Effects Of Nitrogen On Yield And Fiber Quality Formation And Its Physio-Ecological Basis For Cotton (Gossypium Hirsutum L.) Under Water Stress During Flowering And Boll-Forming Stage

Flowering and boll-forming stage is the key yield determinant period of upland cotton. Short-duration drought stress occurring during this stage significantly reduced cotton final productivity and fiber quality.Nitrogen as a mineral nutrient is required in large amounts by plants and nitrogen application could improve drought resistance in plants.The study on the effects of nitrogen on physiological mechanism for cotton yield and quality formation under water stress has obvious implication for improving drought resistance and guiding cultural management in cotton.Pot experiments were conducted to short-term water stress during flowering and boll-forming stage of cotton in 2004～2006 at the research field area of Nanjing Agricultural University.Within each water treatment,three nitrogen levels(0, 240 and 480kg·N ha^-1) were imposed.The study focused on:(1) effects of nitrogen on the dry matter weight,nitrogen accumulation and distribution,yield and fiber quality under water stress and afterward re-watering;(2) responses of net photosynthetic rote(P_n), transpiration rate(T_?) and water use efficiency(WUE) of cotton to nitrogen under water stress and afterward re-watering,and the relationship between chlorophyll fluorescence characteristic,antioxidant enzyme activity,endogenous hormone content and net photosynthetic rate in cotton leaf;(3) effects of nitrogen on root growth,N accumulation, antioxidant enzyme activities and root vigor under soil water stress and afterward re-watering;(4) effects of nitrogen on the cotton fiber thickening and fiber strength development under water stress and afterward re-watering.The main results were as follows:1.Effects of nitrogen on the dry matter weight,nitrogen accumulation and distribution under soil water stress and afterward re-wateringSoil water stress significantly reduced dry matter weight,nitrogen accumulation,but increased nitrogen content in different organs of cotton.Water stress reduced the distributive indexes of dry matter weight and nitrogen accumulation in leaf,but increased the distributive indexes in root,and increased the Root/Shoot ratio.Nitrogen could increased dry matter weight and nitrogen accumulation of cotton,and augmented the water stress indexes.There was notability compensative growth for cotton after re-watering, especially for root,the dry matter weight and nitrogen accumulation in root of Water-stress treatments were higher than that of Well-water treatments.Nitrogen enhanced the compensative capacity.The dry matter weight of reproductive organs,seed cotton yield and fiber quality were all highest at 240 kg N ha^-1 level under water stress.These results suggest that 240 kg N ha^-1 is the optimal nitrogen application rate under soil water stress in our experiments,deficient(0 kg N ha^-1) and excessive nitrogen supply(480 kg N ha^-1) are of disadvantaged to the yield and fiber quality.2.Effects of nitrogen on the net photosynthetic rate,transpiration rate and water use efficiency,and the relationship between chlorophyll fluorescence characteristic, antioxidant enzyme activity,endogenous hormone content and net photosynthetic rate in cotton leaf under soil water stress and afterward re-watering.(1) P_n,stomatal conductance(G_s) and T_? were declined when the soil relative water content went down under soil water stress.Nitrogen reduced P_n,G_s and T_?,but increased WUE in cotton leaf under soil water stress.(2) Nitrogen has significantly effect on chlorophyll fluorescence characteristic,antioxidant enzyme activity,endogenous hormone content in cotton leaf.According the data,240 kg N ha^-1 was the best nitrogen application level for improving Pn among three nitrogen levels. The maximum photochemical efficiency of photosystemⅡ(PSⅡ) photochemistry(F_v/F_m), the quanttun yield of electron transport(φ_PSⅡ),the electron transport rate(ETR) and the photochemical quenching co-efficient(qP) in cotton leaf were all higher at 240 kg N ha^-1 than other two nitrogen level.Under soil water stress,CAT activity,soluble protein content, ZR content,IAA content,GA content,ZR/ABA,IAA/ABA,GA/ABA of 240 kg·ha^-1 cotton was highest among the three nitrogen levels,and MDA content and ABA content of 240 kg N ha^-1 cotton was smallest.Deficient(0 kg N ha^-1) and excessive(480 kg N ha^-1) nitrogen supply were of disadvantaged to the ehiorophyll fluorescence characteristic,antioxidant enzyme activities,endogenous hormone content and photosynthesis under soil water stress, because the deficient nitrogen treatment cotton suffered drought stress and nitrogen deficit stress,and the excessive nitrogen treatment cotton increased the water stress.After re-watering,there was no difference of SOD activity,CAT activity,MDA content and photosynthesis between water-stress and well-water treatments.ABA content in water-stressed cotton leaf were significant lower than that in well-water treatments,but ZR, IAA,GA content,ZR/ABA,IAA/ABA,GA/ABA in water-stressed cotton leaf were significant higher than that in Well-water treatments.Nitrogen supply could promote the SOD activity,CAT activity,POD activity,ZR content,IAA content,GA content,ZR/ABA, IAA/ABA,GA/ABA and P_n,and reduced MDA content and ABA content in water-stress and well-water treatments.3.Effects of nitrogen on the cotton root growth under water stress and afterward re-wateringSoil water stress significantly reduced cotton root dry matter production and N accumulation.The Root/Shoot ratio(R/S) and Root-N/Shoot-N(R_N/S_N),increased with water stress,was smaller at 240 kg N ha^-1 level than at 0 and 480 kg N ha^-1 level.Nitrogen decreased the SOD activity,but increased the activities of POD and CAT.Malondialdehyde (MDA) content was greater in water-stress treatments than in well-water treatments,and MDA content of 240 kg N ha^-1 level was smallest among the three nitrogen levels under soil water stress.After re-watering,MDA content of 240 kg N ha^-1 level was the same as that of 480 kg N ha^-1 level,but less than that of 0 kg N ha^-1 level.The root vigor,debased by soil water stress,was the highest at 240 kg N ha^-1 level.After re-watering,nitrogen promoted the root vigor under two watering managements.The trends of net photosynthetic rate were the same as that of root vigor,but opposite to that of MDA content under soil water stress.These results suggest that appropriate nitrogen supply(240 kg N ha^-1) may contribute to drought resistance of cotton plants by adjusting the antioxidant enzyme activities of root,debasing lipid peroxidation and boosting root vigor under short-duration soil water stress,but excessive nitrogen(480 kg N ha^-1) was deleterious.4.Effects of nitrogen on the cotton fiber development under water stress and afterward re-wateringDuring soil water stress,soil water stress significantly reduced soluble protein content in cotton fiber,increased the antioxidant enzyme activities of SOD,CAT and POD and MDA content,but reduced the fiber thickening key enzymes activities of sucrose synthetase,β-1,3-glucanase,sucrose phosphate synthetase,and IAA oxidase,and the cotton fiber strength.On the 10th day after re-watering,there was no significant difference of SOD,CAT and POD activity between soil water-stressed and well-water treatments,MDA content were declined,and the fiber thickening related key enzymes activities were still lower than Well-water treatments.The result of 2-year experiment showed that 240 kg N ha^-1 was the optimal nitrogen application rate for forming high-strength fiber under soil water stress and afterward re-watering during the flowering and bell-forming stage.The antioxidant enzymes activities were highest,MDA content were the lowest,and the fiber thickening key enzymes activities of sucrose synthetase,β-1,3-glucanase,sucrose phosphate synthetase,and IAA oxidase were the highest at 240 kg N ha^-1 nitrogen level, thus leading to the greatest fiber strength.Deficient(0 kg N ha^-1) and excessive(480 kg N ha^-1) nitrogen supply are of disadvantaged to the fiber thickening key enzymes activities and cotton fiber strength.