Reproductive biology and pollination of Senna sylvestris (Vell.) H.S. Irwin & Barneby (Leguminosae, Caesalpinoideae)

The reproductive biology of Senna sylvestris was studied in the Panga Ecological Station, Uberlândia and in the Macaúbas district of Patrocínio, Minas Gerais state, Central Brazil. S. sylvestris is a shrub or small tree up to 5m which flowers from January to February. Flowers are hermaphrodite, zigomorphous, with an yellow corolla and disposed in paniculate inflorescences. The stigma is wet, non-papillous and crateriform, surrounded by hairs. Anthers are poricidal in a heteranterous androecium, with three staminodia, four small stamens in the upper portion and two larger ones below with another one with a thin anther in between them. S. sylvestris is self-sterile and non-apomictic, with self-sterility involving late-acting self-incompatibility type phenomena. Self- and cross pollen tubes grow to the ovary and penetrate ovules without any detectable diference, but fruits and seeds developed only from cross pollinations using pollen from the two larger anthers. Smaller stamens and even the large one with a thinner anther produced non-viable pollen. These stamens serve as holding places for the visiting bees. Pollen from these anthers may germinate but no fruit resulted from pollination with this kind of pollen. The main visitors and pollinators were large bees Xylocopa brasilianorum, Oxaea flavescens, Bombus morio, and species of Centris and all collect pollen by vibrating the anthers (buzz pollination). Differences in pollinator species richness were observed between sites and study years. Smaller number of pollinators in Macaúbas may be the result of the increased disturbance in that area. However, species richness and number of pollinators did not explain the constantly low fruit set in the studied species.

breeding system; cerrado; Fabaceae; large bees; late-acting self-incompatibility