Effects of Exogenous Melatonin Treatment on Physiological and Biochemical Responses of Hippeastrum ‘Rebecca’ Seedlings Under Short-Term Cold Stress

Short-term low-temperature stress can disrupt multiple physiological and biochemical processes in plants, thereby causing physiological damage. In this study, we aimed to investigate the physiological and biochemical response characteristics of Hippeastrum ‘Rebecca’ seedlings to short-term low-temperature stress under long-term exogenous melatonin (MT) accumulation treatment. The results showed that continuous spraying of 200 μmol/L melatonin effectively promoted the overall growth of Hippeastrum ‘Rebecca’ seedlings, with increased growth indices in the melatonin-treated group. Short-term low-temperature stress significantly impaired the photosynthetic apparatus function and altered chlorophyll fluorescence characteristics in Hippeastrum ‘Rebecca’ seedlings. Compared with the 20 °C normal-temperature condition, under 4 °C low temperature without melatonin treatment, the actual photochemical efficiency of PSII, the maximum photochemical efficiency of PSII under light-adapted conditions, and the photochemical quenching coefficient decreased significantly, whereas the non-photochemical quenching coefficient did not change significantly, indicating that low temperature primarily induced photosynthetic photoinhibition by suppressing photochemical reaction efficiency. Long-term melatonin accumulation treatment alleviated, to some extent, the inhibitory damage caused by low temperature to the photosynthetic apparatus; under 4 °C low temperature, values in the MT-treated group were higher than those in the untreated group. Short-term low-temperature stress triggered a burst of reactive oxygen species, leading to substantial increases in hydrogen peroxide (H2O2) and hydroxyl radical (·OH) contents. Melatonin accumulation treatment significantly suppressed the short-term induced H2O2 accumulation and effectively activated the plant antioxidant system, significantly elevating the activities of superoxide dismutase, glutathione reductase, and catalase; however, melatonin showed no significant scavenging effect on ·OH. Meanwhile, melatonin treatment significantly regulated endogenous hormone homeostasis, upregulating indole-3-acetic acid (IAA) and trans-zeatin riboside (ZTR) contents while significantly decreasing abscisic acid (ABA) content. Long-term melatonin accumulation also altered the accumulation patterns of organic acids under short-term low temperature, significantly increasing the contents of malic acid, succinic acid, and citric acid. In conclusion, long-term with 200 μmol/L melatonin pretreatment (60 days) followed by short-term (24 h) cold exposure alleviated low-temperature inhibition in Hippeastrum ‘Rebecca’ seedlings. This protective effect was attributed to the activation of the antioxidant defense system, the regulation of endogenous hormone homeostasis, and the modulation of organic acid accumulation, collectively alleviating the physiological inhibition induced by low-temperature stress.