Abstract: 【Objective】This study was conducted to investigate the effect of alternating high temperature and drought on the content of Cry1Ac protein in Bt cotton and the underlying physiological mechanism to provide a reference for the safe and stable utilization of insect resistance of Bt cotton in production.【Method】The conventional cultivar Sikang 1 (SK-1) and hybrid cultivar Sikang 3 (SK-3) were used in 2019 and 2020 in experimentally controlled greenhouse, Yangzhou University. From 7：00 am to 7：00 pm two high-temperature treatments [34℃ (A1) and 38℃ (A2)] were imposed on cotton plants, followed by an optimum temperature 28℃ during the remaining night hours. There were two treatments for soil moisture content, which were 50% (B1) and 60% (B2) field capacity. The treatment with 32℃/28℃ and 75% field capacity was set as the control (CK). The leaf Cry1Ac protein content and its physiological mechanism were detected on 4, 7, and 10 days after stress (DAS), respectively.【Result】Compared with CK, the Cry1Ac protein content all decreased under stresses of high temperature and drought, and with the extension of the stress time, greater decrease was observed. The extent of decline for A1B2 was the smallest, followed by A1B1, and A2B1 and A2B1 were the largest. The Cry1Ac protein content for A1B2 was significantly lower than CK after 7 DAS, while significant differences between A1B1, A2B2, A2B2 and CK were detected after 4 DAS. The Bt gene expression level, soluble protein (SP), free amino acid (aa) contents, nitrate reductase (NR), the glutamic pyruvic transaminase (GPT), glutamic oxaloacetate transaminase (GOT), glutamine synthetase (GS) and glutamate synthase (GOGAT) activities showed a downward trend. While the tannin content, the activities of protease and peptidase showed an upward trend. The correlation analysis and path analysis showed SP, aa, NR, GPT, GOT, GS, GOGAT were positively correlated with Cry1Ac protein content. The tannin content, the activities of protease and peptidase were negatively correlated with Cry1Ac protein content. NR, GPT and GS could be key indices for the Cry1Ac protein content.【Conclusion】The interaction of high temperature and drought resulted in the decrease of Cry1Ac protein content in Bt cotton, with greater decrease observed as the stress extended. There was no significant difference between the treatment with 34℃/28℃ and 60% field capacity and CK in 7-10 DAS. The extent of reduction decreased and the period was delayed. NR, GPT and GS could be key indices for the Cry1Ac protein content.

Key words: Bt cotton, Cry1Ac protein, high temperature, drought, physiological mechanism