Abstract: 【Objective】As one of the salt-tolerance crops, there is a great difference among different varieties. So, the objective of this study was to investigate the salt tolerance of cotton seedlings of different genotypes which are grown widely in different eras in Xinjiang, to excavate their own salt tolerance genetic resources, to screen suitable evaluation indicators of salt-tolerance, and to establish a mathematical evaluation model for salt tolerance, which would provide a theoretical basis for discovery and promotion of salt tolerant germplasm, breeding of new salt-tolerant varieties and evaluation on salt tolerance of a large number of cotton varieties.【Method】Seventeen varieties that are widely grown at different decades in Xinjiang of China were selected as test materials in this study. Compared to the control, the mass ratio of NaCl to growing medium was 0.006 in salt stress, and the composition of matrix is 1 part grass charcoal plus 1 part vermiculite. After being disinfected and pregermination, cotton seeds of seventeen different genotypes were to sow in the medium. This is helpful to reflect the actual stress environment in field conditions and explain the mechanisms of the tolerance by means of continuous salt stress at seedling stage. The salt comprehensive evaluation of salt tolerant coefficient (STC) about the emergence rate (ER), fresh weight (FW), dry weight (DW) and plant water content (PWC) of each variety, leaf area of the first true leaf (LA), the net photosynthetic rate (Pn), chlorophyll content (Chl) and relative electric conductivity (REC) in leaves of cotton seedlings of different genotypes under salt stress (0.6%) were evaluated by principal components analysis, hierarchical cluster analysis and regression analysis. Physiological performance characteristics of various salt-tolerant types of cotton varieties were analyzed, too.【Result】The results showed that the 11 single indexes in leaves of cotton seedlings of different genotypes under salt stress could be classified into 6 independent comprehensive components by means of principal components analysis. According to the salt-tolerance comprehensive evaluation value (D value) of different cotton varieties, which was calculated from the subordinate function analysis, the 17 cotton cultivars were divided into four salt-tolerant types by cluster analysis, 3 of the 17 varieties were salt-sensitive type, 6 varieties were weak salt-resistance type, 6 were medium salt-resistance type, and 2 were high salt-resistance type. A mathematical evaluation model for cotton salt tolerance was established by means of regression analysis, and D= -1.192+ 0.402REC+ 0.119LA+ 0.274FW+ 0.086Pn+ 1.019Chl (R2= 0.9921). Five indexes closely related to the salt tolerance, LA, REC, FW, Pn and Chl, were screened, too. Evaluation accuracy of all varieties were more than 94.44%, this shows that the five indexes in the regression equation obviously affect the salt resistance of cotton seedlings, and this equation can be used in salt tolerance evaluation of cotton cultivars. Furthermore, the regression analysis and clustering results were mutual authentication. The results showed that, compared with salt-sensitive cotton varieties, REC of cotton leaves of high salt-resistance type was lower than that of other cultivars after treated with 0.6% salt stress, Pn, Chl, LA and FW were higher, LA of high salt-resistant cotton seedlings especially was nearly 2 times larger than other categories varieties.【Conclusion】Higher salt tolerant varieties, which had lighter injury, higher leaf area and photosynthetic capacities under salt stress, were able to maintain higher accumulation ability of salinity and photosynthetic products, decreased ion toxicity from soil, and enhanced salt tolerance of plants. Determination of the five indicators under the same condition can be used for rapid identification and prediction of other cotton varieties, which is very useful for the breeding, promotion, identification and screening of salt tolerant germplasm.

Key words: cotton , seedling stage , different genotypes , salt tolerance , principal components analysis , comprehensive evaluation , regression analysis , evaluation indicator