Identification of salt tolerance of 121 watermelon (Citrullus lanatus L.) germplasm resources

Abstract:【Objective】The aims of this study were to identify the salt tolerance of watermelon germplasm resources at the seedling stage and select salt tolerant types. Watermelon is an important vegetable crop in China, and its yield and quality decline seriously under salt stress. At present, the research mainly focuses on the identification of salt-tolerant rootstocks and the application of exogenous substances. However, there are few reports on the screening of salt-tolerant germplasm at seedling stage of watermelon. In this study, 121 watermelon germplasm resources were screened for salt tolerance at seedling stage by the multivariate statistical method, in order to provide abundant parental materials for improvement and breeding of watermelon salt- tolerant varieties.【Methods】121 watermelon germplasm materials were cultured until cotyledon expansion, and then transferred into Hoagland's nutrient solution for culture with 15 plants in each pot, and the nutrient solution was changed once every two days. There were two treatments, control group (CK, Hoagland’s nutrient solution) and salt stress group (Hoagland’s nutrient solution was added with 150 mmol · L^-1 NaCl), which had 3 replicates for each treatment with 15 plants in each replicate. During salt stress, the nutrient solution and NaCl were replaced once every 2 days, and the indexes were determined 8 days after treatment. The indexes included shoot fresh weight, shoot dry weight, stem diameter, root length, root surface area and SPAD value. And watermelon accessions were evaluated with correlation analysis, principal component analysis and membership function analysis.【Results】Compared with the control, these indexes of all materials under NaCl stress revealed abundant genetic diversity. The relative value of fresh weight of overground part was 0.132-0.979, with an average value of 0.509. The relative value of dry weight of overground part was 0.156-0.967, with an average value of 0.611. The relative value of stem diameter was 0.721-1.103, with an average value of 0.917. The relative value of root length was 0.193-0.989, with an average value of 0.745. The relative value of root surface area was 0.204-0.985 with an average value of 0.646. The relative value of SPAD was 1.052-1.494, with an average of 1.201. The results showed that the variation range of each index with 121 watermelon materials was different, so single index could not be used to evaluate the salt tolerance of watermelon. Under NaCl stress, salt tolerance indexes of watermelon seedlings had different degrees of correlation. There existed significantly positive correlations with shoot dry weight, in comparison with shoot fresh weight and stem diameter. Root surface area showed significantly positive correlations with shoot fresh weight, shoot dry weight and root length. The principal component analysis showed that the contribution rates of the first three principal components were 30.685% , 24.245% and 19.490% , respectively, and the cumulative contribution rates reached 74.420%, and the eigenvalues were all greater than 1, indicating that the original six related single indicators could be converted into three independent principal components, which covered most of the information of the original indicators. The fresh weight and dry weight of overground part were the most important variables of principal component, which reflected the overground biomass of watermelon seedlings under salt stress. The load (absolute value) of the second principal component in root length and root surface area was larger, reflecting the growth of watermelon seedlings under salt stress, while the load (absolute value) of the third principal component in SPAD was larger, reflecting the physiological changes of watermelon seedlings under salt stress. Principal component analysis showed that the feature vectors of fresh weight of overground part and root length were larger among the three principal components, which could be used as indexes for evaluation of salt tolerance of watermelon seedlings. The membership function value, weight value and weighted membership function value (D value) were calculated according to the formula. The D value range of test materials was 0.165-0.566, indicating that there were extensive differences in salt tolerance among test materials. According to D value, 121 watermelon materials were divided into 4 salt-resistance types: Three accessions were high salt-sensitive materials (D value ＜ 0.3), including PI186489, PI494532 and Dahongzi; 81 accessions were week salt-sensitive materials (0.3 ≤ D value ＜ 0.6) , including Zhengzhou YCL, Huozhou 1, Yanshi 1, Zhengzhou 1, PI482322, Zhengzhouzigua and so on; 34 accessions were medium materials (0.6 ≤ D value ＜ 0.7), including Luofeilin, Grif16135, Kuiketawuzi, Xiaoma Seed, PI161375, 27150, Tomato seed, Meili, etc; 3 accessions were salt tolerant materials (D value ≥ 0.7), including Zaohua, PI490377, Zhongshihong.【Conclusion】Vine fresh weight, and root length were considered as the best indexes to identify salt resistance of watermelon at the seedling stage. High salt- sensitive materials, (PI186489, PI494532, Dahongzi) and salt tolerant materials (Zhongshihong, PI490377, Zaohua) can be used for breeding salt-tolerant cultivars and identification of salt-tolerant gene.