Differential effects of acid rain on photosynthetic performance and pigment composition of the critically endangered Acer amplum subsp. catalpifolium

Acer amplum subsp. catalpifolium is a native deciduous broad-leaved tree species mainly distributed in the rainy zone of west China classified as critically endangered. Acid rain is a frequent phenomenon in this region and its impact on regional forests, especially for susceptible species have major implications on community stability and biodiversity conservation. To understand the basic physiological responses of A. amplum subsp. catalpifolium, an acid rain simulation experiment spanning 18 months was conducted. The aim is to determine the effects of acidity levels (pH 2.5, 3.5, and 4.5), chemical composition (HSO: HNO ratio of 7.5:1 and 5:4.1, respectively), and deposition patterns (soil addition and leaf spraying modes) on photosynthetic performance and stress status of A. amplum subsp. catalpifolium The results showed that simulated acid rain can enhance the peak photosynthetic rate (Pn) and stomatal conductance (Cs) in the diurnal variation of A. amplum subsp. catalpifolium under soil addition mode, and it is attributed to the increased net photosynthetic rate at the light saturation point (Amax), net photosynthetic rate at saturating CO concentration (Pmax) and various chlorophyll fluorescence parameters (ΦPSII, Fv/Fm, Fv/F0, and qP). On the contrary, the photosynthetic performance of A. amplum subsp. catalpifolium was significantly inhibited by acid rain under leaf spraying mode, with increased dark respiration rate (R) and photorespiration rate (R). The reduction levels in Amax and Pmax was higher in the nitric based acid rain compared to sulfuric acid dominant acid rain. In general, improved nutrient availability brought about by acid rain on soil promoted the photosynthetic performance of A. amplum subsp. catalpifolium. Meanwhile, acid rain in direct contact with the leaves negatively affects leaf performance and chemical makeup. Therefore, acid rain patterns (such as fog or snow) in direct contact with leaves should be accounted for in the protection of A. amplum subsp. catalpifolium.