Abstract: Exploring the response mechanisms of Lumnitzera littorea seedlings to salt stress can provide theoretical basis of protecting this endangered species. Here, we examined the effects of different concentrations of NaCl (50, 150, 450, 750 mmol·L^-1) on plant growth, leaf photosynthesis, fluorescence parameters, changes of photosynthetic pigments, antioxidant enzyme activities, chloroplast ultrastructure, and stomatal characteristics. The results showed that under 150 mmol·L^-1 NaCl treatment, net photosynthetic rate (P_n), stomatal conductance (G_s), potential maximum light energy conversion efficiency (F_v/F_m) and actual light energy conversion efficiency (Φ_PSII), contents of total chlorophyll (Chl) and carotenoid (Car), activities of superoxide dismutase (SOD) and peroxidase (POD) were significantly higher than those of other treatments. When the NaCl concentration exceeded 450 mmol·L^-1, values of the above indices decreased significantly, while the intercellular CO² concentration (C_i), contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), the production rate of superoxide anion (O₂^-_·) significantly increased. Leaf malondialdehyde (MDA) content under 750 mmol·L^-1 NaCl exhibited 56.58% higher than that of 150 mmol·L^-1 NaCl. In addition, under 750 mmol·L^-1 NaCl stress, the boundary of chloroplast membrane was gradually blurred, and the starch particles and osmiophilic particles increased and expanded. More importantly, the matrix and grana lamella were gradually loose and deformed. Furthermore, the stomatal opening of leaves was reduced or even closed. Our results indicated that 150 mmol·L^-1 NaCl was the optimal concentration for the growth and development of L. littorea seedlings. When NaCl concentration exceeded 450 mmol·L^-1, photosynthetic capacity was reduced by inducing unbalance of antioxidant system, inhibiting the photosynthetic pigment synthesis, and preceding stomatal closure and damaging the chloroplast, which eventually led to plant wilting.

Key words: Lumnitzera littorea, salt stress, photosynthesis, antioxidant system, chloroplast ultrastructure