## 4 Discussion
### Evaluation of the Chilling Tolerance of Leafy Mustard Cultivars  
These findings indicated that the variation trends of the various physiological indices of the different mustard cultivars were not completely consistent, and the trends of the various physiological indices varied greatly. The correlation analysis revealed different degrees of correlation among the indices and close correlation among the chlorophyll fluorescence parameters. Given that the different degrees of correlation of the indicators resulted in overlapping information and given the different roles played by each indicator in the chilling stress response of mustard, it is impossible to accurately evaluate the chilling tolerance of mustard cultivars using these indicators, and a comprehensive analysis is needed. The chilling tolerance of plants is affected by their own genetic factors and external environmental factors; using a single index to evaluate the chilling tolerance of plants limits the knowledge obtained [42; 43]. It was necessary to analyse multiple physiological indices in an integrated manner to comprehensively and accurately evaluate the chilling tolerance ability of mustard.  
PCA is a method for reducing the dimensionality of a particular dataset. It does this by creating new covariates that are uncorrelated with each other, which are called principal components, and will reduce the problem of solving eigenvalues/eigenvectors [44]. In this study, PCA was used to transform the 13 indicators into six more representative composite indicators, and these indicators focus on the information of the original indicators with nonsignificant differences. The membership function method utilizes the principles of fuzzy mathematics to provide a comprehensive and scientific evaluation of things or phenomena [45]. In this study, the membership function value method in fuzzy mathematics was utilized to calculate the comprehensive evaluation value (D-value) of the chilling tolerance of different cultivars, and the D-value reflects the difference in chilling tolerance among different cultivars. The results showed that SJTKJ Mustard had the strongest chilling tolerance, and DX Mustard had the weakest chilling tolerance. Cluster analysis refers to the classification of any sample category or other prior knowledge in a batch of samples based on the characteristics of the samples. Identical or similar characteristics are classified into one category based on some measure of similarity [46]. In this study, 65 mustard cultivars were divided into four categories according to the strength of chilling tolerance and chilling tolerance evaluation models were established using stepwise regression analysis for non-chilling-tolerant, mildly chilling-tolerant, moderately chilling-tolerant and highly chilling-tolerant cultivars. Four chilling tolerance indices, namely, Fv/Fm, \(\Phi\)PSII, POD activity, and Rfd, were selected based on correlation analysis of each trait index and regression analysis of the D-value of the comprehensive evaluation index. However, in future studies, in addition to physiological indicators and fluorescence characteristics, we should further consider collecting more biomass and leaf, petal, seed phenotypic characteristics, and other indicators of mustard at different reproductive stages, because this is more conducive to a comprehensive understanding of the correlation between the whole life cycle of chilling tolerance in mustard from different germplasm resources, and make a more comprehensive and integrated analysis.