Floral biology and unique pollination system of root holoparasites, Balanophora kuroiwai and B. tobiracola (Balanphoraceae)

Abstract

We investigated the floral and pollination biology of two monoecious root holoparasites, Balanophora kuroiwai and B. tobiracola (Balanophoraceae), in the subtropical forests of southern Japan. Both species secrete nectar from extrafloral nectaries distributed among the flowers, which is mainly consumed by ants, cockroaches, and pyralid moths. Pollen grains were found attached to the bodies of these insects. Pyralid moths of the genera Assara and Nacoleia were observed laying eggs on the inflorescences of B. kuroiwai. In both Balanophora species, pyralid larvae were found feeding on vegetative tissue without exploiting the seeds, and adults emerged from the fruited infructescences. In B. kuroiwai, we assessed pollination success under different experimental conditions by estimating the percentage of styles that had pollen tubes reaching the ovules. This revealed that: (1) the plants were at least sporophytically self-compatible; (2) they were generally pollinated within an inflorescence (geitonogamy); (3) outcrossing occurred, but the rates varied greatly among inflorescences; and (4) ants were probably responsible for the geitonogamy. While ants and flightless cockroaches were the most likely contributors to geitonogamous self-pollination, we consider pyralid moths to be the most likely cross-pollinators of Balanophora species. This is a new example of pollination mutualism involving a plant and its pollinating parasite.