Effects of simulated acid rain on leaf structure and photosynthetic physiology in Michelia shiluensis.

All treatments at pH≤4.5 significantly increased stomatal area, with the area of individual stomata increasing by 47.6% and 76.6% under the pH 3.5 and pH 2.5 treatments, respectively, relative to the control. Under pH≤3.5, the thickness of all leaf anatomical tissues increased significantly, and the mesophyll became more compact. During the stress period, the net photosynthetic rate (Pn) continuously declined, while the intercellular CO2 concentration (Ci) increased during the middle and late stages of stress.

Under pH≤3.5, the maximum photochemical efficiency of PSII (Fv/Fm) remained relatively stable, whereas the photochemical quenching coefficient (qP) and electron transport rate (ETR) increased transiently during the early stage of stress but gradually declined thereafter. In contrast, both parameters remained continuously suppressed under the pH 2.5 treatment. In conclusion, acid rain reduced the 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.