Effects of exogenous hormones on chlorophyll fluorescence parameters and the antioxidant enzyme system in soybean leaves under low-temperature stress

Low-temperature stress is a major abiotic constraint on soybean production. To evaluate how exogenous hormones affect chlorophyll fluorescence and the antioxidant defense system in soybean leaves under low-temperature stress, we used two cultivars with contrasting cold tolerance, Mengdou 16 and Heihe 18. Plants were exposed to 4℃ and treated separately with optimal concentrations of abscisic acid (ABA), salicylic acid (SA), melatonin (MT), brassinolide (BR), and jasmonic acid (JA). Their effects on morphology, photosynthetic performance, osmotic adjustment, and antioxidant enzyme activities were assessed after 2, 4, and 6 days. Membership function analysis was used to identify the most effective hormones for enhancing cold tolerance, and response surface methodology was applied to determine an optimal hormone combination. Low-temperature stress caused substantial injury, with Mengdou 16 showing greater cold tolerance and more stable physiological responses than Heihe 18. All hormone treatments alleviated growth inhibition, and the most effective concentrations were 13.93 mg L⁻¹ ABA, 200.62 mg L⁻¹ SA, and 30.30 mg L⁻¹ JA. Hormone application increased plant height, fresh weight, and leaf area, although responses differed between cultivars. Total chlorophyll content increased by up to 97.60% in Mengdou 16 and 108.28% in Heihe 18. In both cultivars, initial fluorescence (F₀) and non-photochemical quenching (qN) decreased; photochemical quenching (qP) and electron transport rate (ETR) increased in Mengdou 16, whereas qP in Heihe 18 decreased in some treatments. The maximum reductions in F₀ were 34.38% and 24.86% in Mengdou 16 and Heihe 18, respectively, indicating improved photosynthetic energy conversion. Malondialdehyde (MDA) content decreased significantly under all treatments, with maximum reductions of 66.50% (Mengdou 16) and 62.57% (Heihe 18), while proline content increased by up to 24.47% and 19.97%, respectively. Activities of SOD, POD, and CAT were also enhanced, with SOD and CAT peaking at day 4 and POD showing the greatest increase at day 6. Overall, exogenous hormones mitigated low-temperature damage in soybean by improving photochemical performance and strengthening antioxidant and osmotic adjustment systems, providing a theoretical basis and practical guidance for soybean production under low-temperature conditions.