Objective To study the physiological response of different bitter gourd materials at seedling stages to high temperature stress, preliminarily evaluate the heat resistance of this materials, and provide theoretical basis for the subsequent breeding of heat-resistant varieties of bitter gourd.

Method This study used 10 high-generation inbred lines (KG1-KG10) and 6 hybrid varieties (KG11-KG16) of bitter gourd as test materials. Different bitter gourd materials were subjected to high temperature cycling at 40 ℃ /12h during the day and 25 ℃ /12h at night for 3 days. The relative electrical conductivity (REC), malondialdehyde (MDA) content, proline (Pro) content, and antioxidant enzyme activity of seedling bitter gourd leaves were measured. And the heat resistance of these 16 bitter gourd materials was preliminarily evaluated using the membership function method.

Result The relative electrical conductivity (REC) of the leaves of 16 bitter gourd materials increased during the seedling stage under high temperature compared with normal temperature conditions. Among them, the bitter gourd materials with significant differences in increase were KG1, KG2, KG9, KG10, KG11, KG13, KG14, KG15, and KG16. Except for KG1 and KG2, the content of malondialdehyde (MDA) in the other 14 bitter gourd materials decreased. Among them, the bitter gourd materials with significant differences in decrease amplitude were KG3, KG4, KG5, KG6, KG8, KG10, KG11, KG12, KG13, KG14, KG15, and KG16. The content of proline (Pro) also increased and decreased, with 6 bitter gourd materials showing an increase in Pro content in their leaves. The bitter gourd materials with significant differences in increase were KG1, KG2, KG3, KG4, and KG10; However, 10 bitter gourd materials showed a decrease in Pro content in their leaves, KG6, KG7, KG8, KG9, KG11, KG14, KG15, and KG16 showed significant decrease. Among the antioxidant enzyme activities, the peroxidase (POD) activity of leaves of 16 bitter gourd materials increased significantly, while the activities of superoxide dismutase (SOD) and catalase (CAT) increased and decreased. The SOD activity of bitter gourd materials KG5 and KG14 decreased, while the SOD activity of the other 14 bitter gourd materials increased to varying degrees. The bitter gourd materials with significant differences in increase were KG1, KG2, KG3, KG4, KG6, KG7, KG8, KG9, KG10, KG11, KG12, KG15, and KG16; The CAT activity of bitter gourd materials KG8, KG10, KG12, and KG16 significantly increased, while the CAT activity of the other 12 bitter gourd materials decreased to varying degrees. The bitter gourd materials with significant differences in decline were KG1, KG2, KG3, KG4, KG5, KG7, KG9, KG11, KG13, and KG14. Through comprehensive evaluation using the membership function method, it was preliminarily determined that the heat resistance of these 16 bitter gourd materials is in the following order: KG1 > KG2 > KG16 > KG11 > KG10 > KG3 > KG9 > KG12 > KG15 > KG4 > KG13 > KG8 > KG6 > KG5 > KG7 > KG14.

Conclusion Different bitter gourd materials have undergone different changes in six physiological indicators under high temperature stress, further indicating that these six physiological indexes have varying degrees of correlation with the heat resistance of bitter gourd. Preliminary evaluation showed that the KG1, KG2, KG16, KG11, and KG10 bitter gourd materials with better heat resistance have potential application value and can be utilized in subsequent bitter gourd heat resistance breeding.