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Cotton cultivars with brown (Xiangcaimian 2), green (Wanmian 39) and white (Sumian 9) fiber were investigated to study fiber developmental characteristics of natural-colored cotton and the effect of hormones on fiber quality at different stages after anthesis. Fiber lengths of both natural-colored cottons were lower than the white-fibered control, with brown-fibered cotton longer than green. Fiber strength, micronaire and maturation of natural-colored cotton were also lower than the control. The shorter fiber of the green cultivar was due to slower growth during 10 to 30 days post-anthesis (DPA). Likewise, the lower fiber strength, micronaire and maturation of natured-colored cotton were also due to slower growth during this pivotal stage. Indole-3-acetic acid (IAA) content at 10 DPA, and abscisic acid (ABA) content at 30 to 40 DPA were lower in the fibers of the natural-colored than that of the white-fibered cotton. After applying 20 mg L -1 gibberellic acid (GA 3 ), the IAA content at 20 DPA in the brown and green-fibered cottons increased by 51.07 and 64.33%, fiber ABA content increased by 38.96 and 24.40%, and fiber length increased by 8.13 and 13.96%, respectively. Fiber strength, micronaire and maturation were also enhanced at boll opening. Those results suggest that the level of endogenous hormones affect fiber quality. Application of external hormones can increase hormone content in natural-colored cotton fiber, improving its quality.

Core Ideas Cultivar with high square biomass for individual and whole plant had higher insecticidal protein content. The increased square biomass by mepiquat chloride application enhaced Bt protein concentration. The decreased square biomass by nitrogen deficit reduced square Bt protein concentration. The square insecticidal protein concentration relates to its biomass. Bt transgenic cotton has been cultivated in China and many other cotton production areas in the world. However, the insecticidal efficacy is often low for the reproductive organs, especially for the cultivars with a big boll in China. The objective of the study focused on the effect of square biomass on the Bt protein concentration during the squaring period for Bt cotton. In the variety effect study in 2015 and 2016, two hybrid Bt cultivars (Sikang3 and Sumian29) and two conventional Bt cultivars (Sikang1 and Nucotn33B) were compared for square biomass and Bt toxin content from 15 to 25 d after square appearance (DASP) during the cotton growing seasons. Cultivars with high square biomass for individual and whole plant had higher insecticidal protein content. In the cultural practice effect study in 2017, the experiment of mepiquat chloride (MC) dose and different nitrogen deficit extents were designed to further investigate the relationship between square biomass and square Bt protein concentration. The MC application significantly increased square mass and square Bt protein content. In contrast, the nitrogen deficit decreased square mass together with square insecticidal protein concentration. The positive correlation between square biomass (the individual square biomass and the square biomass per plant) and square insecticidal protein content was highly significant in variety study and cultural practice study. The results indicated that increment of square development by variety selection and cultural practice could increase insect resistance during squaring period of Bt cotton.

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