| This thesis used 8 pak choi lines from Agronomy and Biotechnology college of CAU, which have been fixed by several generations of selfing. Hurt percentage on cell membrane and growth under natural heat stress is studied on the pak choi lines which are classified into three levels (hear-tolerant, intermediate and heat-sensitive). And then, the pak choi seedlings were put under two high temperature treatments, 32℃/22℃(d/n) and 36℃/28℃(d/n), respectively. After the treatments, the contents of dissociative proline, dissociative amino, free protein, resolvable sugar and MDA along with the activities of SOD, CAT, POD were assayed by means of spetrophotography. At the same time, the contents of endogenous hormones, including ABA, GA3, IAA, Z+ZR were assayed by using ELISA. The relationship between the physiological and biochemical characters and the heat resistance of pak choi was discussed. The results are as following:Heat damage index and relative growth under the temperature of 32°C/22°C(d/n) showed significant correlation with heat tolerance. Both of them can be used to determine the heat-tolerant ability of pak choi lines, which can be most quick and convenient. The physiological parameters, dissociative proline, dissociative amino, free protein, resolvable sugar and endogenous hormones can also be used to determine the heat-tolerant ability. For the sceen of heat tolerant pak choi, the treatment of 32°C/22°C(d/n) is better than 36°C/28°C(d/n), as the absolute or relative content under the treatment of 32°C/22°C(d/n) of most physiological and biochemical characters, such as dissociative proline, dissociative amino, resolvable sugar and endogenous hormones showed better correlation with heat-tolerant ability. The only exception is free protein. When we use the subject function to determine heat-tolerant ability of pak choi comprehensivly, we find that 196-3, 197-5, 196-8, 198-9 are heat-tolerant lines.
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