Melatonin is a crucial biological hormone associated with many physiological and biochemical processes in plants and also enhances resistance against various abiotic stresses. However, the mechanisms underlying the melatonin-assisted mitigation of salt stress in tomato (Solanum lycopersicum L.) plant are still poorly understood. A hydroponic experiment was conducted to investigate the protective role of melatonin in two tomato cultivars (Roma and FM9) under a highly saline growth medium (160 mM NaCl). The one level of melatonin (1.0 µmol L−1) was applied exogenously, sole, or in combination with the salinity stress. NaCl-induced phytotoxicity significantly (P < 0.05) reduced shoot and root dry matter accumulation, chlorophyll contents, relative water contents (RWC), membrane stability index (MSI), and antioxidant enzymatic activities in both cultivars as compared to the control treatment. Moreover, salt treatment alone increased soluble sugar contents (sucrose and fructose), sodium (Na+) uptake, as well as oxidative damage in the leaves of tomato seedlings. However, exogenous supply of melatonin alleviated salt toxicity in tomato seedlings which were more obvious in Roma cultivar as compared to FM 9 cultivar, as demonstrated by a higher increment in the values of growth indicators, RWC, MSI, gaseous exchange attributes, activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). In addition, melatonin also alleviated salt-induced oxidative stress by suppressing malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents as well as significantly reduced Na+ uptake at the root surface of tomato plants. It can be concluded that melatonin-induced salt tolerance in tomato is due to enhancement of plant water relations, and improved photosynthetic and antioxidant capacity along with ion homeostasis.

中文翻译：

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通过改善两种番茄 (Solanum lycopersicum L.) 栽培品种幼苗的渗透和氧化应激对褪黑激素的耐盐性

褪黑激素是一种重要的生物激素，与植物中的许多生理和生化过程相关，还可以增强对各种非生物胁迫的抵抗力。然而，褪黑激素辅助缓解番茄 (Solanum lycopersicum L.) 植物盐胁迫的机制仍知之甚少。进行了水培实验以研究褪黑激素在高盐生长培养基（160 mM NaCl）下对两个番茄品种（罗马和 FM9）的保护作用。一种水平的褪黑激素 (1.0 µmol L-1) 外源性地、单独地或与盐度应力结合使用。NaCl 诱导的植物毒性显着 (P < 0.05) 降低了地上部和根部干物质积累、叶绿素含量、相对水分含量 (RWC)、膜稳定性指数 (MSI)、与对照处理相比，两个品种的抗氧化酶活性均有所提高。此外，单独的盐处理会增加番茄幼苗叶片中可溶性糖含量（蔗糖和果糖）、钠 (Na+) 的吸收以及氧化损伤。然而，外源供应褪黑激素减轻了番茄幼苗的盐毒性，与 FM 9 品种相比，这在罗马品种中更为明显，这可以通过生长指标、RWC、MSI、气体交换属性、超氧化物活性值的更高增量来证明。歧化酶 (SOD)、过氧化物酶 (POD)、过氧化氢酶 (CAT) 和抗坏血酸过氧化物酶 (APX)。此外，褪黑激素还通过抑制丙二醛 (MDA) 和过氧化氢 (H2O2) 的含量以及显着降低番茄植株根表面的 Na+ 吸收来减轻盐诱导的氧化应激。可以得出结论，褪黑激素引起的番茄耐盐性是由于植物水分关系的增强，以及光合和抗氧化能力的提高以及离子稳态。