Floral and reproductive biology of the medicinally significant rainforest tree, Fontainea picrosperma (Euphorbiaceae)

Fontainea picrosperma (Euphorbiaceae) is a dioecious rainforest tree from northern Australia that is of commercial interest following the recent discovery of the putative anti-cancer agent, tigilanol tiglate, in its seed. Production of tigilanol tiglate will rely on purification from harvested fruit and therefore an understanding of the reproductive characteristics that determine fruit set of this species is critical. Most rainforest plant species rely exclusively on animal vectors to transport pollen between plants for successful reproduction. Flower traits and phenology can facilitate sexual reproduction by attracting pollinators whereas failure to attract pollinators can result in low fruit set due to pollen limitation. Here, we describe the floral morphology, flowering phenology and reproductive biology of F. picrosperma. This species bears small, white, actinomorphic flowers with a shallow receptacle. These floral traits are often associated with generalist insect pollination and are common to other dioecious tropical rainforest flowers. Individual female flowers persisted on the tree for several days longer than individual male flowers. Male panicles contained significantly more flowers than female inflorescences, and male flowers opened sequentially on a panicle whereas female flowers opened almost simultaneously within an inflorescence. F. picrosperma was pollen limited, as hand pollinated female flowers produced almost double the final fruit set (39.6 ± 4.4%) of open pollinated flowers (21.3 ± 3.4%). Optimised production of tigilanol tiglate may therefore rely on improving pollen flow from male to female trees.