| Fiber strength is one of the most important index to evaluate fiber quality, which associates with secondary wall thickening stage and cellulose deposition characters. Nitrogen is one of the regulating factors to the production and fiber quality of cotton. To study the regular mechanism of nitrogen on cotton fiber formation and clarify the optimal nitrogen status for fiber strength could provide a basement in field to improve fiber quality by regulate nitrogen status. A2-year (2008and2009) study was conducted outdoors in Nanjing (118°50’E,32°02’N), which was located in the middle-lower reaches of Yangtze River Valley, China. Cotton cultivars Dexiamian1, Kemian1and NuCOTN33B, that were quite different in fiber strength were used as materials, and different nitrogen rates were applied to carry out different leaf (the subtending leaf of boll) nitrogen concentration condition in the experiment. The objectives in this study were (1) to analyze the effects of leaf nitrogen concentration on related enzymes and fiber strength in cotton fiber during fiber development,(2) to analyze the effects of leaf nitrogen concentration on related substances and fiber strength in cotton fiber during fiber development,(3) to analyze the response of related enzymes and related substances in fiber on leaf nitrogen concentration during fiber development,(4) to analyze the response of source-sink activity on leaf nitrogen concentration during boll growth,(5) to analyze the relationship between nitrogen status and carbohydrates content and the effect of this relationship on fiber quality.The main results were as follows:1. Effects of leaf nitrogen concentration on related enzymes (Invertase, Sucrose synthase, Sucrose phosphate synthase andβ-1,3-glucanase) activity and fiber strength formationBy studying the response of characteristics of nitrogen concentration in subtending leaf of cotton boll to nitrogen application rates and the response of related enzymes’activity to leaf nitrogen concentration, to analyze the mechanism of nitrogen on fiber strength formation and clarify the optimal nitrogen status on fiber strength. The results indicated that the leaf nitrogen concentration was increased with the increasing of nitrogen rates; and the changes of leaf nitrogen concentration followed the equation:YN=αt(YN is nitrogen concentration in the subtending leaf of cotton boll(%); t is days post anthesis(d); a and β are parameters); The enzymes activity and fiber strength all changed following the parabolic equation (Y=ax2+bx+c, Y is enzyme activity or fiber strength, x is leaf nitrogen concentration (%), a, b and c are parameters) with the increasing of leaf nitrogen concentration. The leaf nitrogen concentration also had a significant effect on related enzymes activity and fiber strength, and there was little difference between the optimal leaf nitrogen concentrations that were for acquiring the highest related enzymes and for acquiring the highest fiber strength respectively in each sampling time. And the equations of the optimal leaf nitrogen concentration to the time course of days post anthesis were NDexiamianl=7.2841DPA-0.2771(R2=0.9860,P<0.01), NKemianl=7.1807DPA-0.2989(R2=0.9879, P<0.01), NNUCOTN33B=7.1467DPA-0.2819(R2=0.9755, P<0.01) for Dexiamian1, Kemian1and NuCOTN33B。 So, it is believed that higher fiber strength, accompany with the related enzymes activity could be acquired by regulating the leaf nitrogen concentration to a better level in cotton cultivation.2. Effects of leaf nitrogen concentration on related substances (Sucrose, β-1,3-glucance and cellulose) content and fiber strength formationBy studying the response of related substance content to leaf nitrogen concentration, to analyze the mechanism of nitrogen on fiber strength formation and clarify the optimal nitrogen status on fiber strength. The results indicated that the leaf nitrogen concentration was increased with the increasing of nitrogen rates; and the changes of leaf nitrogen concentration followed the equation:Yn=αt-β (YN is nitrogen concentration in the subtending leaf of cotton boll(%);t is days post anthesis (d); a and β are parameters); The substances’content and fiber strength all changed following the parabolic equation (Y=ax2+bx+c,Y is enzyme activity or fiber strength, x is leaf nitrogen concentration (%), a, b and c are parameters) with the increasing of leaf nitrogen concentration. The leaf nitrogen concentration also had a significant effect on related substances content and fiber strength, and there was little difference between the optimal leaf nitrogen concentrations that were for acquiring the highest related substances and for acquiring the highest fiber strength respectively in each sampling time. And the equations of the optimal leaf nitrogen concentration to the time course of days post anthesis were NdeXiamianl=7.6649DPA-02974(R2=0.9758), Nkemianl=7.2945DPA-0.3086(R2=0.9816) and NNUCOTN33B=7.3418DPA-0.2948(R2=0.9684) for Dexiamian1, Kemian1and NuCOTN33B. So, it is believed that higher fiber strength, accompany with the related substances content could be acquired by regulating the leaf nitrogen concentration to a better level in cotton cultivation.3. The mechanism of leaf nitrogen concentration on related enzymes and related substances in fiber at different fruit branch positionBy studying the response of related enzymes activity and related substance content to leaf nitrogen concentration in lower and upper fruit branch position, to analyze the mechanism of nitrogen on fiber strength formation and clarify the optimal nitrogen status on fiber strength. The results showed that the related enzymes’activity and fiber strength all changed following the parabolic equation (Y=ax2+bx+c, Y is enzyme activity or fiber strength, x is leaf nitrogen concentration (%), a, b and c are parameters) with the increasing of leaf nitrogen concentration, and leaf nitrogen concentration had no significant effect on fiber growth process, and there were similar trend among different fruit branch position. There was little difference between the optimal leaf nitrogen concentrations that were for acquiring the highest related enzymes and for acquiring the highest fiber strength respectively in each sampling time in same fruit branch position. However, there were significant difference of optimal leaf nitrogen concentration between lower and upper fruit branch position, and upper leaf nitrogen concentration was higher than lower. So, it is believed that higher fiber strength, accompany with the related enzymes activity could be acquired by regulating the leaf nitrogen concentration to a better level in cotton cultivation, and the necessity of nitrogen fertilizer, the time and amount of nitrogen fertilizer application should consider the needing rule of fertilization.4. Effects of leaf nitrogen concentration on source-sink activity between the subtending leaf of boll and cotton fiberThis study was conducted to explore the effects of subtending leaf nitrogen concentration on leaf source activity (soluble sugar content, sucrose content and sucrose phosphate synthase activity) and fiber sink activity (sucrose content and sucrose synthase activity) during the development of cotton boll. The results showed that leaf nitrogen concentration have significant effects on leaf source strength and fiber sink strength during the development of cotton boll. At the same days post anthesis(DPA), leaf source activity and fiber sink activity all changed following parabolic equation (Y=ax2+bx+c, Y is leaf source activity or fiber sink activity, x is leaf nitrogen concentration, a, b and c are parameters) with the increasing of the subtending leaf nitrogen concentration. Before45DPA(38DPA for Dexiamian1), there was a tiny difference among optimal leaf nitrogen concentration of above mentioned index. So, we could change the leaf nitrogen concentration and further to improve leaf source strength and fiber sink strength to best status and was benefit in cotton boll. The equation of optimal leaf nitrogen concentration to days post anthesis for Dexiamian1, Kemian1and NuCOTN33B during the development of cotton boll were NDexiamianl=7.2383DPA-02759(R2=0.9829**), NKemianl=7.236DPA-0.3026(R2=0.9876**)、 NNUCOTN33B=7.1706DPA-0.284(R2=0.9788**). After45DPA, there was a bigger difference among index, and leaf source activity and fiber sucrose phosphate synthase need a higher leaf nitrogen concentration than fiber sucrose.5. The relationship between nitrogen status and carbohydrates in the subtending leaf and effect of above relationship on fiber qualityThis study was to explore the relationship between nitrogen status and carbohydrate, and carbohydrate content and fiber quality. The results indicated that leaf (the subtending leaf of boll) N concentration can be more accurately to reflect the N status of the subtending leaves of boll than SPAD reading or free amino acid. Sucrose and non-structural carbohydrate had a quadratic relation with leaf N concentration (P<0.01). The optimal leaf N concentration is around2.50%, and it follows a typical dilution curve. During24-38days post-anthesis (DPA), sucrose in fiber was positively related to sucrose or non-structural carbohydrate of the subtending leaves of boll (P<0.05), while was not correlated with starch or soluble sugar of the leaf (P>0.05). Fiber sucrose had a positive correlation with fiber strength before38days post-anthesis (DPA)(P<0.05), and it is more susceptible than fiber length, fineness and maturity to fiber sucrose. These suggested that (1)24-38DPA is a crucial period for fiber development which might be significant influenced by physiological and ecological factors,(2) sucrose or non-structural carbohydrate in the subtending leaf of boll could be taken as monitoring indices to evaluate sucrose level in the developing fiber, and then to estimate the final fiber quality. |
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