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The secondary wall thickening stage is a key period to cotton(Gossypium hirsutum L.) fiber strength formation. The subtending leaf of cotton boll is the main apparatus to provide photosynthetic product for cotton boll development, and its photosynthetic capacity has important function in development of cotton boll and the formation of the production and fiber quality of cotton. Nitrogen is one of the limiting factor to produce high quality cotton fiber. The study on relationships between the subtending leaf nitrogen concentration and fiber strength formation can provide theoretical basis for regulating quantificationally nitrogen fertilizer application and improving fiber quality. In 2007 and 2008, field experiments were carried out in Nanjing (118°50’E,32°02’N, middle lower reaches of Yangtze River Valley) and five Nitrogen application rates:0 kgN·hm-2,120 kgN·hm-2,240 kgN·hm-2,360 kgN·hm-2,480 kgN·hm-2, used three cultivars (Dexiamian 1, Kemian-1,NuCOTN) as materials. The study focused on:(1) Relationships between the subtending leaf nitrogen concentration and the saccharide compounds content involved in cotton fiber thickening development and fiber strength formation (2) Relationships between the subtending leaf nitrogen concentration and activities of related enzymes(sucrase, sucrose synthetase, sucrose phosphate synthetase) involved in cotton fiber thickening development and fiber strength formation. The main results were as follows:1. Effects of the the subtending leaf nitrogen concentration on the saccharide compounds content fiber strength formation in cotton fiberThe study focused on the effects of subtending leaf nitrogen concentration of boll on the Saccharide compounds content and fiber strength formation in cotton fiber thickening development stage. The result showed that the changes of optimal nitrogen concentration in subtending leaf of cotton boll followed the equation:YN=at-β(YN is nitrogen concentration in the subtending leaf of cotton boll(%); t is boll age(d); a and (3 are the parameters). In fiber thickening stage, the content of sucrose, and cellulose all changed as a parabola-like curve according to the increasing of the nitrogen content in the leaf subtending boll. The curve of the leaf nitrogen for fiber strength increasing was very close to the curve of the optimum leaf nitrogen for sucrose or cellulose accumulation, while it was significantly apart form the curve of the leaf nitrogen forβ-1,3-glucan accumulation. The nitrogen content of the leaf subtending bolls can present the special nitrogen status for fiber development precisely. In fiber thickening stage, there exists an optimum leaf nitrogen curve for sucrose, (3-1,3-glucan and cellulose accumulation and for fiber strength formation. The higher the sucrose exist the higher fiber strength formed. In the early stage of fiber thickening, higher content ofβ-1,3-glucan is benefit for fiber strength formation, while in later stage higher content ofβ-1,3-glucan will make less contribution to fiber strength formation. In one site, the optimum leaf nitrogen content for fiber strength formation in different cotton culitivar was different, in further conclusion, the nitrogen concentration in the subtending leaf of cotton boll impacted obviously on cotton fiber thickening and strength formation in cotton fiber.2. Effects of the subtending leaf nitrogen concentration on activities of related enzymes(sucrase, sucrose synthetase, sucrose phosphate synthetase,β-1,3-glucanase) involved in cotton fiber thickening development and fiber strength formationThe study focused on the effects of subtending leaf nitrogen concentration of boll on the activities of related enzymes involved in cotton fiber thickening and fiber strength formation. The result showed that the nitrogen concentration in subtending leaf of cotton boll has an effects on the related enzymes involved in cotton fiber thickening and fiber strength formation. In different fiber thickening stage, the activities of sucrase, sucrose synthetase, sucrose phosphate synthetase andβ-1,3-glucanase all changed as a parabola-like curve according to the increasing of the nitrogen content in the leaf subtending boll. In fiber thickening stage, there exists an optimum leaf nitrogen curve for sucrase, sucrose synthetase, sucrose phosphate synthetase,β-1,3-glucanase and cellulose accumulation. The curve of the leaf nitrogen for fiber strength increasing was very close to the curve of the optimum leaf nitrogen for sucrose synthetase, sucrose phosphate synthetase and cellulose accumulation, while it was significantly apart form the curve of the leaf nitrogen for sucrase,β-1,3-glucanase. The results showed that sucrose synthetase and sucrose phosphate synthetase presented the highest activities, which could make more contribution to fiber strength formation. In the early stage of fiber thickening, higher activities of sucrase is benefit for fiber strength formation, while in later stage higher activities of sucrase will make less contribution to fiber strength formation.Higher activities ofβ-1,3-glucanase will make more contribution to fiber strength formation in later stage. At 24,31, 38,45 and 52 day post-anthesis, the range of optimal nitrogen concentration in the subtending leaf of cotton boll of fiber strength formation in Dexiamianl, Kemianl, and NuCOTN are about 3.06%,2.73%,2.65%,2.43%; 2.74%,2.51%,2.40%,2.28%,2.16% and 2.94%,2.65%,2.52%,2.44%,2.28%, respectively.
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