Effects of Drought Stress on Physiological Characteristics and Endogenous Hormone Contents in Leaves of Soybean Seedlings

Abstract

Background: Two soybean cultivars with contrasting drought tolerance-Heinong 44 (HN-44, tolerant) and Heinong 65 (HN-65, sensitive)-were used in a pot-culture experiment to investigate the physiological mechanisms underlying their differential responses to water deficit. Methods: Water deficit was imposed by supplementing the nutrient solution with PEG-6000 at four concentrations (0, 7.5%, 15% and 22.5%). Leaf relative water content (RWC), malondialdehyde (MDA) content, antioxidant enzyme activities and osmotic adjustment substances were determined at days 3, 6, 9 and 12 after stress initiation. Endogenous hormone contents were additionally quantified at day 9. Result: Progressive increases in drought severity and duration caused a continuous decline in leaf RWC and a marked elevation in MDA content in both cultivars; however, HN-44 maintained superior water retention and lower membrane lipid peroxidation relative to HN-65. HN-44 also exhibited a stronger and more durable antioxidant enzyme system (SOD, POD, CAT and APX) and greater osmotic adjustment capacity, with significantly higher accumulations of proline (Pro) and soluble sugars (Ss) than HN-65. Under prolonged severe stress, the antioxidant defense system of HN-44 showed less impairment than that of HN-65. Drought treatment elevated the foliar contents of salicylic acid (SA), IAA-glutamate and gibberellin A3 while suppressing 1-aminocyclopropane-1-carboxylic acid (ACC) accumulation in both cultivars. Among all hormonal changes, SA divergence between the two cultivars was most pronounced; the SA increment in HN-44 reached 165.90%, substantially exceeding that observed in HN-65.