We studied the effect of short-term (2 hours at the end of the night) daily temperature reductions to 12 °C (DROP treatment) on the activity of antioxidant (AO) enzymes in cucumber (Cucumis sativus L.) leaves of various ages. Plants were DROP-treated for 6 days during the exponential growth of the second true leaf (DROP 1) or when the second leaf was mature (DROP 2). DROP 3 plants were DROP-treated for 12 days during the DROP 1+DROP 2 period.
In mature leaves (DROP 2) DROP treatment did not cause any changes in the activity of AO enzymes or lipid peroxidation rates, as estimated by the content of malondialdehyde (MDA). In young leaves (DROP 1 and DROP 3) DROP treatment reduced the activity of superoxide dismutase and ascorbate peroxidase and induced a short-term increase followed by a decrease in the activity of catalase and guaiacol-dependent peroxidase. Chilling tolerance was higher in all DROP-treated plants compared to untreated plants. Thus, we found that a daily short-term temperature drop (DROP) modified the activity of AO enzymes in young leaves, but did not affect the activity of the studied enzymes in mature leaves. Changes in the activity of AO enzymes did not correlate with changes in chilling tolerance. Apparently, the increased antioxidant activity in low temperature-treated plants described in the literature for chilling-sensitive species is not involved in cucumber response to temperature DROP, and some other mechanism is responsible for the higher chilling tolerance in DROP-treated plants. The differential response of young and mature leaves points to some developmental controls in the induction of plant adaptations to short-term chilling.