Flowering and pollination patterns of Magnolia denudata with emphasis on anatomical changes in ovule and seed development

Abstract

Magnolia denudata is an excellent ornamental and ecologically important tree that exhibits low fecundity because of seed abortion. To better understand the cause of this low sexual reproduction, pistil receptivity, and pollen quality were investigated using culture medium and histochemical staining, with emphasis on pollen viability at different flowering stages. Cross-pollination, geitonogamy, and self-pollination were undertaken on flowers of sample trees. We found that flowers produce a large number of pollen grains available for effective pollination. Pollen viability varied at different stages, but was highest during stage 3, in which the stamens detach from the axis, the anthers dehisce, and the gynoecium stigma starts to fade. The duration of pistil receptivity was approximately 5 h. These results indicate that poor fertilisation may occur if the optimum period for pollination is missed or the stigma receives poor-quality pollen grains from stages other than stage 3. Although dichogamy avoids selfing, it has led to considerable geitonogamy. Our anatomical analysis of ovule and seed development further revealed that fertilisation occurred in samples with geitonogamy, but that the embryo degenerated in the torpedo stage, demonstrating that aborted seeds were produced by fertilisation, rather than by pseudogamy. Our results provide new insights into the mechanism of reduced seed set under natural conditions.