Influence of repeated acid rain treatment on antioxidative enzyme activities and on lipid peroxidation in cucumber leaves

The effect of repeated treatment with simulated acid rain (AR) on antioxidative enzyme activities, lipid peroxidation and chlorophyll concentration in cucumber plants (Cucumis sativus L.) was investigated. Measurements were carried out one, five and seven days after triple spraying. Ascorbate peroxidase (APx) and glutathione transferase (GST) activities increased considerably after the exposure to AR stress indicating a direct correlation with pH of the used solution. Both enzyme activities were enhanced to the end of the experiment, which could contribute to the scavenging of reactive oxygen species (ROS) and detoxification of potentially toxic products originating from AR stress. Contrary to APx and GST activities, increases in catalase (CAT) and glutathione peroxidase (GSH-Px) activities were observed shortly after AR stress only. Changes in chlorophyll concentration were strongly dependent on pH of the applied AR. It was also found that lipid peroxidation measured as 2-thiobarbituric acid reactive substances (TBARS) concentrations was increasing in cucumber plants to the fifth day after AR treatment. Leaf bleaching and increased TBARS concentration suggested premature senescence of the AR-treated plants. Our results indicated that antioxidant enzymatic response to AR stress was quite sufficient in leaf tissue exposed to AR pH 4.4 but was not effective at pH below 3.0. A hypothesis can be put forward that elevated activities of APx and GST are of importance in plant defence against AR stress. Moreover, GSH-Px and GST activities seemed to be sufficient to keep TBARS concentration at the control level in the final phase of the experiment. (c) 2005 Elsevier B.V. All rights reserved.