Abstract: Michelia shiluensis is a rare tree species in the family Magnoliaceae endemic to southern China. Its natural distribution regions frequently affected by acid rain, exposing it to a high risk of acid rain stress. In this study, 3-year-old seedlings of M. shiluensis were subjected to simulated acid rain treatments at pH 2.5, 3.5, 4.5, and 5.0, with purified water (pH 7.0) as control, for a 50-day experiment. We investigated changes in plant growth, leaf anatomical structure, and photosynthetic physiological parameters. The results showed that acid rain stress significantly inhibited plant growth, and that the effect became more severe with decreasing pH. Under the pH 2.5 treatment, plant height and ground diameter decreased by 82.3% and 87.4%, respectively. All the treatments with pH≤4.5 significantly increased stomatal area, with that of individual stomata being increased by 47.6% and 76.6% under pH 3.5 and pH 2.5 treatments, respectively. Under pH≤3.5, the thickness of all leaf anatomical tissues increased significantly, and the mesophyll became more compact. During the stress period, net photosynthetic rate continuously declined, while the intercellular CO₂ concentration increased during the middle and late stages of stress. Under pH≤3.5, the maximum photochemical efficiency of PSⅡ remained relatively stable, whereas the photochemical quenching coefficient and electron transport rate increased transiently during the early stage of stress but gradually declined thereafter. In contrast, both parameters remained suppressed under the pH 2.5 treatment. In conclusion, acid rain reduced net photosynthetic rate and inhibited the growth of M. shiluensis by disrupting stomatal structure, increasing the resistance to CO₂ diffusion within the mesophyll, and suppressing the photochemical activity of photosystem Ⅱ. Photosynthetic activity was markedly inhibited at pH≤3.5, whereas severe damage to both leaf structure and photosynthetic capacity occurred under the pH 2.5 treatment.

Key words: acid rain, leaf anatomy, stomata, photosynthetic parameter, chlorophyll fluorescence