Effects of acid rain on the leaves antioxidase activity and photosynthesis of Citrus Grandis (L.) Osbeck. ‘Guanximiyou' seedlings

Abstract:【Objective】Acid rain has spread out from Europe beginning in the late 1950 s. The contaminated regions have expanded from North America and Western Europe to developing countries, especially India and China. Acid rain may do harm to plants through changing their pH balance, increasing the leaching loss of nutrient elements, causing the destruction of leaf anatomy, and so on. Citrus grandis (L.) Osbeck.‘Guanximiyou'is a Rutaceae Citrus evergreen tree fruit whose pomelo is very delicious and nutritious and is an unusual species in Fujian Province. Fujian Province lies in the southeast of the China acid rain area and the pollution by acid rain is the main crisis of the pomelo industry. The majority of research on pomelo focused on the genetic quality, the fresh keeping techniques and the ecological planting mode, and so on. The study of the physiological ecology of the pomelo was given less emphasis. This study focuses on the foliar damage, antioxidant enzyme activity, gas exchange parameters and the leaves anatomical characteristics of Citrus grandis (L.) Osbeck.‘Guanximiyou'1-year seedlings under acid rain treatment (pH 5.6, pH 4.5 and pH 2.5) .【Methods】Healthy 1-year seedlings of C. grandis (L.) Osbeck.‘Guanximiyou'were planted in plastic pots with yellowish red soil in a greenhouse, with average day/night temperatures of 25 ℃/20 ℃ and an average relative humidity of 75%. After two months of growing, three levels of acid rain (pH 5.6, pH 4.5 and pH 2.5) were developed. A completelyrandomized design was applied. Each treatment had fifteen replicates, and a 200 mL acid rain solution was watered every two days. The leaves were harvested at 1, 3, 6 and 12 days after acid rain treatment.The malondialdehyde (MDA) , superoxide dismutase (SOD) , catalase (CAT) and peroxidase (POD) of the leaves were measured with kits. The photosynthesis rate (μmol CO₂· m^-2·s^-1) , stomata conductance (mol H₂O·m^-2·s^-1) , intercellular CO₂ concentration (μmol CO₂·mol^-1) and transpiration rate (mmol H₂O·m^-2·s^-1) were measured by using a photosynthetic Monitor (Li—6400 XT) at 1, 3, 6, 12 and 90 day after acid rain treatment. The leaf anatomical structures were observed by using the paraffin section method. After 90 days treatment, 5-10 matured leaves in each treatment were selected for a paraffin section. These leaves were made by permanent slicing through fixing, dehydrating, embedding and so on. The leaves anatomical structures were measured by Image-Pro Plus 6.0. The experimental data were sorted out by Excel2007 and analyzed with one-way ANOVA by SPSS19.0.【Results】The leaves treated by pH 2.5 and 4.5 acid rain did not show any visible injury and the surface layer of the cells were arranged in neat rows.And the pH 2.5 acid rain significantly decreased the thickness of the palisade cells and significantly increased the thickness of the sponge tissues compared with the pH 5.6 treatment. pH 4.5 acid rain did not have any significant effect on the thickness of the palisade cells and sponge tissues compared with the pH 5.6 treatment. The leaves did not show any oxidative damage due to the increase in SOD and CAT until after the third day being treated by pH 2.5 acid rain. pH 4.5 acid rain caused oxidative damage after six days of being treated by acid rain and the activities of SOD, CAT and POD were higher than the control treatment (pH 5.6) . The photosynthesis rates were decreased by pH 2.5 and 4.5 acid rain during the initial period of acid rain stress. After three months of acid rain stress, the photosynthesis rate with the pH 2.5 treatment was lower than the control treatment and it was speculated this was because of non-stomas factors (oxidative damage, the decrease of palisade cell and so on) . The photosynthesis rates in the pH 4.5 acid rain treatment were not affected after three months. The stomata conductance and transpiration rates in the pH 2.5 treatment were always significantly lower than the control treatment throughout the treatment process. The results showed that pH 2.5 treatment significantly restrained stomatal conduction and affected water absorption and transport. Stomata conductance in the pH 4.5 treatment was always significantly lower than the control treatment on the first day and the 12 th day. And stomata conductance in the pH 4.5 treatment was not significantly different from the control treatment on the third day, the ninth day and the 90 th day. The transpiration rate in the pH 4.5 treatment was significantly lower than the control treatment most of the time.【Conclusion】During the initial phases of the experimentation, the acid rain treatment did not cause any oxygen issues due to the increasing of the antioxidases activity. With a prolonged treatment time, acid rain treatment induced lipid peroxidation damage. pH 2.5 acid rain treatment did not cause any visible blade damage, but significantly decreased the thickness of the palisade tissues compared with the pH 5.6 treatment. pH 2.5 acid rain treatment significantly decreased the net photosynthetic rate compared with the pH 5.6 treatment. Under the pH 4.5 acid rain treatment, the net photosynthetic rate was not significantly decreased due to the ability to repair itself.