Melatonin is a signalling molecule in plants which act as a growth regulator and stress buffering agent but little is known about its defensive role in heat stress tolerance in rice. In this study we analysed the acclimation response of two contrasting heat responsive rice genotypes NERICA-L-44 (NL-44, heat stress tolerant) and Pusa Basmati 1121 (PB-1121, heat stress susceptible) to exogenous melatonin treatment under heat stress. Heat stress induces melatonin biosynthesis in both the genotypes and maximum increase was observed in NL-44. Application of exogenous melatonin further increases melatonin biosynthesis and endogenous content significantly in PB-1121. To understand melatonin’s role in alleviating effects of heat stress, some of the heat stress associated traits such as membrane stability index, hydrogen peroxide content, malondialdehyde content, total antioxidant capacity and enzymatic antioxidants like superoxide dismutase, catalase, ascorbate peroxidase and peroxidase were analysed. We observed that melatonin significantly enhanced membrane stability by reducing hydrogen peroxide mediated membrane damage. Higher activity of antioxidative enzymes and increased total antioxidant capacity helped to maintain reactive oxygen species homeostasis by scavenging mechanism. Exogenous melatonin increased pollen viability, spikelet fertility and grain yield under heat stress treatment. The findings of this study indicates that heat stress induced increase in melatonin biosynthesis may be involved in heat stress tolerance in rice. Exogenous melatonin treatment can further improve heat stress tolerance in susceptible rice genotype (PB-1121) by activating antioxidative mechanism.

中文翻译：

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褪黑激素通过增强水稻 (Oryza sativa L.) 的抗氧化防御来诱导对热应激的适应和花粉活力

褪黑激素是植物中的一种信号分子，充当生长调节剂和应激缓冲剂，但对其在水稻耐热应激中的防御作用知之甚少。在这项研究中，我们分析了两种不同的热反应性水稻基因型 NERICA-L-44（NL-44，耐热应激）和 Pusa Basmati 1121（PB-1121，热应激敏感）在热应激下对外源褪黑激素处理的适应反应。热应激会诱导两种基因型的褪黑激素生物合成，并且在 NL-44 中观察到最大的增加。外源褪黑激素的应用进一步显着增加了PB-1121中褪黑激素的生物合成和内源含量。为了了解褪黑激素在缓解热应激中的作用，分析了一些与热应激相关的特征，如膜稳定性指数、过氧化氢含量、丙二醛含量、总抗氧化能力和酶抗氧化剂，如超氧化物歧化酶、过氧化氢酶、抗坏血酸过氧化物酶和过氧化物酶。我们观察到褪黑激素通过减少过氧化氢介导的膜损伤显着增强膜稳定性。抗氧化酶的较高活性和总抗氧化能力的增加有助于通过清除机制维持活性氧稳态。外源褪黑激素在热应激处理下增加了花粉活力、小穗育性和谷物产量。这项研究的结果表明，热应激诱导的褪黑激素生物合成增加可能与水稻的热应激耐受性有关。外源褪黑素处理可以通过激活抗氧化机制进一步提高敏感水稻基因型（PB-1121）的热应激耐受性。