Flowering and Fruiting Phenology of Magnolia hypoleuca

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The floral biology of Magnolia hypoleuca, a tall tree species widely distributed in Japan, was studied in a deciduous broad-leaved forest in Hokkaido near the northern limit of distribution of the species. The flower is large, protogynous and nectarless. Its petals close after the female stage and again after the male stage. The mean duration of flowering of an individual flower is 3-4 days, while the flowering period of an individual tree lasts up to 40 days. The main pollinators of the flowers were beetles. Success of pollination is assumed to be affected by weather conditions at anthesis. Average fruit sets (ratios of fruits to flowers) were 13-25%. Hand-pollination of 44 flowers produced a significantly higher fruit set (28 fruits) than control flowers (33/133). These results suggest that the fruit set is limited by pollination. The frequency of infertile ovules was higher in controls than in hand-pollinated fruits, indicating that the seed set was also limited by pollination. We propose a mimicry hypothesis in which the non-rewarding female-stage flower mimics the rewarding male-stage flower to explain the short life-span of an individual flower, the long flowering period of an individual tree, and the coexistence of flowers of various stages on a single tree.
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