Abstract:

High cadmium concentration and its bioavailability in the sediment of Three Gorges Reservoir Region have a negative effect on the soil-plant ecosystem in the hydro-fluctuation zone of the region. Salix variegate, a native riparian plant species, can be used widely in vegetation restoration of the hydro-fluctuation zone in this region. Although some plants have evolved mechanisms to limit Cd translocation to the shoot thereby avoiding cadmium toxicity, leaf Cd concentrations in excess of 5-10 mg ·kg^-1 dry matter are toxic to most plants. In order to reveal the photosynthetic adaptive mechanism of S. variegata to cadmium contaminated soils, the photosynthetic chlorophyll contents, gas exchange parameters and chlorophyll fluorescence characteristics of S. variegata seedlings were measured under treatments with five different concentrations of heavy metal Cd²⁺. The cadmium treatments were 0(CK), 10, 20, 50, 100 mg ·kg^-1. The results showed that: 1) Chl a, Chl b, Total Chls and carotenoid contents of treated S. variegata seedlings were significantly lower than that in controlled plants except for in 10 mg ·kg^-1, while Chl a/b ratio in above 50 mg ·kg^-1 Cd²⁺ treated seedlings was significantly lower than controlled plants (P<0.05), indicating that the inhibitory effect of cadmium on Chl a was more severe than on Chl b, especially above 50 mg ·kg^-1. 2) The net photosynthetic rate (P_</em>n), stomatal conductance (G_</em>s) and transpiration rate (Tr) of S. variegata seedlings treated with cadmium significantly decreased compared to the control (P<0.05). Meanwhile, intercellular CO₂ concentration (C_i) of S. variegata seedlings increased gradually and stomatal limitation (L_s) decreased. These results suggested that the predominant reason for decline of P_n of S. variegata was non-stomatal factors. Cadmium would induce closure of stomata and result in reduction in G_s, but was not the direct or solely reason for the reduced net photosynthetic rate. S. variegata seedlings had a high photosynthetic adaptation to cadmium because the net photosynthetic rate in plants exposed to 10 mg ·kg^-1 cadmium still was 84.9% of the control level. 3) Changes in photosynthesis of S. variegata affected by cadmium stress were also determined using chlorophyll fluorescence parameters. The decrease of F_v/F_m and q_P in 10-20 mg ·kg^-1 treatments were not significantly different from the control, indicating that the photoinhibition and the decreased degree of PS II reaction center were slight. The ratio of F'_v/F'_m, ΦPS Ⅱ and ETR were significantly inhibited by cadmium, especially above 50 mg ·kg^-1, demonstrating a severe fall in efficiency of the photochemical reduction of Q_A, and the reaction center capturing excitation energy. NPQ increased with increasing cadmium concentration, which might be responsible for tolerance of S. variegata seedlings to heavy metal cadmium due to form protecting mechanism through elevating its ability of dissipating heat. This research revealed that S. variegata seedlings had relatively high tolerance to cadmium, and would be a promising species for phytoremediation of those cadmium contaminated areas in the hydro-fluctuation zone of the Three Gorges Reservoir Region.

Key words: Salix variegata, cadmium, gas exchange parameters, chlorophyll fluorescence