Effect Of Nitrogen Rates On Physiological Mechanism Of Fiber Development In Superior And Inferior Cotton Bolls

Cotton(Gossypium hirsutum L.), the world’s foremost important fiber crop, has an inherently plastic and indeterminate growth characteristic, suffering varieties of the environments and the physiological age of cotton in different anthesis date. N(nitrogen) is one of the regulating factors to the production and fiber quality of cotton. For improving yield and fiber quality through N, the physiology basis of nitrogen regulating the difference of formation between superior and inferior bolls was studied in this research. In2009-2010, pot experiments were carried out in Nanjing (118°50’E,32°02’N, middle lower reaches of Yangtze River Valley). Two cotton cultivars with different fiber quality (Kemian1and NuCOTN33B) were chosen, and three nitrogen rates (0kgN·ha-1,240kgN·ha-1,480kgN·ha-1), standing for low, optimum and high nitrogen application level, respectively, were applied. The study focused on:(1) Effect of N rates on lint yield, fiber quality and carbohydrates related with fiber developmet of superior and inferior cotton bolls;(2) Effect of N rates on activities of enzymes related with fiber development of superior and inferior cotton bolls;(3) Effect of N rates on enzymes gene expression involved in fiber development in superior and inferior cotton bolls. The main results were as follows:1Effect of N rates on lint yield, fiber quality and carbohydrates related with fiber developmet of superior and inferior cotton bolls to NThe absolute differential value of the inner (between the first and second fruiting node) BW(boll weight)(1.1%for NuCOTN33B and7.3%for Kemian1) and SR(shed rate)(8.4%for NuCOTN33B and16.7%for Kemian1) were typically lower than that between inner and distal FN (fruiting nodes)14.3,26.9%for BW and22.1,29.2%for AR, respectively, and quality of fiber FN1was of advantage compared with that of FN3, especially Str under 240kgN·ha-1(p<0.05), so the former was called superior boll and the latter was calledinferior boll. Compared to superior boll, Mic of inferior boll of cotton grown under480kgN·ha-1reduced by10.0%for NuCOTN33B,14.3%for Kemian1. Boll weight of superior boll was significant higher than inferior boll, but the trend of shed rate was reserved (p<0.05) at240kgN·ha-1. Under the treatment of N rates, absolute differential value of sucrose transformed rate between superior and inferior boll was negatively correlated with the differential value of Mic value, and absolute differential value of cellulose accumulating rate between superior and inferior boll was positively correlated with the differential value of Str (p<0.05). Compared with inferior boll under0kgN·ha-1and240kgN·ha-1, sucrose transformed rate at480kgN·ha-1were greater, that was47.6%for NuCOTN33B,46.2%for Kemian1. α-1,3-glucan transformed rate showed same tendency, which was31.2%for NuCOTN33B,45.5%for Kemian1. Sucrose and (3-1,3-glucan contents in inferior bolls under480kg N·ha-1at upper fruiting branches were higher than that of other nitrogen levels. The result stated that the changing trend of enzymes regulating carbohydrate could reflect the physiological mechanism of fiber development about the difference between superior boll and inferior boll. Additionally, excess N (480kgN·ha-1) could not only increase the retention rate, boll weight per plant and yield contribution rate, but also promote sucrose and β-1,3-glucan transforming thoroughly in late stage of fiber development, which increased the content of cellulose, and optimized Mic and Str finally, the above state was only suited to inferior bolls.2Effect of N rates on activities of enzymes related with fiber development of superior and inferior cotton bollsCompared with inferiors, invertase, sucrose synthetase, sucrose phosphate synthetase and β-1,3-glucanase involoved in fiber development of superior bolls under240kgN·ha-1retained higher activities in early stage of fiber development, while until38DPA, the trend was reversed. The activity of enzymes related with sucrose metabolism, which were in bolls at middle and upper fruiting branches, was significantly correlated with leaf N concentration (P<0.05or P<0.01) in late stage of fiber development. The result stated that because superior and inferior bolls fiber retain high enzymes activity in different stages, superior bolls could provide more energy and substances for cellulose accumulating quickly and steady, while inferiors also could provide those for cellulose steady and dural accululating. Leaf N concentration could improve enzymes activities, including invertase, sucrose synthase and sucrose phosphate synthetase, and promote sucrose metabolism. Additionally, the higher fruiting branch was, the greater compensational effect of nitrogen was. Therefore, at late stage of fiber development, adding N(480kg N·ha-1) appropriately could retain high enzymes activity of inferior bolls fiber located at middle and upper fruiting branches, and promote sucrose metabolism.3Effect of N rates on gene expression of enzymes involved in fiber development in superior and inferior cotton bollsThe amount of gene expression of enzymes involved in cotton fiber development in superior bolls at the initiation of secondary wall thickening was more than that at24day post anthesis. At10day post anthesis, the amounts of gene expression of sucrose synthase and sucrose phosphate synthetase in superior bolls were less than that in inferiors, but invertase, β-1,3-glucanase, β-1,4-glucanase and Expansin were reversed. At17day post anthesis, the amount of gene expression of sucrose synthase, sucrose phosphate synthetase, invertase and (3-1,3-glucanase in superior bolls was less than inferiors, but β-1,4-glucanase and Expansin were reversed. The amount of gene expression of enzymes involved in cotton fiber development in superior bolls was less than inferiors at24day post anthesis, meanwhile, gene expression of sucrose synthetase was maximum under480kg N·ha-1. When leaf nitrogen concentration was limited to3.03-2.37%, relative of mRNA gene expression was beyond60%. The difference of sucrose synthetase gene expression between superior and inferior bolls was larger than other enzymes.These results stated that in the early stage of gene expression of enzymes related with fiber development, enzymes could work coordinately of superior bolls for cellulose synthsis and cell-wall elongation although the amount of gene expression of partly enzymes in superior bolls is lower than inferiors. However, enzymes could not make sure cell-wall elongation in inferior bolls in spite of inferiors is higher than superiors. And up to24day post anthesis, all of enzymes involved in cotton fiber development in inferior bolls could work coordinatly, especially under480kgN·ha-1, resulting in Mic increased and Str decreased. In addition, the response of gene expression in fiber development to leaf nitrogen concentration was less than plant physiological age. Optimal leaf nitrogen concentration was beneficial to gene expression.