The Study On Reproduction Biology In Robinia Neo-mexicana Var.luxurians

Robinia neo-mexicana var. luxurians is a variety of Mexico locust in Robinia, a small or medium sized tree, produced in North American originally, also called Robinia ambigua idahoenisis, which is a good tree species for urban landscape and ecological protection. The amount of flowers in Robinia neo-mexicana var. luxurians is very large, but the seeds production is extremely low, and its sexual reproduction is too difficult to achieve.Contrapose this phenomenon, this research had done many observations, experiments,determinations and analysises by means of reproduction biology to find the causes of this species' low seeds production.(1) We used scanning electron microscopy(SEM) in this experiment to observe the pollen'morphology of Robinia neo-mexicana var. luxurians in different periods of development, and had taken photos for analysis, the results show that the average size of the pollen is32.08?m×19.55?m(poleaxis length × equatorial axes length), the shape of the pollen is long spheroidicity with three germinal furrows, a little square, and on the surface of the pollen there are some ornamentations, folds and germination apertures. The size and shape of pollen,and the amount and shape of ornamentations on the surface have some changes in different periods of development; there are three germination furrows run through the two ends of the pollen, and they are relatively long and narrow, the place nearby the germination furrows is relatively smooth.(2) Acid dissolution method and cellulose dissolution method were both used to determine the pollen amount in a single flower of Robinia neo-mexicana var. luxurians, compared the two methods, we find that the effect of acid dissolution method is not successively enough,the result of cellulose dissolution method is more accurate. The result shows that the maximum pollen amount in a single flower is 66133.33, the minimum amount is 50066.67,and the average amount is 59494.82, which is much more than the ordinary Robinia pseudoacacia.(3) In vitro-culture method of liquid MS(Murashige and Skoog) medium was applied to determine the viability of fresh pollens and pollens which were preserved for one day, two days, four days, six days, eight days, fifteen days under normal atmospheric temperature and low temperature(4?). Then we find that the pollen of Robinia neo-mexicana var. luxurians could begin to germinate after two hours' culture, the pollen germination rate reaches to the maximum after eight hours' culture, the pollen tube elongation stops to increase after fourteen hours' culture. A small amount of pollen can still germinate after twenty-four hours' culture,but the vitality of pollens is very low, and the pollen tubes could not elongate fully. The germination vitality of fresh pollen is good, and the pollen activity will cut down soon when preserved in vitro. Generally speaking, when preserved for about 15 days, the pollen activity is very poor, and preservation under low temperature is better to continue the pollen activity.(4) We cultured pollens in vivo. We pollinated gynodioecious Robinia neo-mexicana var.luxurian and high-quality clone of Robinia pseudoacacia(R. pseudoacacia ‘Luci 152') with pollen of Robinia neo-mexicana var. luxurian, applied aniline blue stain method to observe the germination process of the pollen tubes, and then analyzed the pollination compatibility of them. The result show that the pollen of Robinia neo-mexicana var. luxurian can germinate in both Robinia neo-mexicana var. luxurian and R. pseudoacacia ‘Luci152' treated as the female parent, but the former germinates later and elongates slower, which illustrates that the pollen of Robinia neo-mexicana var. luxurian has affinity with both the stigmas of Robinia neo-mexicana var. luxurian and R. pseudoacacia ‘Luci 152', but the affinity with the former is worse.(5) We used SEM to observe the morphology of the stigma of Robinia neo-mexicana var.luxurian in different periods of development, and had taken photos for analysis. We find that the surface of the stigma has mucilage and bulges, and the conduction of mucilage secretion occurs regular changes, what can reflect the receptivity of the stigma. It has much mucilage at twelve hours before flowering and the receptivity is the best. At forty-eight hours after flowering, the stigma begins to break, the receptivity is already very bad, and the whole receptive period of the stigma is about three days from twenty-four hours before flowering to twenty-four hours after flowering.(6) We had done anatomical observations of the ovary with dissecting microscope, andconducted the investigation on the ovary structure of Robinia neo-mexicana var. luxurian, and counted the ovule number in an ovary. The result shows that the ovary of Robinia neo-mexicana var. luxurian has one room, base with handle, outside with one carpel, and the ovules are coated in the carpel. The ovules lie in the convex outside of arc-shaped ovary, and array on single line linearity. The maximum ovule number in a single flower is 26, the minimum number is 21, and the average number is 23.1.(7) According to the average number of the pollen and ovule in a single flower, we can obtain the P/O(the ratio of pollen amount and ovule number) of Robinia neo-mexicana var.luxurian, which is 2575.53. With Cruden's standard, we make a conclusion that the mating system of Robinia neo-mexicana var. luxurian is predominantly outcrossing. By measuring the diameter of the inflorescence and the size of the flower, observing the flowering behavior and so on, we calculate that the OCI of Robinia neo-mexicana var. luxurian is 4. With Dafni's judgement standard, we also can make a conclusion that the breeding system of Robinia neo-mexicana var. luxurian is partial self-incompatibility, and pollinators are needed for out-crossing.(8) We used artificial controlled pollination method to determine the possibility of Robinia neo-mexicana var. luxurian producing seeds. We carried out some experiments such as the self-pollination, cross-pollination(as male parent, as female parent and natural crossing),competitive pollination, parthenogenesis, and we also designed the natural pollination experiment as a control. The result shows that natural pollination and cross-pollination as male parent can produce seeds with low frequency; self-pollination can produce a little seeds at the beginning, but fall off at the later stage; the phenomenon of flowers and fruits falling off is greatly serious at early stage after pollination for both parthenogenesis and competitive pollination; the effect of cross-pollination as female parent is also very bad, no seed is produced at last generally. There may be some influence of operations of artificial pollination,but by overall consideration, Robinia neo-mexicana var. luxurian is indeed a plant species that is difficult to produce seeds.(9) By applying gravity slide method and periodic observation method to determine the condition of wind pollination and insect pollination separately, we find that the wind pollination for Robinia neo-mexicana var. luxurian is extremely weak, but the insectpollination occupied most. In addition, by the experiment of observing the morphology of the pollen of Robinia neo-mexicana var. luxurian, we also find there are some features for insect pollination on the surface of the pollen, such as ornamentations, folds and germination apertures and so on. In consequence, the insect pollination is the main form of pollination, and the Italian bees are the major pollinators.