High temperature is considered a critical abiotic stressor that is increasing continuously, which is severely affecting plant growth and development. The use of heat-resistant rootstock grafting is a viable technique that is practiced globally to improve plant resistance towards abiotic stresses. In this experiment, we explored the efficacy of bitter melon rootstock and how it regulates photosynthesis and the antioxidant defense system to alleviate heat stress (42 °C/32 °C) in cucumber. Our results revealed that bitter-melon-grafted seedlings significantly relieved heat-induced growth inhibition and photoinhibition, maintained better photosynthesis activity, and accumulated a greater biomass than self-grafted seedlings. We measured the endogenous polyamine and hydrogen peroxide (H₂O₂) contents to determine the inherent mechanism responsible for these effects, and the results showed that heat stress induced a transient increase in polyamines and H₂O₂ in the inner courtyard of grafted seedlings. This increment was greater and more robust in bitter-melon-grafted seedlings. In addition, the use of polyamine synthesis inhibitors MGBG (methylglyoxal bis-guanylhydrazone) and D-Arg (D-arginine), further confirmed that the production of H₂O₂ under heat stress is mediated by the accumulation of endogenous polyamines. Moreover, compared with other treatments, the bitter-melon-grafted seedlings maintained high levels of antioxidant enzyme activity under high temperature conditions. However, these activities were significantly inhibited by polyamine synthesis inhibitors and H₂O₂ scavengers (dimethylthiourea, DMTU), indicating that bitter melon rootstock not only maintained better photosynthetic activity under conditions of high temperature stress but also mediated the production of H₂O₂ through the regulation of the high level of endogenous polyamines, thereby boosting the antioxidant defense system and comprehensively improving the heat tolerance of cucumber seedlings. Taken together, these results indicate that grafting with a resistant cultivar is a promising alternative tool for reducing stress-induced damage.

中文翻译：

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苦瓜砧木通过调节光合作用和抗氧化防御途径，提高了黄瓜的耐热性。

高温被认为是不断增长的重要非生物胁迫源，严重影响植物的生长发育。耐热砧木嫁接的使用是一项可行的技术，已在全球范围内实践以提高植物对非生物胁迫的抗性。在本实验中，我们研究了苦瓜砧木的功效以及它如何调节光合作用和抗氧化防御系统，以减轻黄瓜的热应激（42°C / 32°C）。我们的研究结果表明，苦瓜嫁接苗比自嫁苗显着缓解了热诱导的生长抑制和光抑制作用，保持了更好的光合作用活性，并积累了更多的生物量。我们测量了内源性多胺和过氧化氢（H ₂ O ₂）含量来确定造成这些效应的内在机制，结果表明，热胁迫在嫁接幼苗的内部庭院中引起了多胺和H ₂ O ₂的瞬时增加。在苦瓜嫁接的幼苗中，这种增加更大并且更坚固。另外，使用多胺合成抑制剂MGBG（甲基乙二醛双胍hydr）和D-Arg（D-精氨酸），进一步证实了H ₂ O _2的产生在热胁迫下，内源性多胺的积累介导了这种作用。此外，与其他处理方法相比，苦瓜嫁接的幼苗在高温条件下保持了高水平的抗氧化酶活性。然而，这些活性被多胺合成抑制剂和H ₂ O ₂清除剂（二甲基硫脲，DMTU）显着抑制，表明苦瓜砧木不仅在高温胁迫下保持了较好的光合活性，而且还介导了H ₂ O _{2的产生。}通过调节内源多胺的含量高，增强抗氧化防御系统，全面提高黄瓜幼苗的耐热性。综上所述，这些结果表明用抗性品种嫁接是减少应力引起的损害的有前途的替代工具。