Effect of exogenous growth regulators on physiological characteristics of cold resistance of Rhizophora stylosa seedling with different ages under low temperature stress.

Low-temperature stress is a critical abiotic stress limiting the regional distribution, ecological community patterns, and cultivation industry of Rhizophora stylosa. To assess the role of plant growth regulators in enhancing the cold resistance and recovery of R. stylosa seedlings under low-temperature stress and rewarming conditions, we evaluated the physiological responses of 1-year and 2-year old R. stylosa seedlings to low-temperature stress (day 10 ℃/night 5 ℃) and subsequent rewarming (day 30 ℃/night 20 ℃) in a climate chamber. Seedlings were treated with four types of plant growth regulators: 6-benzylpurine (6-BA, 50, 100, and 150 mg·L^-1), naphthaleneacetic acid (NAA, 50, 100, and 150 mg·L^-1), gibberellin (GA₃, 100, 200, and 400 mg·L^-1), and auxin (IAA, 100, 200, and 400 mg·L^-1), with water as the control (CK). We analyzed the changes in antioxidant enzyme activities, osmotic adjustment substances, and malondialdehyde (MDA) content in the leaves. The results showed that low-temperature stress disrupted physiological homeostasis, and increased MDA content by 40.0% and 37.8% in 1- and 2-year old seedlings, respectively. Application of growth regulators significantly enhanced the activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase) and the soluble protein and soluble sugar content while reduced MDA content. The maximum reduction in MDA content was 35.1% and 42.5% in 1-year-old seedlings and 37.5% and 34.8% in 2-year-old seedlings during stress and rewarming, respectively, indicating improved resistance and recovery. The 2-year-old seedlings had stronger adaptability to low-temperature compared to the 1-year-old seedlings. GA₃(100 and 200 mg·L^-1), IAA (150 mg·L^-1), 6-BA (400 mg·L^-1), and NAA (50 mg·L^-1) were most effective in mitigating low-temperature stress and promoting recovery in R. stylosa seedlings. These findings offered insights to enhance the cold resistance and resilience of R. stylosa under climate-induced abiotic stress.