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Saline‐alkali stress (SS) is a common abiotic stress affecting crop cultivation worldwide, seriously inhibiting plant growth and biomass accumulation. Melatonin has been proven to relieve the inhibition of multiple abiotic stresses on plant growth. Therefore, soybean cultivars Heihe 49 (HH49, SS‐tolerant) and Henong 95 (HN95, SS‐sensitive) were pot‐cultured in SS soil and then treated with 300 μM melatonin at the V1 stage, when the first trifoliate leaves were fully unfolded, to investigate if melatonin has an effect on SS. SS increased reactive oxygen species (ROS) accumulation in soybean leaves and thereby induced DNA oxidative damage. In addition, SS retarded cell growth and decreased the mesophyll cell size, chloroplast number, photosynthetic pigment content, which further reduced the light energy capture and electron transport rate in soybean leaves, and affected carbohydrate accumulation and metabolism. However, melatonin treatment reduced SS‐induced ROS accumulation in the soybean leaves by increasing antioxidant content and oxidase activity. Effective removal of ROS reduced SS‐induced DNA oxidative damage in the soybean leaf genome, which was represented by decreased random‐amplified polymorphic DNA polymorphism, 8‐hydroxy‐20‐deoxyguanine content, and relative density of apurinic/apyrimidinic‐sites. Melatonin treatment also increased the volume of mesophyll cells, the numbers of chloroplast and starch grains, the contents of chlorophyll a and b and carotenoids in soybean seedling leaves treated with SS, thereby increasing the efficiency of effective light capture and electron transfer and improving photosynthesis. Subsequently, carbohydrate accumulation and metabolism in soybean leaves under SS were improved by melatonin treatment, which contributes to providing basic substances and energy for cell growth and metabolism, ultimately improving soybean SS tolerance. [ABSTRACT FROM AUTHOR]

Salt stress is a common abiotic stress that negatively affects crop growth and yield. However, there have been significant differences found on the effect degree and management mechanism in plants under neutral salt stress and alkaline stress. In this study, two soybean cultivars, Heihe 49 (HH49, saline-alkali stress tolerant) and Henong 95 (HN95, saline-alkali stress sensitive), were hydroponically cultured and treated with salt solutions of 25, 50, and 75 mM Na+ in the form of NaCl, Na2SO4, NaHCO3, and Na2CO3. Plants treated with alkaline stress (NaHCO3 and Na2CO3) showed a greater decrease in root growth and root activity of both soybean cultivar seedlings than that under neutral salt stresses (NaCl and Na2SO4) with 25–75 mM Na+ concentration. Alkaline stress (25–50 mM Na+ content) activated a higher ability of antioxidant defense (by enhancing the activists of superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX)) and increased the content of soluble sugars to a higher level than that under neutral salt stresses. However, 75 mM Na+ content salt treatments reduced antioxidant enzyme activities and osmotic regulating substance content. Furthermore, alkaline salt and neutral salt stress was able to induce DNA damage and cell cycle arrest in HH49 and HN95 seedling roots. Treatment with Na2CO3 induced the least random amplification polymorphic DNA (RAPD) polymorphism in soybean seedling roots among all salt treatments, which could have been related to the early cell cycle arrest. [ABSTRACT FROM AUTHOR]