Thermogenesis, Flowering and the Association with Variation in Floral Odour Attractants in Magnolia sprengeri (Magnoliaceae)

Magnolia sprengeri Pamp. is an ornamentally and ecologically important tree that blooms at cold temperatures in early spring. In this study, thermogenesis and variation in the chemical compounds of floral odours and insect visitation in relation to flowering cycles were studied to increase our understanding of the role of floral thermogenesis in the pollination biology of M. sprengeri. There were five distinct floral stages across the floral cycle of this species: pre-pistillate, pistillate, pre-staminate, staminate and post-staminate. Floral thermogenesis during anthesis and consisted of two distinct peaks: one at the pistillate stage and the other at the staminate stage. Insects of five families visited M. sprengeri during the floral cycle, and sap beetles (Epuraea sp., Nitidulidae) were determined to be the most effective pollinators, whereas bees (Apis cerana, Apidae) were considered to be occasional pollinators. A strong fragrance was released during thermogenesis, consisting of 18 chemical compounds. Although the relative proportions of these compounds varied at different floral stages across anthesis, linalool, 1-iodo-2-methylundecane and 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol were dominant. Importantly, we found that the floral blends released during the pistillate and staminate stages were very similar, and coincided with flower visitation by sap beetles and the two thermogenic episodes. Based on these results, we propose that odour acts as a signal for a reward (pollen) and that an odour mimicry of staminate-stage flowers occurs during the pistillate stage.