Pumpkin Rootstocks Improve the Low-temperature Resistance of Bitter Gourd through Physiological Regulation

Bitter gourd ( Momordica charantia L.) is one of the Cucurbitaceae species that is sensitive to low temperatures. Grafting onto pumpkin rootstock is an effective technique to increase the chilling tolerance of the Cucurbitaceae species upon exposure to low-temperature stress. However, research on the mechanism by which pumpkin rootstock increases chilling tolerance of bitter gourd remains limited. In this study, bitter gourd ‘Baoyu No.2’ was used as the scion, and pumpkin ‘Dongfang Changsheng’ served as the rootstock; self-rooted seedlings are used as a control. The grafted and self-rooted bitter gourd seedlings were exposed to low-temperature stress (10 °C) for 10 days and allowed to recover for 1 day at 25 °C/20 °C (D/N). Changes in chilling damage index, osmotic regulatory substance contents, antioxidant enzyme activity, and chlorophyll fluorescence induction kinetics were determined. We found that the malondialdehyde (MDA) and proline (PRO) levels in the leaves of both self-rooted and grafted bitter gourd seedlings, as well as the activity of antioxidant enzymes, including superoxide dismutase (SOD) and peroxidase (POD), tended to increase overall with increasing duration of low-temperature stress. After 10 days of low-temperature stress, the PRO content of grafted seedlings increased by 312.77% compared with the self-rooted seedlings. The SOD activity increased by 46.19%, and the POD activity increased by 4.66%. Under low-temperature stress, the chlorophyll fluorescence induction kinetics (OJIP) curves of both self-rooted and grafted seedlings clearly changed. As the low-temperature stress duration increased, the maximum fluorescence intensity (P) decreased compared with 0 day, and the J-I-P curve tended to flatten. However, the decline and flattening for grafted seedlings were weaker than those for self-rooted seedlings. After 3 days of low-temperature stress, the F v /F m of the self-rooted seedlings reached its lowest point of 0.63, which was 24.10% lower and 18.18% lower than that noted at 0 day and for the grafted seedlings, respectively. The PI ABS of the self-rooted seedlings reached its lowest point of 0.61, which was 86.76% lower than that noted at 0 day and 68.88% lower than the grafted seedlings. Furthermore, after 3 days of low-temperature stress, the absorption per active reaction center ( ABS/RC ) of the self-rooted seedlings increased by 25.19% compared with 0 day and 26.54% compared with the grafted seedlings. Moreover, the dissipation energy per active reaction center ( DI o /RC ) of the self-rooted seedlings was 179.43% of that noted at 0 day and 108.45% greater than that of the grafted seedlings.