Exogenous Melatonin Increased Soybean Yield Under Saline-Alkali Stress by Enhancing Plant Carbohydrate Metabolism and Transportation.

Saline-alkali (SA) stress is one of the most widespread abiotic stresses affecting plant growth and productivity worldwide. Melatonin is an auxin-like analogue that can improve plant tolerance to multiple abiotic stresses. Herein, two soybean cultivars, Heihe 49 and Henong 95 were pot cultured and treated with 80 mM SA stress solution (mixed with NaCl:Na₂CO₃:NaHCO₃:Na₂SO₄ = 1:1:9:9) at the V1 stage. The 0-300 μM melatonin was foliar sprayed at the V3 stage to explore the effects of exogenous melatonin treatments on soybean plant growth at the grain-filling stage under SA stress. Compared with the control, SA stress significantly reduced the growth, biomass accumulation and yield of both soybean cultivars. As expected, melatonin treatments significantly increased the plant height, stem diameter, shoot dry weight and pod number of soybean plants under SA stress. In addition, MT treatments significantly increased the chlorophyll content, gas exchange and chlorophyll fluorescence parameters of soybean leaves under SA stress, which were conducive to the accumulation of soluble sugars, sucrose, fructose and starch in the leaves, pod skins and grains. MT treatments promote the carbohydrate metabolic cycle by increasing the activity of sucrose and starch-metabolizing enzymes and upregulating the expression levels of correlative genes in the leaves, pod skins, and grains of both soybean cultivars. MT treatments upregulated the expression levels of GmSWEETs and GmSUC2 in soybean tissues to increase the transportation of sucrose into grains, promote grain development, and finally increase soybean yield under SA stress.