Effects of simulated acid rain on the photosynthetic physiology of Acer ginnala seedlings

Extensive areas in China have been receiving high levels of acid rain. Acid rain affects plant growth by reducing the chlorophyll content, destroying leaf structure, and hindering photosynthesis. Acer ginnala Maxim. has a high economic, ornamental, and medicinal value. To clarify the acid rain resistance mechanism of maple trees, the chlorophyll content, gas exchange parameters, and chlorophyll fluorescence parameters were measured in the leaves of 2-year-old Acer ginnala seedlings 0, 15, and 30 days after simulated acid rain at pH 5.6, 5.0, 4.5, 4.0, 3.5, and 2.5. The results showed that the relative content of chlorophyll gradually decreased with the increasing acidity. The net photosynthetic rate (Pₙ) decreased with the increasing acidity, and the difference among the treatments was significant except for between pH 5.0 and the control. The stomatal conductance at pH < 5.0 was significantly lower than that of the control, and the intercellular carbon dioxide (CO₂) concentration of the leaves with the pH < 4.5 treatments was significantly lower than that of the control. As the acidity increased, both Fᵥ/Fₘ and Fᵥ/F₀ (Fᵥ, variable fluorescence; Fₘ, maximum fluorescence; F₀, initial fluorescence) decreased. The difference in the photosynthetic performance index based on the absorbed light energy (PIₐbₛ) between the pH 5.0 treatment and the control was not significant during the three periods after the simulated acid rain stress, whereas in the other treatments, this index was significantly lower than that of the control. Our results indicated that under mild acid rain (pH > 4.0) stress, the chlorophyll content, gas exchange parameters, and chlorophyll fluorescence parameters of Acer ginnala changed little, whereas in the other treatments, especially the severe acid rain (pH < 3.0) treatment, these indices showed significant changes that had a severe impact on plant growth and development.