JIA-2019-11

2502 NIE Jun-jun et al. Journal of Integrative Agriculture 2019, 18(11): 2492–2504 increased the lint yield of Bt cotton varieties. The bolls located on the lower and middle fruit branches or the inner fruit positions contributed more to the yield than those located in the upper and outer fruit positions (Dong et al . 2010). A higher cotton yield was measured under a high boll load in the inner and middle fruit positions, and the boll weight was consistently higher at the same density (Jones and Wells 1998; Clawson et al . 2006; Ahmad et al . 2011; Zhi et al . 2016). Better boll retention in the first position on the first fruiting branch is an agronomic advantage, conferring a higher average boll weight and earlier production (Ungar et al . 1987). In our experiment, which was similar to the results of the above studies, the increased lint yield and boll weight were attributed mainly to the optimized boll setting among the different Bt cotton varieties. Previous studies have shown that 70–90% of total harvestable bolls come from the inner canopy (Jenkins et al . 1990; Wang et al . 2016). The results of our study were in line with those previous researches. The higher contribution rate of inner bolls in J169 and L21 was the main reason of the improvement of lint yield. The inconsistent contribution rate of spatial bolls in the five-year was attributed to the changeable weather. The contribution rate of spatial bolls to lint yield per plant was different from the spatial boll number, which might be related to the effect of boll weight on yield. 4.3. Effects of within-plant distribution bolls on fiber quality Cotton fiber quality is affected not only by genetic and environmental factors, but also by the positions and the resources captured by each fruit (Liakatas et al . 1998; Zhao and Oosterhuis 2000; Pettigrew 2001; Read et al . 2006; Richard et al . 2006; Yeates et al . 2010). The fiber quality of L21 was much better than that of Jiman 169 and 99B, which was chiefly caused by the increase in the number of bolls in lower and middle positions. This conclusion is in line with Mao et al . (2015) who found that an improvement in fiber strength under a high plant density and through the application of mepiquat chloride resulted from the increase in the number of bolls in the lower and middle positions. Similarly, Teague et al . (2011) reported that topping earlier than the reference time greatly increases ber quality. 5. Conclusion The lint yield of J169 was the highest and the most stable, followed by L21 and 99B. Differences in the fiber quality of the three cultivars were inconsistent with lint yield, and the fiber quality of L21 was much better than that of J169 and 99B. Differences in lint yield and quality were caused by a higher number of large bolls in the inner position in the loosest-type of J169, fewer large bolls, and a higher number of bolls in the lower and middle positions in the tightest-type of L21, and the lowest number of large bolls but a high number of distal bolls in the looser-type of 99B. The differences in lint yield and its components during the five years were principally attributed to the climate conditions, such as temperature and rainfall. Our overall results support the finding that the increase in lint yield was due to the increase in boll weight, and the large bolls and high fiber quality were attributed to the optimal distribution of bolls within the canopies of the different Bt cotton varieties. Acknowledgements This research was supported by the National Natural Science Foundation of China (31601253), the ModernAgro- industry Technology Research System, China (SDAIT-03), and the Natural Science Foundation of Shandong Province, China (ZR2016CQ20). References Ahmad M, Khan N U, Mohammad F, Khan S A, Munir I, Bibi Z, Shaheen S. 2011. Genetic potential and heritability studies for some polygenic traits in cotton ( G . hirsutum L.). Pakistan Journal of Botany , 43 , 1713–1718. Biles S P, Cothren J T. 2001. Flowering and yield response of cotton to application of mepiquat chloride and PGR-IV. Crop Science , 41 , 1834–1837. Blaise D. 2006. Yield, boll distribution and fiber quality of hybrid cotton ( Gossypium hirsutum L.) as influenced by organic and modern methods of cultivation. Journal of Agronomy and Crop Science , 192 , 248–256. Chen Y, Li Y B, Hu D P, Zhang X, Wen Y J, Chen D H. 2016. Spatial distribution of potassium uptake across the cotton plant affects ber length. 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