Pollination Biology in a TropicalAcaciaHybrid (A. mangiumWilld. ×A. auriculiformisA. Cunn. ex Benth.)

Floral biology and pollination mechanism of the Acacia hybrid ( A. mangium Willd. × A. auriculiformis A. Cunn. ex Benth.) growing in Thailand are investigated using light and electron microscopy. The hybrid is andromonoecious. A floral spike consists of about 150 loosely arranged flowers. Flowers are cream coloured, fragrant and have no floral nectaries. The pistil has a solid style with a smooth, wet stigma and amphitropous ovules with immature integuments. The anther consists of eight loculi, each bearing only one 16-grain polyad. The flowers are weakly protogynous. Anthesis is complete at 0500–0600 h but peak female receptivity begins at 0200–0300 h and is completed that day. The stigmatic exudate is of the lipophilic type and is secreted from the stigmatic cells by a holocrine mechanism. Pollen is the main floral reward for the insect pollinators. There are several floral characteristics which facilitate pollen transfer from anthers and deposition on stigmas. Apis mellifera and Ceratina sp. are the most effective pollinators because they are the most common visitors and carry a heavy load of hybrid polyads. However, their behaviour in foraging for pollen in the same tree and weak protogynous dichogamy may promote self-pollination in the hybrid. The hybrid has low pollination success due to low pollinator number. An increase in exposure time of flowers to pollinators or pollinator number may increase pollination success but may not affect the rate of pollen deposition on stigmas due to the relatively small size of the stigma in relation to the polyad. Copyright 1998 Annals of Botany Company