Sexual Reproduction And Abortion Mechanism Of Sorbus Pohuashanesis

Sorbus pohuashanesis (Hance) Hedl is a valuable ornamental and economic forest tree species with multiple functions of wood, medicine and fruit in northern China. The tree species is suitable to plant in northern China because of its cold-tolerance and adaptation. Generally, S. phohuashanesis propagated by seeds, nevertheless, because of the lack of mother trees (scattered distribution in forest) and abortion during sexual reproduction, the seeds were difficult to be gotten. Therefore we conducted this research in order to understand the sexual reproduction process of S. phohuashanesis and the abortion during this process by studying the series of procedure of development from flowering to fruiting and clarify the abortion mechanism according to three aspects of embryology, physiological and biochemical dynamic and habitat situation. The study would be benefit for not only improving the quantity and quality of seeds of S. phohuashanesis, but also providing a basis for selecting suitable development stage of zygotic embryo for somatic embryogenesis and determining the organ function and systematic development process of zygotic embryo of S. phohuashanesis. In addition, the study could also promote the research about developmental biology of woody plants and provide a basis for protection and utilization of S. phohuashanesis. The main results were as follows:(1) We found inflorescence primordium of the growth cone of S. Phohuashanesis differen-tiating in flower buds at the end of July in the preyear of flowering. In mid-August, the in-florescence primordium was significantly raised to develop into the inflorescence group and the differentiation of flower primordium happened in late August, at the same time every small flowers could be observed protruding; the inflorescence axis differentiation in the buds was also obviously observed, later the flower buds entered winter dormancy. The flower buds enlarged earlier than leaf buds in theyear of flowering, the number of stamen were usually16, the anther had two pollen sac, both of them had one aperture. The pollen grains were triangle and were dispersed within two days after flowering. The ovary was bottle-shaped with2-5locule, in which1-2ovule exist. The fruits became saffron or nacarat and so on in autumn.(2) The pollen dispersed by wind mainly, insects affected fruit setting not significantly. The difference of temporal expression of male and female existed, however, which didn't affect the sexual reproduction. The low pollen vitality resulted by undesirable climate and the lack of pollinator were the important factors which affected reproduction and population distribution.(3) The fertilization pattern of S. phohuashanesis was chalazogamy. When embryo was developing, the nuclear endosperm was absorbed by embryo and then formed the non-endosperm seed. After fertilized ovum formed (6d after flowering on15th May), the zygotic embryo development of S. phohuashanesis experienced proembryo (15d after flowering), globular embryo (21d after flowering), heart-shaped embryo (26d after flowering), torpedo-shaped embryo (30d after flowering) and cotyledonary embryo (40d after flowering), then the zygotic embryo matured (60d after flowering). Abortion were observed during different developmental stages, especially in proembryo stage, when the endosperm distributed along intine of embryo sac and was too young to offer enough nutrition for embryo development.(4) The soluble sugar, starch, soluble protein, SOD, CAT, POD and MDA changed in different ways. SOD, CAT and POD activity reached to highest in heart-shaped stage, which might account for the high abortion in this stage. IAA, GA and ZT contents in abortive samples were lower than that in the normal ones, but the ABA content in the former ones was higher than that in the latter ones. Therefore, we predicted that IAA, GA and ZT might promote development of zygotic embryo and growth of fruit, and inhibited abscission of fruit, on the contrary, ABA promoted abscission of fruit.(5) In cultural habitat, the number of locule and non-abortion seeds were more than that in wild habitat, which might because the former occupied advantageous climatic condition and got well management; in the same habitat (cultural or wild), the non-abortive seeds in2008were more than that in2009, which might also because of the advantageous climatic condition for bud differentiation and development and seed formation in2008(the temperature in2008were higher than that in2009when bud was differentiating and developing. In addition, the temperature and precipitation from summer to autumn in2008(no rain when flowering) were higher than that in2009).(6) Pistil abortion during flower bud stage was one of reasons for abortion of S. phohuashanesis, however, abortion during flower stage and the initial stage of fruit formation account for a large proportion of abortion. Non-abortive seeds decreased following seeds became maturation. The nacarat fruits aborted severely compared to saffron fruits, therefore produced less seeds than the latter. This finding offered basis for selection of mother trees for seed collection, which was used for breeding of S. phohuashanesis.