Day/night temperature fluctuations delay the high-temperature-induced decline in Cry1Ac endotoxin concentration in cotton squares.

INTRODUCTION: High-temperature stress significantly impacts Cry1Ac endotoxin efficacy in Bacillus thuringiensis (Bt) cotton, yet the effects of realistic day/night temperature fluctuation remain unclear. METHODS: Hence, a 2-year (2021-2022) plot experiment was carried out to investigate the response of square Cry1Ac endotoxin concentration to fluctuating high temperature (HDT/NNT, 38/27 °C) compared with a control treatment (CK, 32/25 °C) in cultivars 'Sikang 1' and 'Sikang 3'. RESULTS: The results indicated that the Cry1Ac endotoxin concentration remained stable under CK but decreased significantly under HDT/NNT only after 7 days of stress, with the decline intensifying over time. In 2021, the reduction was 14.18% and 30.88% on the 7th day and 10th day for S1, respectively, and 11.32% and 25.21% for S3. Physiologically, this reduction coincided with decreased soluble protein content and glutamate-oxaloacetate transaminase and glutamic-pyruvic transaminase activities, alongside increased free amino acid content and protease and peptidase activities. DISCUSSION: We hypothesize that normal night temperatures provide temporary physiological compensation against daytime high-temperature stress damage, maintaining a dynamic balance between Cry1Ac synthesis and degradation. However, as high-temperature stress persists between 4 and 7 days, the cumulative damage exceeds the nighttime compensatory capacity limit, which quantitatively and qualitatively disrupts this metabolic equilibrium by day 7, accelerating endotoxin loss. In summary, prolonged high-temperature stress (approximately 1 week) at the peak square stage in the field environments requires close monitoring and timely agricultural interventions.