Planting density and leaf-square regulation affected square size and number contributing to altered insecticidal protein content in Bt cotton

Lower insect resistant efficacy of reproductive organs has been observed widely during development in Bt cotton. The objective of this study was to investigate the effects of altered square development of Bt cotton cultivars on Bt toxin concentration, insecticidal efficacy and seed cotton yield, as well as its underlying mechanism by studying the nitrogen metabolism in this process. Three experiments were conducted at the Yangzhou University Farm, Yangzhou, China during 2014–2016 cotton growth seasons. Two cultivars including Sikang3 (hybrid) and Sikang1 (conventional) with five planting densities were compared for square number, square volume and Bt toxin content during square stage in 2014 and 2015. The planting densities had a significant effect on all the three phenotypes. Low planting density with higher square number per plant and square volume exhibited higher insecticidal protein contents in both cultivars. In addition, higher amino acid and soluble protein content, GPT and GOT activities, lower protease and peptidase activities were also detected in the low planting density treatment. In 2015, leaf cut (LC) and square spraying with GA3 treatments were applied to Sikang1 and Sikang3. The square number per plant, square volume, and insecticidal protein content were decreased by LC, but increased by GA3 application. In both experiments, the correlations between square growth and amino acid, soluble protein contents, GPT, GOT activities were positive, but the correlation between square growth and protease and peptidase activities were negative. In 2016, the insect harm and seed cotton yield were investigated under three planting density (1.50, 4.50 and 7.50 plants m−2), leaf cut and GA3 application. The lower boll worms number and hazard square rate with higher yield were observed at low planting density and GA3 application, and contrary effects were detected at high planting density and leaf cut treatment. Therefore, the results of our present study indicated that the square insecticidal protein content and insecticidal efficacy were significantly affected by square growth, and higher square growth ability benefit insect resistance of cotton plants during square stage.