Study On Seed Breeding Technology And Heat Tolerance Of Rhododendron L.

The genus Rhododendron L. is the largest genus in the Ericaceae. It has a lot of differentkinds and colors. It also has a extremely high ornamental value and landscape application value.But rhododendrons are mostly distributed in high altitude area, and they prefer cool environment,high temperature becomes one of the main limiting factors for their introduction anddomestication. In the process of introduction and cultivation, breeding stocks when grafting andbreeding new varieties, still have to based on seed cultivation. Therefore, this paper focusing onthe seed breeding technology and heat resistance of Rhododendron, mainly did the followingresearch work. Firstly, the standard germination test, cold water-immersion germination method,high temperature aging treatment method, ultraviolet absorption spectrophotometry andsimulated field seedling germination test, these five methods were compared in order to find outthe most suitable determination method for Rhododendron seed vigor. Secondly, the influence ofdifferent storage temperatures、substrates and different pH treatments on seed vigor of differentrhododendrons were studied. Finally, Seedlings of four species in Rhododendron L. were undercondition of artificial simulated high temperature stress. By determining heat injury index, heattolerance of four species was studied. At the same time, by observation and comparison thedifferences of leaf anatomy and ultrastructure with electron microscope, we hope to provide atheoretical basis for breeding new heat resistant varieties of Rhododendron from the perspectiveof cytology. The paper leads to the conclusions as follows:(1) In the comparison of the five kinds of determination methods for rhododendron seedvigor, we found the standard germination test and ultraviolet absorption spectrophotometry (seedsoaked for8hours) can accurately reflect the seedlings emergence in the field. And their resultshad significant or extremely significant correlation to the results of simulated field seedlinggermination test. Besides their coefficient of variation was relatively small and the experimentalresult was relatively stable, so these two methods could be used for the determination ofRhododendron seed vigor. Compared with ultraviolet absorption spectrophotometry, the standardgermination test was time-consuming and difficult to count accurately.(2)In general, Rhododendron seeds were suitable for preservation at low temperature, whichwas good for maintaining the germination rate of Rhododendron seeds. Seed germination rate ofRhododendron simsii is highest when storaged at minus20℃condition in the refrigerator, whilethe seed germination rate of Rhododendron mariesii is highest when storaged at minus80℃condition in the refrigerator. The suitable matrix for seed germination and seedling survival ofRhododendron was peat, with its surface covered with a layer of sphagnum moss, in whichRhododendron seed germination rate and seedling survival rate both achieved at a high level. Forthese plants of the genus Rhododendron, the optimum pH range for their seed germination was5~6.5roughly. The study found that if the pH of solution to cultivate Rhododendron seeds was greater than6.5or less than5, then the seed germination rate decreased significantly.(3) Phytotron identification method、principal component analysis and membership functionanalysis were reliable method for heat tolerance identification of Rhododendron, their resultswere in correspondence, the heat tolerance of four species of Rhododendron showed thefollowing rank order: Rhododendron ovatum> Rhododendron simsii> Rhododendron mariesii>Rhododendron fortunei. Stomatal density was positively correlated with heat resistance ofRhododendron, heat injury index was negatively correlated with heat tolerance of Rhododendron,Stomatal short diameter had a negative correlation with the heat resistance to a certain extent,these three indicators can be used as the index of heat tolerance identification of Rhododendron.(4)Under high temperature stress, chloroplast was the most sensitive organelle to hightemperature in all the mesophyll cell organelles, it was mainly manifested as the expansion ofthylakoids of chloroplast, stroma lamellae was fuzzy and began to degrade when the situationwas serious. Thermal stability of mitochondria was better than chloroplast, but it also occurredpartly degradation when the stress was serious. According to the heat resistance of differentspecies of Rhododendron, mesophyll cells resulted in different degree of plasmolysis,plasmolysis was found in mesophyll cells of R.fortunei significantly, and plasmolysisphenomenon didn′t occur in mesophyll cells of R.ovatum. Effects of high temperature stress onthe nucleus was not significant, but karyopyknosis phenomenon appeared when thermal stresswas serious. Although the mesophyll cell microstructure identification was relatively complex, itwas more accurateand scientific, and it could provide theoretical basis for heat resistant breedingfrom the perspective of cytology.