Melatonin is an indolamine bioactive molecule that regulates a wide range of physiological processes during plant growth and enhances abiotic stress tolerance. Here we examined the putative role of exogenous melatonin application (foliar or root zone) in improving drought stress tolerance in soybean seedlings. Pre-treatment of soybean seedlings with melatonin (50 and 100 µM) was found to significantly mitigate the negative effects of drought stress on plant growth-related parameters and chlorophyll content. The beneficial impacts against drought were more pronounced by melatonin application in the rhizosphere than in foliar treatments. The melatonin-induced enhanced tolerance could be attributed to improved photosynthetic activity, reduction of abscisic acid and drought-induced oxidative damage by lowering the accumulation of reactive oxygen species and malondialdehyde. Interestingly, the contents of jasmonic acid and salicylic acid were significantly higher following melatonin treatment in the root zone than in foliar treatment compared with the control. The activity of major antioxidant enzymes such as superoxide dismutase, catalase, polyphenol oxidase, peroxidase and ascorbate peroxidase was stimulated by melatonin application. In addition, melatonin counteracted the drought-induced increase in proline and sugar content. These findings revealed that modifying the endogenous plant hormone content and antioxidant enzymes by melatonin application improved drought tolerance in soybean seedlings. Our findings provide evidence for the stronger physiological role of melatonin in the root zone than in leaves, which may be useful in the large-scale field level application during drought.

中文翻译：

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外源褪黑素通过促进植物生长和大豆植物抗氧化防御系统诱导干旱胁迫耐受性


褪黑素是一种吲哚胺生物活性分子，可调节植物生长过程中的多种生理过程并增强非生物胁迫耐受性。在这里，我们研究了外源褪黑激素施用（叶面或根区）在提高大豆幼苗的干旱胁迫耐受性中的假定作用。研究发现，用褪黑激素（50 和 100 µM）对大豆幼苗进行预处理可显着减轻干旱胁迫对植物生长相关参数和叶绿素含量的负面影响。与叶面处理相比，根际施用褪黑激素对干旱的有益影响更为明显。褪黑素诱导的耐受性增强可归因于光合活性的改善、脱落酸的减少以及干旱通过降低活性氧和丙二醛的积累而引起的氧化损伤。有趣的是，与对照相比，褪黑素处理后根区茉莉酸和水杨酸的含量显着高于叶处理。褪黑激素的应用刺激了主要抗氧化酶的活性，例如超氧化物歧化酶、过氧化氢酶、多酚氧化酶、过氧化物酶和抗坏血酸过氧化物酶。此外，褪黑激素还可以抵消干旱引起的脯氨酸和糖含量的增加。这些发现表明，通过施用褪黑激素改变内源植物激素含量和抗氧化酶可以提高大豆幼苗的耐旱性。我们的研究结果提供了证据，证明褪黑激素在根区的生理作用比在叶子中更强，这可能有助于干旱期间的大规模田间应用。