A Study On Reproductive Biology In Magnolia And Its Systematic Characterization

Magnoliaceae plants, originated mostly from China, are the endangered and sensitive species to global climate change. This family occupies an important position on the evolution of angiosperm system. By using morphological observation, anatomical and embroyological analysis, hand-pollination, infrared thermometer and infrared imaging, the features in relation to reproductive development and systematic evolution of Magnolia denudata and Magnolia sprengeri were investigated, based on analysis of the floral character, pollination biology, breeding system and thermogenisis occurred in Magnolia. In the present experiments, the mechanism of ovule abortion of M. denudata under natural condition and the reason of incomplete carpel development of Subgenus Yulania were revealed. Furthermore, the co-evolution of Magnolia flower buds with dense trichomes, heat generation occurred at anthesis and evolution mechanism of two pollen tubes occurred in pollen germination in Magnoliaceae were uncovered. The main results were summarized as follows:1) Pollen viability of M. denudata varied at different stages, but was at its highest in stage 3, during which time the stamens detach from the axis, the anthers dehisce, and the gynoecium stigma starts to fade, and pollen germination rate was fastest and nearly 1.5 hours for germination. The duration of pistil receptivity was very short and approximately five hours. These results indicated that poor fertilisation may occur if the optimum period for pollination was missed or the stigma receives poor-quality pollen grains from stages other than stage 3.2) The duration of dichogamy, i.e., from the state of a receptive stigma to mature stamens, was approximately 36 h, although dichogamy avoids selfing, but for the dichogamous M.denudata in early spring in northern China, more numerous floral displays resulted in a longer sequence of visits by pollinators to flowers within the same tree and increased geitonogamous self-pollination. Therefore, protogyny of Magnolia might have little effect in reducing geitonogamy.3) Reproductive process in Magnolia is cross-pollination in a strict sense, and hand-pollination experiments demonstrated that fruits and seeds from cross-pollinated samples grew well, whereas fruits of geitonogamous samples were aborted in later development. The amount and elongation rate of self-pollinated pollen tubes in the styles did not differ from those that were cross-pollinated. Using UV microscopy, both self and cross pollen tubes were found to reach the base of the style along style canal. In addition, zygote and endosperm nucleus developed, but the embryo degenerated in the torpedo stage, our anatomical analysis of ovule and seed development further revealed that fertilisation occurred in samples with geitonogamy, but the embryo degenerated in the torpedo stage, demonstrating that aborted seeds were produced by fertilisation, that is late-acting self-incompatibility.4) Subgenus Yulania carpels are not fully developed due to some ovule sterility. For self-incompatibility mechanism, the incomplete development of carpels was found to be the effect by ovule abortion caused by self-pollination and geitonogamy, and aggregate follicle fruits are not plump. Moreover, the ovule abortion of Subgenus Yulania was late acting self-incompatibility that was post zygotic self-imcompatibility. Incompleted syncarp fruits of primitive angiosperm led to effective self-incompatibility and the evolution can not proceed smoothly.5) There was a significant heat production occurred during anthesis in Magnolia. As the Magnolia plants have dense trichomes on well-developed bracts which tightly packed around floral buds, measurements by thermal couple probe demonstrated that pubescent bracts played a role in keeping the interior of buds warmer than environment, and pubescents avoided the heat spreads out; In its budding stage, the temperature of buds was higher than that of branch and leave buds; In floral stage, the temperature of gynoecium was higher during stigma receptivity period as well as pollen stage. During anthesis, the gynoecium had more control than the stamens. Thermogenisis in plant floral stage, maintain a high temperature, not only to protect pollen development, pollen tube growth, fertilization and etc. during reproduction development, but also can promote evaporation of floral scent, to attract insects visiting flowers, all of which increase their own reproductive success.6) It is commonly observed that there were two tubes from an individual pollen during the germination in Magnolia, Michelia, Manglietia, Liriodendron and other genuses in Magnoliaceae. Given the link between gymnosperm and angiosperm, it is supposed that some original features are maintained in pollen germination pattern in Magnoliaceae. These results have implications for understanding the systematic evolution of primitive angiosperm.