The relationship between photooxidative stress and leaf antioxidants involved in leaf age of higher plants.

Abstract

Photosynthetic cells are prone to oxidative stress via active oxygen species (AOS) caused by the highly energetic reactions and an abundant O2 supply in photosynthesis. On the other hand, AOS have an important role in plant tissues during the senescence process, leading to irreparable metabolic dysfunction and cell death. Higher plants contain antioxidants and antioxidant enzymes which function to interrupt the cascades of uncontrolled oxidation. It seems likely that the tolerance capacity to oxidative damage depends on the balance between the AOS-generation and defense systems in the cells. Here we studied the relationship between oxidative stress tolerance and levels of antioxidants in different aged leaves of plants. Leaf discs (1.0 cm2) were taken from the upper leaves (leaf 3, 4), the middle leaves (leaf 5, 6), and the lowest leaves (leaf 7, 8) counting from the top of tobacco plants (Nicotiana tabacum cv Xanthi) grown under normal conditions for 8 weeks. Levels of total AsA and GSH of leaf 8 were 44% and 33% compared to those of leaf 3, respectively. Enzyme activities of cytsolic APX, choloroplastic APXs, and GR of leaf 8 were 54%, 80% and 39%, respectively. There were no changes in the chlorophyll contents in different aged leaves. To evaluate the tolerance capacity to photooxidative stress, leaf disks were floated to PQ (0.5 µM) and then exposed to the moderate light intensity (90 µE m-2 s-1). The destruction of chlorophyll was significantly observed in the lowest leaves, but not in the upper leaves after 24 hr.