Abstract: We assessed the regulatory role of melatonin (MT) on photosynthetic efficiency, carbon assimilation, and hormone metabolism of Kandelia obovata seedlings under different low periods. To investigate the protective role of MT in photosynthesis and endogenous hormone balance, we sprayed 0.2 mmol·L^-1 MT on K. obovata seedlings subjected to low-temperature stress (8 ℃/-3 ℃) for 2, 4, and 6 days. The results showed that, with increasing stress duration, the regulatory role of MT on photosynthesis of K. obovata seedlings first increased and then decreased, reaching a maximum under low-temperature stress for 4 days. After 4 days of low-temperature stress, spraying MT increased net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s), actual photochemical efficiency of PSⅡ (F_v/F_m), maximum photochemical quantum yield of PSⅡ (Ф_PSⅡ), and photochemical quenching coefficient (q_p) by 13.67, 2.40, 1.48, 2.03, 1.42 and 2.00 times, respectively. Spraying MT decreased the intercellular CO₂ concentration (C_i) and non-photochemical quenching coefficient (NPQ) of seedlings by 76.29% and 50.36%, respectively. Furthermore, spraying MT increased leaf photorespiration rate (R_p), CO₂ saturation point (CSP), carboxylation efficiency (CE), and maximum RuBP regeneration rate (J_max) in seedlings by 1.28, 1.09, 4.24, and 12.70 times, respectively. In addition, leaf endogenous MT, auxin (IAA), gibberellin (ABA), and abscisic acid (GA) were increased by 1.23, 1.24, 1.17, and 1.61 times respectively in seedlings sprayed with MT under low-temperature stress for 4 days. In conclusion, foliar spraying MT could improve photorespiration rate and photosynthetic carbon assimilation ability of K. obovata seedlings under low-temperature stress, increase endogenous hormone content, enhance photosynthetic capacity, and reduce the damage of low-temperature stress. However, such mitigation effects were dependent on the duration of low-temperature stress. Endogenous MT was positively correlated with P_n, T_r, F_v/F_m, Ф_PSⅡ, R_p, CE, J_max, IAA, GA, and ABA, indicating that endogenous MT plays an important role in regulating photosynthetic efficiency, carbon assimilation, and hormones.

Key words: melatonin, low temperature stress, Kandelia obovata, endogenous hormone, photosynthetic carbon assimilation