Plant Growth Regulators Enhance Saline–Alkali Tolerance by Upregulating the Levels of Antioxidants and Osmolytes in Soybean Seedlings

Soil salinization has become a global problem and seriously endangers crop growth and yield improvement. In the present study, soybean (Hefeng 50) seedlings were used as test materials to study the mitigation effect of foliar spraying of different plant growth regulators (PGRs) [50 mg L−1 indole-3-butyric acid potassium salt (IBAK), 50 mg L−1 chitosan oligosaccharide (COS), 2 mg L−1 abscisic acid (ABA), 30 mg L−1 5-aminolevulinic acid (ALA), and 1.2 mg L−1 brassinolide (BR)] on oxidative stress caused by the mixed Saline–Alkali concentration of 110 mmol L−1. The results showed that the application of PGRs promoted the growth of Saline–Alkali stressed plants, where the maximum increase in shoot FW was treated with COS, and in root FW, root length, root surface area, and root volume were obtained with the IBAK treatment. Treatments ABA, ALA, and BR had higher net photosynthetic rates, and the chlorophyll content was considerably increased under COS and BR treatments compared with Saline–Alkali treatment. Moreover, PGRs markedly enhanced the activities of antioxidant enzymes, the concentration of ascorbate (AsA), glutathione (GSH), proline, soluble protein, soluble sugar, sucrose, and starch, and the ratios of AsA/DHA and GSH/GSSG, but reduced the concentration of malondialdehyde (MDA), electrolyte leakage (EL), hydrogen peroxide (H2O2), and superoxide radical (O2·−) in soybean seedlings compared with Saline–Alkali treatment. The principal component analysis revealed that the ranking of PGRs enhancing Saline–Alkali tolerance of soybean seedlings was BR > IBAK > ABA > COS > ALA, and the most effective treatment was BR, which may be assigned to more vigorous antioxidant defense and osmotic adjustment.