 species of bees that visited flowers of sunn hemp, four species of Megachile (M. lanata, M. bicolor, M. disjuncta and M. anthracina) and one species of Xylocopa (X. fenestrata) were effective pollinators and constituted more than 75% of visits. Species of Apis never came in contact with stigma or anther lobes and were not involved in pollination. Open pollination resulted in 78% pod set. However, when flowers were prevented from pollinator visitation, the pod set was still 62%. In the absence of pollinators, filaments of long stamens elongated and reached the level of stigma which presumably led to delayed autonomous self-pollination, a bet-hedging strategy by the plant to set seeds in the absence of pollinators. • Sunn hemp flowers bear dimorphic stamens (short-stamen and long-stamens). • Viability of pollen from two stamens type varied across floral longevity period. • When nearing senescence, the filaments of long stamens grew nearly five times longer to reach stigmatic surface. • In the absence of pollinators, long stamen elongation led to delayed autonomous self-pollination. • Floral visits by Megachile and Xylocopa species resulted higher pod set than Apis Species. [ABSTRACT FROM AUTHOR]