Abstract: We investigated the effects of low-temperature stresses on the photosynthesis of 14-month-old seedlings of Aegiceras corniculatum. These seedlings were subjected to different low-temperature stresses (day/night temperatures of 17 ℃/12 ℃, 12 ℃/7 ℃, 7 ℃/2 ℃, and 5 ℃/-2 ℃) for four days. We examined the impacts of low temperatures on gas exchange parameters, antioxidant enzyme activity, hydrogen peroxide, and endogenous phytohormone content in the leaves, as well as the melatonin synthesis pathway. Results showed that when the temperature exceeded 12 ℃ (day)/7 ℃ (night), the decline in the net photosynthetic rate was primarily attributed to stomatal limitation. However, when the temperature fell below 7 ℃ (day)/2 ℃ (night), the reduction in photosynthesis could be attributed to both stomatal and nonstomatal limitations. Severe low-temperature stress not only diminished net photosynthetic rate, stomatal conductance, activities of superoxide dismutase and peroxidase, and hormone contents in leaves but also impeded the formation of intermediate products in the melatonin synthesis pathway and the activities of related synthases. Concurrently, severe low-temperature stress elevated intercellular CO₂ concentration and the contents of malonaldehyde and hydrogen peroxide in leaves. The findings suggest that the inhibition of severe low-temperature stress on the photosynthesis of A. corniculatum seedlings is primarily linked to the blockade of endogenous melatonin biosynthesis pathway, oxidative homeostasis, and the imbalance of plant hormones.

Key words: low temperature stress, Aegiceras corniculatum, photosynthesis, antioxidant enzyme, endogenous phytohormone, melatonin