Floral morphology and pollination system of Syzygium guineense (Willd.) DC. subsp. macrocarpum (Engl.) F. White (Myrtaceae), a subspecies with high nectar production

Abstract

The floral morphology and pollination of S. guineense subsp. macrocarpum were investigated on ten trees selected for a study of floral phenophases in the natural forest of Bassila, located in the Sudano-Guinean zone (Benin). For the floral analysis, the two stages considered were flower buds for preflowering data and open flowers for flower data. To determine the pollination mode and identify main pollinators, two stages were considered, namely flower bud and initiated fruits. The fruit set, autogamy and allogamy rate were assessed. Results show that several flower morphological traits predispose this species to selfing and cross pollination, including hermaphroditism and a long anther
stigma distance. The symmetry is actinomorphic with a floral receptacle adnate to the ovary. The latter is inferior, bicarpellary and bilocular syncarpous and contains 18
22 ovules on an axile placenta. Only a single ovule is fertilized and becomes a seed. Visitors frequently registered on the flowers of S. guineense subsp. macrocarpum are exclusively insects belonging to six orders, including Diptera (26%), Hymenoptera (24%), Lepidoptera (19%). The mean fruit set was 0.47 § 0.10 for the open-pollination experiments and 0.27 § 0.12 for autogamous self-pollination. The autogamy rate was 57.45% and the allogamy rate 42.55%. This study confirms that S. guineense subsp. macrocarpum is predominantly autogamous. Self-compatibility is not for exceptional species, but a dominant characteristic of the genus Syzygium. The viability of the pollen grains and receptivity of the stigma at each flowering stage should be further studied.