Effect of wild watermelon rootstock grafting on high temperature tolerance of watermelon seedlings

【Objective】In order to further explore the effect of rootstock grafting on the high temperature tolerance of watermelon seedlings, this study aims to compare and analyze the growth performance of self-rooted watermelon seedlings and watermelon seedlings grafted on wild watermelon rootstock Ye zhuang No. 1 under high temperature stress, reveal the mechanism of grafting technology in improving the high temperature tolerance of watermelon seedlings, and provide scientific basis and technical support for stress-resistant cultivation of watermelon.【Methods】In this study, the self-rooted seedlings of Zaojia watermelon variety was used as a control, and the watermelon rootstock variety Yezhuang No. 1 as the rootstock for grafting. The grafted seedlings and self-rooted seedlings were subjected to high temperature treatment at 42 ℃ for 5 days. By measuring and recording the length and thickness of the hypocotyl under scion, the relative chlorophyll content and leaf color parameters of cotyledons and true leaves, the photosynthetic fluorescence parameters of true leaves, as well as the content of stress related substances such as malondialdehyde (MDA) and antioxidant enzyme activity such as peroxidase (POD), the effects of grafting on watermelon seedling growth under high temperature stress were comprehensively evaluated.【Results】Agronomic traits performance: Both high temperature and grafting can significantly increase the thickness of the hypocotyl below the scions, but had little effect on the length of the hypocotyl. Grafted seedlings showed higher relative chlorophyll content (SPAD) between cotyledons and true leaves under high temperature treatment, and the leaf color was more vibrant green,indicating that grafting can alleviate chlorophyll degradation and leaf chlorosis caused by high temperature. In addition, the changes in brightness (L* value), red green value (a* value), and yellow blue value (b* value) of the cotyledons and true leaves on grafted seedlings also showed a positive effect of grafting on leaf color. Photosynthetic fluorescence parameters: Under high temperature stress, the actual quantum efficiency (Y (Ⅱ)), photosynthetic electron transfer rate (ETR), and photochemical quenching coefficient (qP) of photosystem Ⅱ (PSⅡ) in grafted seedlings were significantly higher than those in self- rooted seedlings, while the non photochemical quenching coefficient (NPQ) was lower. These results indicated that grafted seedlings can maintain higher light energy capture and conversion capacity under high temperature conditions, maintain relatively stable electron transport chain activity, and reduce the degree of damage to the photosystem. At the same time, although the PSⅡ maximum photochemical efficiency (Fv/Fm) and potential photochemical efficiency (Fv/Fo) of grafted seedlings decreased, they were still higher or close to the level of self-rooted seedlings, further verifying the heat tolerance advantage of grafted seedlings. Physiological indicators related to adversity: In terms of physiological indicators related to adversity, grafted seedlings showed higher chlorophyll a, chlorophyll b, and total chlorophyll contents under high temperature stress, as well as lower MDA content and POD activity. These results indicated that grafting can enhance the antioxidant capacity of watermelon seedlings, reduce the degree of membrane lipid peroxidation, and maintain the stability and integrity of cell membranes. In addition, although there was no significant difference in soluble sugar content between grafted seedlings and self-rooted seedlings under high temperature treatment, grafted seedlings were able to maintain a relatively stable physiological state under high temperature stress.【Conclusion】Grafting enhanced high- temperature adaptability by coordinating morphology and pigment metabolism: Grafted seedlings exhibited more significant morphological adaptive adjustments under high temperatures, such as a significant increase in hypocotyl thickness, which may be related to auxin mediated cell wall remodeling. At the same time, grafted seedlings also exhibited advantages in pigment metabolism, such as higher chlorophyll content and more stable leaf color, which helped maintain light capture capacity and photosynthetic electron transfer efficiency. Stability of Photosynthetic Mechanisms and Heat Resistant Core Mechanisms Revealed by Fluorescence Parameters: Grafted seedlings can maintain more efficient light energy conversion efficiency and relatively stable electron transfer chain activity at high temperatures, thanks to the stability of their photosynthetic mechanism. Specifically, higher qP values and lower NPQ values indicated that grafted seedlings can more effectively utilize light energy for photochemical reactions and reduce energy loss caused by non photochemical quenching. In addition, the PSⅡ damage of grafted seedlings was relatively mild, further demonstrating the heat resistance of their photosynthetic mechanisms. Membrane system protection and oxidative stress balance: Grafting significantly alleviated the phenomenon of membrane lipid peroxidation induced by high temperature, mainly due to the higher antioxidant capacity and stable chlorophyll content of grafted seedlings. Chlorophyll, as a key pigment in photosynthesis, its stability is crucial for maintaining the integrity of membrane systems. At the same time, the lower POD activity of grafted seedlings also indicated that their basic antioxidant capacity was sufficient to eliminate reactive oxygen species (ROS), thereby reducing oxidative stress damage to the membrane system. Physiological integration effect of grafting heat resistance: Compared to cross rootstock grafting, this rootstock grafting may have unique advantages, such as optimized material transport, signal transmission fidelity, and integration of circadian rhythms. These advantages helped graft seedlings maintain a more stable physiological state and higher growth vitality under high temperature stress. This study indicates that rootstock grafting significantly improves the high-tem-perature tolerance of watermelon seedlings by enhancing hypocotyl stability, maintaining chloroplast pigment homeostasis, optimizing photochemical efficiency and heat dissipation balance, inhibiting membrane lipid peroxidation, and coordinating antioxidant enzyme activity through multiple mechanisms. These findings provide important theoretical basis and technical support for the stress- resistant cultivation of watermelons, and have broad application prospects and promotion value.