Evaluation And Genome-wide Association Study Of Low Temperature And Weak Light Tolerance In Zucchini Core Germplasm

Zucchini?Cucurbita pepo L.?is an important melon vegetable in the world,whose cultivation area and yield in our country are among the highest in the world.In recent years,the protected cultivation area of zucchini is second only to cucumber,and it has become an important cultivated crop for farmers'income increase and agricultural industrial structure adjustment.However,in the production process of off-season protected areas in the north,due to the long-term low temperature and weak light conditions,in order to ensure the yield and quality,it is necessary for the zucchini varieties to have low temperature and weak light resistance.At present,there are few varieties of zucchini tolerant to low temperature and weak light,and there is a lack of efficient breeding techniques.At the same time,there are few reports on the molecular mechanism of low temperature and weak light tolerance of zucchini,which seriously hinders the cultivation of fine varieties.In this study,110 zucchini germplasm materials were identified and evaluated at seedling stage,and the germplasm materials with low temperature and weak light tolerance were screened out.Then genome-wide SNP data were obtained by re-sequencing,and population structure and relationship analysis were performed.Finally,genome-wide association analysis was used to obtain SNPs significantly associated with low temperature and weak light tolerance of zucchini and to screen potential candidate genes.The results provided a basis for the improvement of low temperature and low light tolerance evaluation technology,the discovery of low temperature and low light tolerance related genes and the selection of new varieties with good low temperature and low light tolerance,and also provided a reference for the analysis of low temperature and weak light molecular mechanism.The main results are as follows:1.Seven phenotypic traits of 110 zucchini germplasm materials were statistically described and analyzed.The coefficient of variation?CV?analysis showed that the CV of stomatal conductance of zucchini seedlings was the largest and the CV of net photosynthetic rate was the smallest.The CV of leaf area,hypocotyl,stomatal conductance and transpiration rate of zucchini seedlings under low temperature and weak light exceeded 50%,indicating that there were abundant phenotypic variations in these four traits.The CV of net photosynthetic rate was less than 25%,indicating that the genetic characteristics of this phenotypic trait were relatively stable.The results of correlation analysis showed that there were complex correlations between the seven phenotypic traits.Principal component analysis showed that the seven phenotypic traits could be classified into two principal components,and the cumulative contribution rate of the two principal components was 75.03%,indicating that the two principal components could represent most of the information of the seven low temperature and weak light related phenotypic traits of zucchini.2.Under low temperature and weak light stress,there were significant differences in leaf area variation,hypocotyl variation,photosynthetic rate?Pn?,stomatal conductance?Cond?,intercellular CO₂ concentration?Ci?,transpiration rate?Tr?and chilling injury index among different zucchini germplasm seedlings.Taking all these 7 characters into account,9 zucchini germplasms?D54?D73?D79?D61?D80?D71?D64?D76 and D59?were selected as low temperature and weak light resistant materials,among which D54 and D79 were with strong resistance to low temperature and weak light.3.A total of 2,476,696 pairs of SNP markers were used to analyze the population structure,relationship and linkage disequilibrium of 110 zucchini germplasm materials.The population structure analysis showed that the 110 zucchini germplasm materials could be divided into three subpopulations.The relationship analysis showed that only 7.81%of the total material with relationship coefficient greater than0.20,indicating that the relationship between these 110 zucchini germplasm materials was relatively far away.The linkage disequilibrium analysis showed that the LD attenuation value of the genome was 18.3 kb,and the physical distance range of each chromosome's LD attenuation was uneven.4.Using a mixed linear model,a total of 34 SNP loci associated with zucchini's low temperature and weak light tolerance were detected by genome-wide association analysis.These 34 SNPs were distributed on 8 different chromosomes.The genes were annotated and candidate genes were excavated in the regions extending 300 kb in the upstream and downstream of the significant SNP.Finally,the Cp4.1LG20g04830 was a potential candidate gene related to the photosynthetic rate after low temperature and weak light treatment.It was speculated that Cp4.1LG04g14030 gene may be a potential candidate gene to control stomatal conductance and transpiration rate of zucchini after low temperature and weak light treatment.