Reproductive biology of a tropical Acacia hybrid (Acacia mangium Willd. x A. auriculiformis A. Cunn. ex Benth.)

The Acacia hybrid (Acacia mangium x A. auriculiformis, Leguminosae: Mimosoideae) has created considerable interest for plantations because of its adaptability and growth performance when compared to the parental species. This study concentrated on sexual reproduction, and seed and seedling quality using light and electron microscopy, histochemistry, and seed and seedling tests.; Two peak flowering periods in the hybrid appear to coincide with high rainfall and temperature, whereas two fruit-maturation periods occur during a windy dry season. The hybrid is andromonoecious. A floral spike consists of about 150 loosely arranged flowers. Flowers are cream colored and fragrant and have no floral nectaries. The pistil has a solid style with a smooth, wet stigma and amphitropous ovules with immature integuments at pollination. 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 to the stigmas. Apis mellifera and Ceratina sp. are the most effective pollinators. They are the most common insect visitors and carry a heavy load of hybrid polyads. However, their behavior in foraging for pollen in the same tree may promote self-pollination. The 16-pollen polyads have the highest viability at anthesis (over 80%) but lose viability within 3 days. In vivo pollen germination occurs within a few hours and pollen tubes grow up to 16 μm/min, reaching the ovarian chamber 7 to 8 hr after pollination. In vivo pollen tube growth is supported by the stylar secretion that may be stimulated by pollination and an ovarian secretion which is independent of pollination.; Fertilization in the hybrid occurs within 3 days after pollination. One of the two synergids is the site of pollen tube penetration and its degeneration is triggered by the pollen tube penetration of the nucellus. Endoplasmic reticulum is likely involved in the polar nuclear fusion but not in the fusion of sperm nuclei with the egg and polar nuclei. Because no sperm cytoplasmic fusion occurs during karyogamy, the hybrid, therefore, possesses maternal cytoplasmic inheritance. The hybrid zygote is metabolically inactive and has a two-month dormant period due to delays in embryo nutrition.; The hybrid has very low reproductive success (0.0054). Low fruit set in the hybrid (2%) was attributed primarily to insufficient pollination (65% of total) and early fruit abortion (33% of total). Low seed set (24%) is mainly caused by failure of pollen tube penetration of the ovules (over 70%). (Abstract shortened by UMI.)...