Abstract: 【Objective】 Low-temperature is one of the main factors affecting the normal growth and development of cotton plants. Identification of chilling resistance of cotton is the prior condition for the mechanism of cold resistance, which also can provide certain theoretical references for cold resistance breeding. 【Method】 The temperature conditions for cotton cold resistance identification were screened at germination stage following 7 d recovery at normal conditions (28℃, day/night, 14 h/10 h) after 7 d treatment at different low temperatures of 0℃, 4℃, 10℃, 15℃ with 4 upland cotton varieties (lines) with different cold resistance levels. Screening of days of cold resistance identification for 0℃ low-temperature treatment at germination stage was performed by 7 d recovery after the respective treatments of 1, 2, 3, 4, 5, 6, and 7 d under 0℃ low temperature. Two upland cotton varieties (lines) with different cold resistance levels was treated for 1, 2, 3, 4, 5, 6, and 7 d under 4℃ low temperature and recovered 7 d in order to evaluate cold resistance of cotton at budding stage. Evaluation of cold resistance of cotton at cotyledon stage was carried out at 4℃for 24 h and normal growth was recovered for 7 d with 47 cotton varieties. The biochemical indexes of bud and cotyledon under low temperature were measured at budding and cotyledon stages with the reagent kit developed by Nanjing Jiancheng Bioengineering Institute. 【Result】 The study established the standards of identification of cold-resistance by screening the stress conditions at different growth periods, including germination stage, budding stage and cotyledon stage. The relative cotyledons unfolding rate under normal condition growth for 7 d after 0℃ treatment for 4 d can be served as a cold resistance identification index of germination stage. The relative cotyledons unfolding rate under normal condition growth for 7 d after 4℃ treatment for 5 d can be used as chilling resistance identification index of the budding stage. Chilling resistance index under normal condition growth for 7 d after 4℃ treatment for 24 h can be used for evaluating the chilling resistance of cotton at cotyledon stage. The activities of SOD and POD in the bud of cold-resistant variety Yu2067 were increased at the early stage of low-temperature treatment and then decreased to a steady, while the activities of two enzymes in the bud of chilling-sensitive cotton line Hengmian No.3 decreased at first and then increased to steady. The activity of CAT in the bud of two varieties all showed a uptrend at first and then downtrend, but the activity of CAT in Yu2067 was always higher than that in Hengmian No.3 at later stage of low-temperature treatment. Besides, the soluble protein contents in the bud of two varieties were similar at first, but the content in Yu2067 started to be higher than that in Hengmian No.3 at later stage of low-temperature treatment. After the treatment of 4℃ for 24 h at cotyledon stage, the activities of SOD, POD and CAT were all showed a down-trend in Yu2067 while they all went up in Hengmian3. But the soluble protein content in Yu2067 went up while declined in Hengmian No.3. 【Conclusion】 Specific identification ways are needed at different growth stages correspondingly. The standards of identification of cold-resistance levels at different stages are different. Different materials have different cold resistance at different growth stages. Based on the variation trends of enzyme activity of three enzymes, it was speculated that the cold-resistance of cotton at budding stage had relation with antioxidant enzymes activity, while the cold-resistance of cotton at cotyledon stage had little relation with three antioxidant enzyme activities, but may be closely with the soluble protein content.

Key words: Gossypium hirsutum L., growth stages, cold resistance, identification ways, biochemical characteristics