Decline in leaf photooxidative-stress tolerance with age in tobacco

To clarify the mechanism of the decline in oxidative stress tolerance of leaves during the course of senescence, we studied the differences in the levels of various antioxidants and antioxidant enzymes in leaves of different ages. In 8-week-old tobacco plants, even though the chlorophyll content had not changed, the protein content and the photosynthetic capacity were significantly decreased in the older leaves compared to the younger leaves. The older leaves had constitutively higher levels of H2O2. Analyses of the components of the active oxygen species-scavenging system revealed that older leaves had significantly lower levels of ascorbate, glutathione and most of the antioxidant enzymes, especially catalase, Cu/Zn-superoxide dismutase, and ascorbate peroxidase (APX). When leaves of different ages were treated with 0.5 μM methylviologen under moderate light intensity at 150 μmol photons m−2 s−1, the older leaves lost more chlorophyll and sustained ion leakage. During the induced photooxidative stress, chloroplastic APX, a key component of the water-water cycle, was particularly destroyed in the older leaves. These findings suggest that the decline in the levels of antioxidants and antioxidant enzymes associated with leaf senescence lead to lower photooxidative-stress tolerance, which might in turn accelerate the propagation of senescence.