Effects Of Soil Salinity On Growthand Its Physiological Mechanism For Cotton(Gossypium Hirsutum L.)

Salinity is considered one of the major limiting factors for plant growth and agricultural productivity. Cotton is one of the most important economic crops in China, which has been reported to be salt tolerant. With the reduction of field area, more and more cotton was planted on saline soil. Thus it is necessary to increase cotton production. The study on the effects of soil salinity on physiological mechanism for cotton yield formation has obvious implication for improving salt resistance and guiding cultural management in cotton.1. Fourteen cotton cultivars were used to evaluate of salt tolerance and select the salt tolerance indices of cotton (Gossypium hirsutum L.) at germinating and seedling stage.2Two cotton cultivars with different salt-tolerance with different salinity levels were imposed, The study focused on:(1) effects of soil salinity on ions transportation and distribution and plant growth of cotton;(2) effects of soil salinity on root growth, antioxidant enzyme activities, root vigor, the changes of root plasma membrane lipid and fatty acid;(3) responses of photosynthesis of cotton to soil salinity, and the relationship between chlorophyll fluorescence characteristic, antioxidant enzyme activity, endogenous hormone content, fatty acid composition and net photosynthetic rate in cotton founctional leaf. The main results were as follows:1. Evaluation of salt tolerance and selection of salt tolerance indices of cotton (Gossypium hirsutum L.) at germinating and seedling stageBy artificial simulated salt stress with NaCl aqueous solution, we compared salt tolerance of13cotton cultivars at their germinating stage and seedling stage according to subjection values of salt toxicity coefficient of indices determined and the sum subjection values and classified the13cultivars at the two stages by cluster analysis using their single salt toxicity coefficient subjection values and the sum subjection values. Results indicated that, the appropriate concentration of NaCl aqueous solution, for the assessment of salt tolerance, was150mmol L-1; and the capacities of salt tolerance were of differences at different growth stages among these cultivars; of which, CCRI-44and CCRI-177were two salt-tolerant cultivars, showing a stabilized capacity at both germinating and seedling stage; yet CCRI-102, Sumian12and Simian3were the salt-sensitive ones despite of unsteady properties; meanwhile CCRI-103, Dexiamian1and NuCOTN33B were the ones with the steady properties at the medium levels. That the salt toxicity coefficient of germination rate, germination energy, germination index, vigor index and fresh weight could serve as the indicators to determinate salt tolerance in the cultivars at germinating stage. And the coefficient of plant height, leaf expansion rate, shoot dry weight, root dry weight, root vigor and net photosynthetic rate could function as the indexes in the assessment of salt tolerance at seedling stage.2. Effect of soil salinity on yield firmation and its physiological mechanism for cotton (Gossypium hirsutum L.)(1) In2008-2009, two cotton cultivars with different salt-tolerance with different salinity levels were imposed to determinate the effects of soil salinity on ions absorption and transportation and plant growth of cotton. Results showed that, Soil salinity significantly decreased the dry matter weight of cotton plant, the impact on aerial parts was more significant than on roots. Soil salinity could weaken the water adsorption of roots and suppress water transportation from roots to leaves. Soil salinity had an action on ions distribution in plants, displaying a rising tendency of Na+, Cl-and Mg2+contents and a deducing one of that of K+and Ca2+as the salinity level increasing, meanwhile, the ratios of K+/Na+, Ca2+/Na+and Mg2+/Na+in tissues decreased. As an important ions tank for plants, stem and branch could enhance the salt tolerance magnitude by pumping out K+, Ca2+and Mg2+while pumping in Na+, yet its inability of manipulating the transportation of Cl-to leaves caused a relatively high Cl-content in leaves. With the rising of salt densities, the transportation of K+、Ca2+and Mg2+from roots to ground shoots weakened, while that of selective transportation of K+, Ca2+and Mg2+from ground shoots to leaves and cotton flower buds and bolls was strengthened. Due to the factors as mentioned above, the figures of K+/Na+, Ca2+/Na+and Mg2+/Na+were maintained less significant drops as compared to those at the relatively higher salinity levels. Thus, the relatively higher salt tolerance of CCRI was largely accountable to the high abilities, of stem and branch intercepting Na+, of leaves excluding Cl-, as well as of the transportation of K+, Ca2+and Mg2+from stem and branch to leaf.(2) In2008-2009, two cotton cultivars with different salt-tolerance with four treatments (CK, DT, ST and SD) were set to determinate the effects of soil salinity and drought and their combination on cotton root physiological characters and growth. Results showed that, Drought, salinity and their combination treatments reduced cotton root plasma membrane phospholipid content but increased Galactoselipid lipid content, which resulting in decrease in ratio of phospholipid and Galactoselipid. DT, ST and SD decreased activity of antioxidant enzyme, which lead to increasing of lipid peroxidation and changing in membrane fatty acid composition. The relatively high activity of antioxidant and ratio of phospholipid and Galactoselipid was helpful to maintain a relatively high plasma membrane H+-ATPase andCa2+-ATPase. SD had most significant impact on root physiological characters and growth, followed by DT and ST.(3) Soil salinity had obviously inhibitory effects on the net photo synthetic rate, especially, of Sumian12. The content of superoxide anions(O2-) and hydrogen peroxide (H2O2) increased with the increasing of salinity level. The content of soluble protein and activities of superoxide dismutase (SOD) and catalase (CAT) increased first and then decreased, activity of peroxidase (POD) showed a continuous increase trend. Activity of IAA oxidase increased at5.80dS"1of salinity level, but decreased with the increase of salinity level. The content of IAA and GA3increased at5.80dS-1, but decreased at higher salinity level. With the soil salinity level increasing, the content of ABA gradually increased, while the content of ZR sharply decreased. The ratios of IAA/ABA、GA3/ABA and ZR/ABA showed dropping trend with increased salt concentration, However, the ratios of CCRI-44were higher than those of Sumian12subject to salt stress. Those results mentioned above suggested that salinity-tolerance cultivar had higher activities of SOD and POD and stronger ability to regulate endogenous hormone contents of functional leaves for adapting salt stress, which was benefit to keep a higher Pn.When in saline environments, the contents of saturated fatty acids (palmitic acids and stearic acids) and unsaturated fatty acids (oleic acids) increased, while the contents of high-unsaturated fatty acids (linoleic acids and linolenic acids) decreased; that of short-chain fatty acids (palmitate acids) increased, yet that of long-chain fatty acids decreased; the ratios of the contents of unsaturated fatty acids by saturated ones became larger with a lowering DBI indexes. The disparities of lipoxygenase activities accounted mainly for multiple grades of membrane lipid peroxidation amongst species. When the figures of lipoxygenase activities was smaller than13.70ds m-1, membrane lipid peroxidation was triggered by the combined action of active oxygen and lipoxygenases; otherwise, it was mainly accountable to the rising activities of lipoxygenases. The deduction of DBI indexes initiated the improvement of photo-inhibition of in PS II in cotton leaves and the decreased the conversion efficiency of solar radiation, so the lowered net photosynthetic rates were caused. Soil salinity that higher than0.35%made the diurnal variation patterns of the net photosynthetic rate changed from a one-peak curve to a constantly decreasing one gradually. The relatively low intensity of oxidation catalyzed by CCRI-44was helpful to maintain a relatively low degree of the contents of unsaturated fatty acids and membrane lipid peroxidation intensity. And the relatively high contents of unsaturated fatty acids was one of the important reasons for the higher net photosynthetic rates of CCRI-44than that of Sumian12.