Nano-Iron Alleviates Carbohydrate Deficiency and Improves Yield of Soybean Under Saline-Alkali Stress by Regulating Photosynthetic Carbon Assimilation.

In salinized soils, high salt and pH inhibit iron availability via chelation and ion competition, inducing iron deficiency stress in plants. Compared with traditional iron fertilizers, nano-iron has characteristics like easy absorption and targeted slow release, showing great potential in improving plants' saline-alkali tolerance. Herein, a spherical-like nano-iron (average particle size of 20-50 nm, zeta potential of -25 mV, and with oxide layer) was synthesized. Soybean cultivars Hefeng 50 (HF50) and Henong 95 (HN95) were grown in pots filled with soil treated with mixed saline-alkali solution (NaCl: Na2SO4: Na2CO3: NaHCO3 = 1: 9: 1: 9, pH = 8.9) to investigate the effects of foliar application of 0-300 mg l-1 nano-iron on soybean growth. Foliar nano-iron significantly alleviated saline-alkali stress induced growth inhibition, with 200 mg l-1 as the optimal concentration. Nano-iron enhanced antioxidant enzymes activities in soybean leaves and roots under saline-alkali stress, reduced cellular H2O2 and O2 - contents, mitigated membrane lipid peroxidation, and improved osmotic adjustment capacity. It up-regulated the expression levels of genes related to chlorophyll biosynthesis, light absorption, electron transport, and carbohydrate metabolism, increased chlorophyll content, promoted the efficiency of light absorption and utilization in soybean leaves, accelerated carbohydrate biosynthesis, and provided a material and substrate basis for plant stress-resistant physiological regulation. Ultimately, the 200 mg l-1 nano-iron treatment significantly increased the number of seeds per plant, seed weight per plant, and 100-seed weight of both soybean cultivars. This study provides the basis for developing nano-iron-based soybeans saline-alkali tolerant cultivation technologies.