## 3 Results
### Establishment of Regression Equations and Screening of Identification Indices  
As shown in Table 6, the correlations of SOD activity, Fv/Fm, \(\Phi\)PSII, NPQ, qP, and Rfd with the overall evaluation value of chilling tolerance (D) were highly significant, and the correlations of POD activity and CAT activity with the overall evaluation value (D) were significant. The correlation coefficients between the chilling tolerance coefficients of \(\Phi\)PSII, qP, and Rfd and the overall evaluation value (D) were greater, with correlation coefficients of \(0.737\), \(0.703\), and \(0.693\), respectively. The optimal regression equation was established using the value of D as the dependent variable and the chilling tolerance coefficients of the individual indices as the independent variables. The optimal regression equation was established as follows: Y = \(-0.333\) + \(0.004\)\(\times\) X\({}_{1}\) \(-0.002\)\(\times\) X\({}_{2}\) + \(0.003\)\(\times\) X\({}_{3}\) + \(0.019\)\(\times\) X\({}_{4}\) + \(0.06\)\(\times\) X\({}_{5}\) + \(0.011\)\(\times\) X\({}_{6}\) + \(0.012\)\(\times\) X\({}_{7}\) + \(0.479\)\(\times\) X\({}_{8}\) + \(0.11\)\(\times\) X\({}_{9}\) + \(0.018\)\(\times\) X\({}_{10}\) + \(0.044\)\(\times\) X\({}_{11}\), where Y is the predicted value of the overall evaluation of chilling tolerance, X\({}_{1}\) represents chlorophyll, X\({}_{2}\) represents soluble sugar, X\({}_{3}\) represents proline, X\({}_{4}\) represents SOD activity, X\({}_{5}\) represents POD activity, X\({}_{6}\) represents CAT activity, X\({}_{7}\) represents MDA, X\({}_{8}\) represents Fv/Fm, X\({}_{9}\) represents \(\Phi\)PSII, X\({}_{10}\) represents NPQ, and X\({}_{11}\) represents Rfd. The coefficients of each indicator represent the weight of their impact on the comprehensive evaluation value (D) of chilling tolerance. The determination coefficient of the regression equation is R\({}^{2}\) = 0.999 (\(p\) < 0.001), there is no multicollinearity problem (tolerance > 0.2, VIF < 5), and these 11 indicators have an impact on the comprehensive evaluation value (D) of chilling tolerance. The correlation analysis and regression analysis results between the chilling tolerance index and the comprehensive evaluation value (D) of chilling tolerance indicate that the Fv/Fm, \(\Phi\)PSII, POD activity, and Rfd represent the preferred indicators for identifying chilling tolerance in mustard. Physiological Differences between Most Chilling-Tolerant and Least Chilling-Tolerant Cultivars of Mustard  
The comprehensive analysis showed that No. 65 (SJTKJ) was the most chilling-tolerant cultivar and No. 58 (DX) was the least chilling-tolerant cultivar. As shown in Table 7, all the indices (except Pro and Fv\({}^{\prime}\)/Fm\({}^{\prime}\)) of SJTKJ were greater than those at 25 \({}^{\circ}\)C under the 5 \({}^{\circ}\)C treatment, whereas only the Chl content, SOD activity, and CAT activity of DX were greater than those under the 25 \({}^{\circ}\)C treatment. All of the other indices were less than those under the 25 \({}^{\circ}\)C treatment, and all the indices (except Pro content and CAT activity) of SJTKJ were greater than those of DX under the 5 \({}^{\circ}\)C treatment. As shown in Figure 3, under 5 \({}^{\circ}\)C treatment, the new leaf portion of SJTKJ showed orange colour, indicating that the low temperature had less effect on the PSII maximum light quantum efficiency (Fv/Fm) of the new leaf, suggesting that the new leaf was more photosynthetically capable. whereas the steady-state fluorescence decay rate (Rfd) was mostly red at the leaf margins. The middle portion of DX leaves under the 5 \({}^{\circ}\)C treatment was largely green and blue in colour, with larger decreases in Fv/Fm and Rfd. The corresponding values of all fluorescence