Identification and Evaluation of Thermotolerance in Broccoli Seedlings Based on a Multi-Trait Phenotyping System

To establish a systematic approach for evaluating heat tolerance at the seedling stage in broccoli, we investigated 14 representative cultivars cultivated in China. Physiological indicators such as electrical conductivity, malondialdehyde, proline, and chlorophyll content were measured before and after heat stress, alongside phenotypic scoring of heat injury, to characterize the differential thermotolerance among genotypes. The results indicated that significant differences (p < 0.05) in heat tolerance at the seedling stage were observed among different broccoli cultivars after heat stress treatment at 40 °C. Among them, Yanxiu exhibited the strongest heat tolerance, followed by Meiqing, Naihanyouxiu, Meiao 7172, Feicui 5, Guowang 11, Zheqing 80, Zhongqing 15, Zhongqing 318, Zhongqing 319, and Qianghan. Lvxiong 90, Zhongqing 11, and Zhongqing 16 were the least heat tolerant. Pearson’s correlation analysis demonstrated that the seedling heat tolerance of different broccoli cultivars was significantly negatively correlated with electrical conductivity, with a correlation coefficient of 0.542 (p < 0.05). In this study, a rapid and robust method for determining the heat resistance response of broccoli was established, providing a scientific basis and technical support for the identification of the heat resistance of new broccoli cultivars and the selection and breeding of heat-tolerant cultivars in China.