Sowing cotton directly after harvesting wheat in the Yangtze River Valley of China requires early mature of cotton without yield reduction.  Boll-setting period synchronisation and more yield bolls distributed at the upper and middle canopy layers are also required for harvesting.  The objective of this study is to quantify the individual and interaction effects of plant density and plant growth regulator mepiquat chloride (MC) on temporal and spatial distributions of yield bolls, as well as yield and yield components.  During the 2013–2016 cotton growing seasons, the experiments were conducted on a short-season cotton cultivar CRRI50 at Yangzhou University, China.  Various combinations of plant density (12.0, 13.5 and 15.0 plants m^–2) and MC dose (180, 270 and 360 g ha^–1) were applied on cotton plants.  The combination of 13.5 plants m^–2 and 270 g ha^–1 MC resulted in the greatest boll number per unit area, the highest daily boll setting number and more than 90% of bolls positioned within 45–80 cm above the ground.  In conclusion, appropriate MC dose in combination of high plant density could synchronize boll-setting period and retain more bolls at the upper and middle canopy layers without yield reduction in the system of direct-seeded cotton after wheat harvest, and thus overcome the labor-intensive problem in current transplanting cropping system. 

Plant density is the cultivation practice usually employed to manipulate boll distribution, boll setting and yield in cotton production.  In order to determine the effect of plant density on the insecticidal protein content of Bacillus thuringiensis (Bt) cotton plants, a study was conducted in Yangzhou University of China in 2015 and 2016.  Five plant densities (PD1–PD5, representing 15 000, 30 000, 45 000, 60 000, and 75 000 plants ha–1) were imposed on two Bt cotton cultivars, Sikang 1 (the conventional cultivar, SK-1) and Sikang 3 (the hybrid cultivar, SK-3).  The boll number per plant, boll weight and boll volume all decreased as plant density increased.  As plant density increased from 15 000 to 75 000 plants ha^–1, seed Bt protein content increased, with increases of 66.5% in SK-1 and 53.4% in SK-3 at 40 days after flowering (DAF) in 2015, and 36.8% in SK-1 and 38.6% in SK-3 in 2016.  Nitrogen (N) metabolism was investigated to uncover the potential mechanism.  The analysis of N metabolism showed enhanced soluble protein content, glutamic-pyruvic transaminase (GPT) and glutamate oxaloacetate transaminase (GOT) activities, but reduced free amino acid content, and protease and peptidase activities with increasing plant density.  At 20 DAF, the seed Bt toxin amount was positively correlated with soluble protein level, with correlation coefficients of 0.825** in SK-1 and 0.926** in SK-3 in 2015, and 0.955** in SK-1 and 0.965** in SK-3 in 2016.  In contrast, the seed Bt protein level was negatively correlated with free amino acid content, with correlation coefficients of –0.983** in SK-1 and –0.974** in SK-3 in 2015, and –0.996** in SK-1 and –0.986** in SK-3 in 2016.  To further confirm the relationship of Bt protein content and N metabolism, the Bt protein content was found to be positively correlated with the activities of GPT and GOT, but negatively correlated with the activities of protease and peptidase.  In conclusion, our present study indicated that high plant density elevated the amount of seed Bt protein, and this increase was associated with decreased boll number per plant, boll weight and boll volume.  In addition, altered N metabolism also contributed to the increased Bt protein content under high plant density.

Cotton bolls exhibit the lowest insecticidal efficacy among all organs of Bt cotton, which would ultimately affect the yield formation.  The objective of this study was to investigate the effects of different urea concentrations on the seed Bt protein contents, seed cotton yield and the corresponding protein metabolism mechanism.  The experiments were conducted during 2017–2018 cotton growing seasons.  Two cultivars, Sikang 3 (hybrid, SK3) and Sikang 1 (conventional, SK1), were treated with six urea concentrations and their seed Bt protein contents were compared during boll formation period.  The urea spray concentration had a significant effect on the seed Bt toxin content and seed cotton yield.  Spraying of either 5 or 6% urea led to higher insecticidal protein contents and higher seed cotton yield for both cultivars.  Moreover, the highest amino acid and soluble protein contents, as well as GPT and GOT activities, and lower protease and peptidase activities were observed at the 5 to 6% urea levels.  Significant positive correlations between the seed Bt toxin and amino acid contents, and between the seed Bt toxin content and GPT activities were detected.  The lower boll worm number and hazard boll rate were also observed with the 5 to 6% urea treatments, which may be the reason why nitrogen spraying increased the seed cotton yield.  Therefore, our results suggested that the seed Bt toxin content and insect resistance were impacted markedly by external nitrogen application, and 5 to 6% urea had the greatest effect on insect resistance.

To clarify the effect of the N deficit on the amount of square Bt insecticidal protein, different N application rates (0, 75, 150, 225, and 300 kg ha^–1) were imposed on the conventional cultivar Sikang 1 (SK-1) and hybrid cultivar Sikang 3 (SK-3) during 2015–2016 cotton growth seasons.  Under different N application rates, the square number per plant, square volume and square dry weight reduced when the N rates decreased from conventional rate (300 kg ha^–1) to 0 kg ha^–1.  And the square Bt protein content decreased accordingly.  The analysis of N metabolism showed that soluble protein content, GPT and GOT activities decreased, free amino acid, peptidase and protease activities increased under N deficit.  Correlation analysis indicated that the reduced Bt protein content under N deficit was related to altered N metabolism.  In conclusion, square development and the amount of square Bt toxin both decreased under N deficit, indicating that promoting the square development under appropriate N application rate would also promote the insect resistance during squaring stage.