Copper induced oxidative stress in tea (Camellia sinensis) leaves

Tea [Camellia sinensis L. (O.) Kuntze] is an economically important plantation crop of India but is prone to attack by several fungal pathogens. Copper based fungicides are being used for decades to control fungal diseases in tea which may lead to accumulation of copper in the soil. The biochemical responses to increasing concentrations of copper (50 to 700 microM) were investigated in the leaves of two cultivars of tea commonly grown in the Darjeeling hills. Exposure to excess Cu resulted in increased lipid peroxidation (level of TBARS increased from 3.5 micromol g(-1) f.wt. in control to 12 micromol g(-1) f.wt. in TS-520 plants exposed to 700 mM of Cu), reduced chlorophyll content (from 83.7 microg g(-1) f.wt. in control to 22.5 microg g(-1) f.wt. in TS-520 plants exposed to 700 mM of Cu), higher levels of phenolic compounds(total phenol content increased from 4.54 mg g(-1) f.wt. in control to 5.79 mg g(-1) f.wt. in TS-520 plants exposed to 400 mM of Cu) and an increase in peroxidase enzyme levels. Two new peroxidase isozymes (POD1 and POD2) were detected in plants exposed to Cu. In addition, biochemical responses in two tested cultivars, TS-462 and TS-520 differed significantly. TS-520 was found to be more sensitive to increasing concentrations of Cu. Superoxide dismutase activity increased progressively from 2.55 U mg(-1) protein in control to 5.59 U mg(-1) protein in TS-462 but declined from 4.75 U mg(-1) protein in control to 3.33 U mg(-1) protein in TS-520 when exposed to Cu concentrations higher than 400 microM. Asharp increase in the activity of ascorbate peroxidase (from 0.53 units in control to 2.37 units in plants exposed to 400 mM of Cu) was noticed at the 10th day of exposure in the more tolerant cultivar. On the other hand, catalase levels increased only marginally (from 8.4 to 10.1 units in TS 520 and 8.7 to 10.9 units in TS 462) in both the cultivars. From this study, it appears that Cu exposure led to the production of reactive oxygen species in the leaves resulting in significant lipid peroxidation. Tea plants try to mitigate this oxidative damage through accumulation of phenolic compounds and induction of antioxidant enzymes.