A. mangium flower is regular in symmetry, consisting of five sepals, five petals, numerous stamens and one gynoe-cium. It has a mild and sweet fragrance, which is particularly distinct in the early morning when individual flowers are in boom. (Zakaria 1993). Stigma is non-papillate, measure 63 microns in diameter and forms a cup shaped depression at the tip of style. Stigma and anthers lie in same plane. The anther is bilobed and measures 183 microns. Each lobe has four separate loculi with each loculus enclosing a polyad (Composite pollen grains). The polyad is spherical in shape with diameter of 30-40 microns and each polyad consists of 16 pollens. On an average, there are about 113 stamens per flower. The ovary is sessile, normally with minute hairs, with 12-14 ovules per ovary. Flowers are generally hermaphroditic. However, in some inflorescence staminate flowers are also present (Zakaria and Kamis 1991).
A. mangium is generally an outcrossing species with the tendency toward selfing (Zakaria 1993). In A. mangium andromonoecy-spatial separation of sexes is not prominent. In terms of temporal separation of sexes, protogynous dichogamy is not prevalent. Anthesis occurs very early in the day, with flowers opening in the preceding night at about 21:00 hr. The synchronous emergence of styles and stamens, the immediate anther dehiscence, and stigma receptivity after anthesis signify that the flowers of A. mangium are homog-amous (Zakaria 1993). Zakaria (1993) also found that the species index of self-incompatibility (ISI) rating was 0.38, which could lead it to be classified as an out-crossing species with some degree of selfing despite being partially self-incompatible. Its partial self-incompatibility is probably due to the presence post zygotic lethal genes as in case some other Acacia species.
A. mangium requires biotic agents to transfer pollen from anthers to the stigmas. Pollinators are mainly entomophilic, with Trigona and Apis spp., as the consistent pollen vectors. The most active time of day for these pollinators is between 07:30 and 11:00, after which their activity decreases; and very few pollinators are observed in the day. In spite of dense and conspicuous inflorescence, A. mangium fails to attract a more varied spectrum of pollinators, probably because it lacks floral nectaries (Zakaria 1993).
A. mangium has a chromosome number of 2n = 26 as same as in A. auriculiformis, so it often readily hybridizes with A. auriculiformis. Hybrids of A. mangium × A. auriculiformis have the potential to become an important source of planting material for plantation forestry. The hybrid seems to be more resistant to heart rot than A. mangium. Moreover, the hybrid has the straight bole and stem of A. mangium and the self-pruning ability of A. auriculiformis (Zakaria 1993). F1 hybrid trees between A. mangium and A. auriculiformis in Vietnam produced 300-500% greater wood volume than the parental species at 2.5-3 years and at 4.5 years old hybrids, on average, twice the wood volume of A. mangium (Le 1996). Sedgley et al. (1992) found that the cross A. auriculiformis × A. mangium was more successful than the reciprocal, but fertile seed was produced following interspecific pollination in both directions. Vacuum drying of pollen and storage in a deep freeze is recommended for the medium length storage (3 years) of pollen used in crossing programmers of these species (Harbard and Sedgley 1994).