Synchronization between temporal variation in heat generation, floral scents and pollinator arrival in the beetle-pollinated tropical Araceae Homalomena propinqua

Inflorescences of beetle-pollinated plants, such as Araceae, show temporal heat generation, the role of which varies among plants. Homalomena propinqua is a common understory herb in the lowland dipterocarp forests of Borneo. Two species of beetle, Parastasia bimaculata Guerin (Scarabaeidae) and Dercetina sp. (Chrysomelidae), are known to visit inflorescences of this species and subsequently transfer its pollen. In this study, to examine the role of heat generation by inflorescences of H. propinqua, especially as an attractant of pollinators, measurements of the temperature of spadices, behavioral experiments with beetles and chemical analyses of floral scents were carried out. As a result, spadices of H. propinqua exhibited heat generation from 06:30 hours (dawn) until 09:30 hours on both the first and second days of flowering, and this paralleled the arrival of the beetle pollinators. Behavioral tests showed that both beetle pollinators were attracted to spadices significantly more than spathes. Floral scents of H. propinqua were composed of 18 compounds, the main one being 2-butanol, a highly volatile low molecule compound. The amount of floral scents increased temporally between 07:00 hours and 08:00 hours on the first day of flowering, which was consistent with the arrival of pollinators and heat generation by the spadices. These results indicate that the heat generated by spadices may promote temporal volatilization of floral scents, especially highly volatile compounds such as 2-butanol, and as a result temporally attracting beetle pollinators.