Spatio-temporal variation in sexual reproduction of the tropical seagrass Enhalus acoroides (L.f.) Royle in Cape Bolinao, NW Philippines

Abstract

The Indo-Pacific seagrass Enhalus acoroides (L.f.) Royle can reproduce by vegetative growth as well as sexually, by the production of seeds. To assess the role of environmental conditions in the occurrence of sexual reproduction in this dioecious species, spatio-temporal variation in flowering was investigated in the reef flats off Cape Bolinao, north-western Philippines. Flowering occurred year-round, but the intensity varied temporally and correlated with mean water temperature. Spatially, differences in flowering intensity correlated with available light as affected by turbidity and water depth. Exposure duration of female flowers to air seems to be crucial for pollination and subsequent seed setting, resulting in higher numbers of fruits in shallower sites. Contrary to previous hypotheses, no effect of tidal levels on the release of male flowers was observed. It is suggested that production of gas bubbles in photosynthesis plays a key role. The great discrepancy between observed flowering and the presence of peduncle scars on the rhizomes at the deeper sites suggests that early abortion is common here. The use of these scars to quantify flowering will lead to an overestimation of reproductive effort in deep sites. The rare occurrence of flowering in deep sites supports the contention that light availability is a key factor in successful flowering in E. acoroides. Allocation of biomass and nutrients (N, P) over various plant components suggests that sexual reproduction involves high cost in E. acoroides.