Abstract: In this study, the effect of soil acid stress on photosynthetic characteristics and ultrastructure of Sedum aizoon L. leaves were studied by simulating soil acidification under different soil pH values (3.4, 4.6, 5.5). The results showed that with the intensification of soil acidification, the dissolution of aluminum ions in soil increased rapidly, while the total chlorophyll content in leaves decreased gradually and the relative cell conductivity and malondialdehyde content increased gradually. Under mild (pH 5.5) and moderate (pH 4.6) acidification, plant photosynthetic and fluorescence parameters were not significantly different from CK (pH 6.6), and chloroplast ultrastructure was intact. Under severe (pH 3.4) acidification, photosynthesis and fluorescence parameters showed obvious threshold response, P_n, C_i, T_r, G_s, F_o, F_m, Φ_PSⅡ, qP, F_v/F_m fell rapidly, NPQ and L_s were significantly higher, and the chloroplast was no longer uniformly distributed along the cell membrane, and the cluster aggregated in the cytoplasm, the granum lamella appeared obvious gap, parts of the internal capsule membrane blurred. Thus, the reason of soil acidification leading to membranous peroxidation of plant leaves might be that it promoted the increase of aluminum ions in soil. Mild and moderate soil acidification did not cause changes in leaf photosynthesis and ultrastructure, while severe soil acidification would lead to destruction of chloroplast structure and function and decrease of photosynthetic capacity. At the same time, the plant can defend against the harm of soil acidification by inducing the increase of osmotic substances and the enhancement of antioxidant enzyme activity.

Key words: soil acidification, Sedum aizoon L., photosynthesis, ultrastructure