Involvement of the thylakoidal NADH-plastoquinone-oxidoreductase complex in the early responses to ozone exposure of barley (Hordeum vulgare L.) seedlings.

A possible implication of the plastid NADH-plastoquinone-oxidoreductase (Ndh) complex in the response against ozone-mediated oxidative stress in barley (Hordeum vulgare L.) leaves was investigated. After a 4 h treatment, exposure of barley seedlings to moderate ozone concentrations produced leaf-age-dependent increases in lipid peroxidation, peroxidase, and Ndh complex activities in the thylakoid membranes. A significant amount and activity of the Ndh complex were detected in mature barley leaves, but not in young barley leaves. In fact, young barley leaves behaved like ndh-deficient leaves in most of the aspects studied. When plants were exposed to photo-oxidative light after ozone fumigation, the recovery of Fv/Fm was lower in young leaves than in mature leaves. Ozone treatment significantly decreased non-photochemical quenching (qN) in young leaves, but not in mature leaves. Mature leaves showed higher levels of the energy (DeltamuH+) dependent (qE) component of qN. Treatment with antimycin A, an inhibitor of cyclic electron flow, increased the decay of qN produced by ozone in young leaves, but not in mature ones. The reduction state of plastoquinone increased after ozone treatment in mature dark-adapted leaves and was strongly quenched by far red light. It is proposed that the function of the Ndh complex helps the maintenance of qN, probably through the poising of the redox steady-state level of the intersystem carriers and then by optimizing the rate of cyclic electron flow. This should constitute an age-dependent early response in barley leaves, by contributing to minimize photoinhibition in the presence of ozone and high light.