Heat-Production and Cross-Pollination of the Asian Skunk Cabbage Symplocarpus renifolius (Araceae)

Abstract

The Asian skunk cabbage Symplocarpus renifolius has an exothermic spadix on which about 100 flowers bloom in very early spring when effective pollinators such as bees and drone flies are inactive. This species is protogynous; female phase and male phase took 6.8 ± SD 5.8 days and 16.7 ± 5.7 days, respectively, with a short transitional phase of bisexuality (2.1 ± 0.9 days). The spadices produced heat 24 hours/day throughout female and bisexual phases, but temperature dropped quickly after the beginning of male phase. Although self-compatibility was expected from the flower structure, the basipetal flowering, and the absence of effective pollinators, bagging tests demonstrated that they rarely produce seeds without crossing. The spadices were visited by small numbers of invertebrates throughout the flowering season. Of these invertebrates, house flies, rove beetles, and mosquitos were the likeliest pollinators, since they are probably attracted both to the pollen produced in male phase and to the stench or carbon dioxide in female phase. On two female spadices with immature male flowers, we fortuitously collected a rove beetle and a mosquito that carried some pollen grains; these had to have been transported from other S. renifolius spadices. This infrequent and ineffective pollination appears to explain why as low as 13% of spadices set seeds in a natural population. We examine alternative hypotheses to explain production of heat in spadices of skunk cabbage.