Leaf age- and paraquat concentration-dependent effects on the levels of enzymes protecting against photooxidative stress

Antioxidant protective enzymes are usually induced in leaves under conditions of increased active oxygen generation, such as high light intensity, low CO, fixation rate or in the presence of paraquat, which transports electrons from photosynthetic machinery to oxygen to form O-2 (-). However, at high photooxidative stress, even protective enzymes can be destroyed and leaf cells become dead. The protective role of several chloroplastic activities was evaluated at increasing photooxidative stress in barley leaves of different ages. We investigated the effects of different paraquat concentrations (combined with low and high light intensities) in expanding and aged-senescent leaves on the activity of plastid peroxidase and on the activity and protein levels of plastid superoxide dismutase (SOD), glutathione reductase (GR) and NADH dehydrogenase of the complex including polypeptides encoded by plastid ndh genes. The chloroplastic GR was the most sensitive to inactivation when photooxidative stress increased. SOD was preferentially induced in young-expanding leaves while NADH dehydrogenase and peroxidase were preferentially induced in adult-senescent leaves. The results suggest a limited role of GR in the protection against photooxidative stress and a close relation between the actions of Ndh complex and peroxidase. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.