Analysis Of Genetic Diversity And Evaluation Of Resistance To Powdery Mildew And Tolerance To Low Temperature Of Cucumber Core Collections

Cucumber is one of the most important vegetables not only in China but also in the whole world. Due to artificial selection and hybridization of cultivars,the diversity of cucumber germplasm resources was threatened and the genetic background became increasingly narrow. In recent years,as the cultivated area of protected land such as vinyl house and green house expanding,the influence of the low temperature and weak light on cucumber production was enhanced. In this study, experiments were performed based on the core germplasm population which was constructed on the basis of the cucumber germplasm resources preserved in the national genebank of the vegetable genetic resources. In order to understand the genetic diversity distribution as well as the genetic relationships among the core germplasm, genetic diversity was analyzed in morphological and molecular level respectively through identification of agronomic traits and SSR markers. In addition, by means of evaluation tolerance to low-temperature at seedling stage and the resistance to powdery mildew at adult stage under natural stress conditions,the resistance distribution of the core germplasm resources was explained and the resistant accessions were screened out. which set the foundation for elite germplasm innovation,resistance breeding and further discovery of elite genes by the method of association analysis. The main results were as follows:1. Results of identification of 31 morphological traits of cucumber core germplasm resources at their seedling and adult stage showed that the population possessed relatively abundant genetic diversity in morphological level. The mean morphological diversity index of domestic germplasm was 1.51,a bit higher than 1.45 of foreign germplasm. Result of cluster analysis showed that 307 cucumber accessions were divided into two distinct parts including north China type and south China type,and most foreign germplasm was classified into the latter type. The mean diversity indexes of cucumber germplasm originated for 7 different districts indicated that most cucumber germplasm resources of our country possessed abundant genetic diversity while the diversity level of the resources from northwest and south China was relatively low.2. 367 alleles were detected among 240 cucumber core germplasm using 81 pairs of SSR primers. Result of cluster analysis showed that foreign germplasm could be distinguished from domestic gemplasm distinctly while the south China type germplasm could be separated from the north China type. Result of genetic diversity analysis of cucumber germplasm from different origins showed that the genetic diversity of the cucumber germplasm from the southwest areas was relatively abundant,with the number of allele, the number of effective allele, genetic diversity index. A narrow foundation of genetic diversity was detected in the cucumber germplasm resources from the northwest areas. Above results were consistent with that of the morphological analysis.3. Resistance distribution of 263 cucumber core germplasm in three different seasons showed the normal distribution and partly deviated to susceptible region. The season of spring in 2009 witnessed the most obvious deviation. The resistance of most cucumber germplasm was influenced by the environmental conditions and only 18 germplasm expressed stable resistance in different seasons. Based on the resistance of cucumber core germplasm at their adult stage in three seasons, 23 germplasm with high resistance to powdery mildew were screened out, with the main type of them as north China type.4. Under the natural low temperature stress, five indexes including the seedling survival rate, cold damage index, number of leaves, plant height, content of chlorophyll were evaluated among 263 cucumber core germplasm. All indexes related with low-temperature tolerance corresponded to normal distribution, with the seedling survival rate and the cold damage index deviating to low-level (or high-level) region significantly. The result of correlation analysis showed that the cold damage index showed the significantly negative correlation with other four indexes including the seedling survival rate, number of leaves, plant height and content of chlorophyll. Principal component analysis of the 5 indexes showed that the first principal component was cold damage index and the seedling survival rate while the second principal component was number of leaves and the cumulative contribution rate of the two components was 74.36%.Low-temperature tolerance of the cucumber core germplasm was evaluated through membership function and 28 cucumber germplasm with high tolerance to low tempe rature were screened out.