| Chinese cabbage is that our country people are delighted for one kind of vegetable eating. The position in Chinese vegetable is very important, also occupies extremely large portion on the marketplace. In recent years, living standard with development of the economy, the people, urgent need achieves anniversary of fresh Chinese cabbage of high grade produces and supplies. However, most area of our country, the precocity Chinese cabbage cultivars are easy to accept high temperature damage. Adopt the tradition breeding method, it is having become a batch of heat-tolerant new cultivars of Chinese cabbage, but Chinese cabbage showing heat resistance because of cultivars type and the geographical distribution diversity. A few scholars are aimed at different geographical feature condition at home and abroad, such as heat-tolerant cultivation and breeding, physiological and biochemistry characteristic, but some result is not consistent, and about that Chinese cabbage heat resistance system research is less, the research specifically for Shanghai area Chinese cabbage heat resistance has not seen report.In Shanghai area, the 10 portion of new heat-tolerant cross combination have made use of crossbreeding approach, being hit by a selective breeding from the 13 portion of heat-tolerant parent. Under summer high temperature, we selected 5 portion of new heat-tolerant cross combinations. Combining with heat-tolerant and heat-sensitive cultivars, there are studied on heat-tolerance identification method, physiology and biochemistry characteristic and chlorophyll fluorescence characteristic in seedlings, in order to be suitable to the heat-tolerant new cultivars in Shanghai, and in order to clarify the possible mechanisms of Chinese cabbage. The main results were presented as follows.1. Under summer high temperature, the proline contents of different cultivars at seedling stage were greater than those at other developmental stages, and sensitive to high temperature. The soluble protein and malonaldehyde contents and CAT activity of different cultivars in developmental stage all presented a "high-low-high" tendency, and the CAT activity of heat-tolerant cultivars was significantly different from that of heat-sensitive ones (P<0.05). The SOD activity presented a downward trend, but the POD activity an upward trend, and there was a significant difference between the heat-tolerant cultivars and heat-sensitive ones (P<0.05). Main agricultural and quality characters of different cultivars were not difference. These results suggest that the variations of proline, soluble protein and malonaldehyde contents and enzyme activities are probably related to biochemical mechanism of heat tolerance of Chinese cabbage.2. Under the controlled heat stress 36℃and 40℃, results showed that the activities of supeioxide dismutase (SOD) and peroxidase (POD) were a upward trend ; catalase (CAT) activity was a upward and then downward trend in 36℃,but it was a downward trend in 40℃; However, Soluble protein content and chl a and chl b were a downward and then upward trend. Malonaldehyde content was a upward and the downward trend. Electrolyte leakage was a upward trend. Meanwhile the avtivities of SOD, POD, CAT and Soluble protein, chl a and chl b content of the heat-tolerant cultivars were higher, but malonaldehyd -e content and electrolyte leakage were lower compared with heat-sensitive cultivars as a whole.3. Under the controlled heat stress 36℃and 40℃, results showed that the Fo and qN were a upward trend ; Fm,Fv/Fo,Fm/Fo,qP,ETR,Yield and OPS II were a downward and then upward trend in 36℃and 40℃as a whole; Meanwhile it was similar trend in the same temperature of different cultivars. The heat-tolerant cultivar 6-3-10 was stable compared with controlled cultivars, and 6-3-10 was higher heat tolerence in heat temperature.
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