Abstract: Salix viminalis has an outstanding capacity to accumulate cadmium (Cd), but with poor tolerance to high concentrations of Cd. Investigating the regulation function of melatonin (MT) on photosynthetic rate of S. viminalis exposed to high concentrations of Cd would help reveal the alleviation effect of MT on woody plants exposed to Cd and provide supporting measures for phytoremediation of heavy metals. Cutting seedlings of clonal S. viminalis were used as experimental material. Three treatments were set as blank (BT), Cd stress (CT), and foliar spraying MT followed by Cd stress (MCT). We examined the regulatory modes of MT on photosynthetic gas exchange parameters, chlorophyll fluorescence parameters under dynamic light, and the content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in S. viminalis under Cd stress. The chlorophyll a (Chla) and Rubisco contents of S. viminalis exposed to Cd were decreased by 20% and 21% on the 7th day after treatment, respectively. The chlorophyll b (Chlb), stomatal conductance (G_s), intercellular CO₂ concentration (C_i), net photosynthetic rate (P_n) of S. viminalis exposed to Cd were decreased by 32%, 35%, 23%, 21% on the 14th day after treatment. Under the treatment of spraying 50 μmol·L^-1 MT, Chl a, Chlb G_s, C_i, Rubisco content were increased by 30%, 28%, 47%, 35%, 25% respectively. P_n and photosystem Ⅱ (PSⅡ) maximum photochemical efficiency (F_v/F_m) of S. viminalis exposed to Cd stress were decreased by 36% and 16% on the 21st day after treatment. After MT spraying, P_n and F_v/F_m were increased by 39% and 13%, respectively. Photoinhibition of S. viminalis exposed to Cd was aggravated, while spraying MT alleviated its photoinhibition. Under Cd stress, MT promoted photosynthetic gas exchange of S. viminalis by increasing stomatal conductance, promoted light harvesting by maintaining chlorophyll concentration, improved photosynthetic capacity by enhancing light energy conversion efficiency and photosynthetic electron transfer rate, and promoted CO₂ assimilation by increasing Rubisco content. Moreover, MT promoted growth of S. viminalis exposed to Cd stress by increasing photosynthetic intensity.

Key words: melatonin, cadmium (Cd), Salix viminalis, photosynthesis rate