Tea was planted in Lahijan by Kashefalsataneh in 1930. The main concern about important commercial plants such as tea is the formation of ice crystals in low temperatures. This can damage the live cells leading to lowering the quality of the plant and eventually its death. Formation of reactive oxygen species (ROS) and oxidative stress is the result of various environmental stresses leading to freezing. Investigating the variations in any of these factors could help to understand the mechanism of freeze resistance in ever-green plants. The aim of the present research was to investigate lipid peroxidation, the presence of antifreeze protein and variations in the activity of some antioxidant enzymes, including superoxide dismutase (SOD), ascorbate peroxidize (APX) and catalyse (CAT) in tea leaves subjected to 20, 0, -2, -5 and -8°C in tea leaves from the north of Iran. The results showed formation of an antifreeze protein with MW of about 20 KD in response to cold stress. It was also found that the activity of SOD, APX and CAT increased in tea leaves due to cold stress. The activity of SOD increased down to -8°C. APX and CAT increased their activity down to -5°C. On the other hand, the lipid per oxidation factor, MDA, was also elevated in response to the cold stress.

Pennyroyal (Mentha pulegium L.) from the Lamiaceae family is a medicinal plant which has great antioxidant properties. Environmental stresses such as drought can result in changes in the activity of antioxidant enzymes and the content of some biochemical factors in plants. In this investigation, the effects of drought stress on the activity of supe-roxide dismutase, ascorbate peroxidase, catalase, peroxidase, malondialdehyde and soluble sugars content in pennyroyal shoots and roots were evaluated. To create the water deficit, 24-day seedlings of pennyroyal were placed in 1/2 strength Hoagland solution, containing PEG 6000 (0 and 5% (w/v)) for 24 hours. The results showed that drought stress incr-eased catalase activity and lipid peroxidation and decreased superoxide dismutase and peroxidase activity of the shoots. On the other hand, the activity of catalase and peroxidase increased in the roots. Ascorbate peroxidase activity showed no significant difference in the shoots and the roots. Moreover, drought stress significantly increased the amount of so-luble sugars of glucose, galactose, xylose and rhamnosus in the shoots. Therefore, the increased activity in antioxidant enzymes as well as the amount of soluble sugars under drought stress might be a sign of tolerance of M. pulegium under low levels of drought.