Durian Floral Differentiation and Flowering Habit

Flower bud differentiation and the fl owering habit of durian (Durio zibethinus Murray) 'Mon Thongʼ from budbreak to anthesis were investigated at the Chantaburi Horticultural Research Center in Thailand. Clusters of fl ower buds appeared at the end of November on primary or secondary scaffold branches near where a fl ower cluster occurred the previous year. Anatomical observations revealed that the development of fl oral organs was acropetal; the fi ve fused epicalyx forming a large, elongated envelope enclosing the sepals, petals, stamen and fused multi-carpellate pistil. Floral organ development was completed in early January. The mature fl ower bud more than doubled in size one day before anthesis, with anthesis starting around 1600 HR and ending ≈1900 HR. The anthers did not dehisce until the completion of fl owering. This change induced heterostyly in this cultivar, which promoted out-crossing by reduc- ing the possibility of self-pollination. Aromatic nectar that attracted insects to the fl ower was secreted during anthesis. This is the fi rst report to have clarifi ed the overall fl owering process in durian and provides the basic information for elucidating reproductive biology of durian in future research. Durian (Durio zibethinus Murray), one of the most impor- tant fruits in Southeast Asia, is one of 27 species in the family Bombacaceae (Kostermans, 1958). Within the genus Durio, only durian is widely distributed and cultivated; a few species bear edible fruit (Soegeng-Reksodihardjo, 1962). The durian is a native of Malaysia and Borneo, but its distribution has spread from Sri Lanka to New Guinea and from Hawaii to northern Australia (Yaccob and Subhadrabandhu, 1995). Although the fruit has a strong odor, its fl avor has gained wide acceptance in all parts of the world. In recent years, Thailand has been the major producer of durian, with yields increasing from 426,245 metric tons in 1987 to 780,393 metric tons in 1994 (Subhadrabandhu and Ketsa, 2001). Durian has an unusual reproductive biology, being a cauli- fl orous plant (Davis and Bhattacharya, 1974) with most fl ower clusters appearing on primary or secondary scaffold branches. The clusters consist of 5 to 30 fl owers that are hermaphroditic, nocturnal, and strongly odorous (Davis and Bhattacharya, 1974). Valmayor et al. (1965) observed that all fully developed buds in the Philippines opened ≈1700 HR and all fl ower organs except the pistil abscised by 2300 HR. Most fl owers abscised after anthesis, resulting in low fruit set. There are several reports that pollination of durian fl ower is mediated by bats at night (Baker, 1969; Soepadmo and Eow, 1977; Subhadrabandhu et al., 1991), though fl owers are visited by insects during anthesis. The species is thought to be self-incom- patible (Somsri, 1987; Subhadrabandhu et al., 1991; Valmayor et al., 1965), although Subhadrabandhu et al. (1991) stated that orchards planted with a single cultivar in Thailand show no signs of reduced fruit set. Basic information on fl oral differentiation and anthesis is lacking in durian. Therefore, this study focused on the development of fl oral organs from bud emergence to anthesis using the scanning electron microscope (SEM) and stereomicroscope, and investigated the fl owering process of durian fl ower as the fi rst step for elucidating their reproductive biology.