Roles for redox regulation in leaf senescence of pea plants grown on different sources of nitrogen nutrition

Leaf senescence and associated changes in redox components were monitored in commercial pea (Pisum sativum L. cv. Phoenix) plants grown under different nitrogen regimes for 12 weeks until both nodules an leaves had fully senesced. One group of plants was inoculated with Rhizobium leguminosarum and grown with nutrient solution without nitrogen. A second group was not inoculated and these were grown on complete nutrient solution containing nitrogen. Leaf senescence was evident at 11 weeks in both sets of plants as determined by decreases in leaf chlorophyll and protein. However, a marked decrease in photosynthesis was observed in nodulated plants at 9 weeks. Losses in the leaf ascorbate pool preceded leaf senescence, but leaf glutathione decreased only during the senescence phase. Large decreases in dehydroascorbate reductase and catalase activities were observed after 9 weeks, but the activities of other antioxidant enzymes remained high even at 11 weeks. The extent of lipid peroxidation, the number of protein carbonyl groups and the level of H2O2 in the leaves of both nitrate-fed and nodulated plants were highest at the later stages of senescence. At 12 weeks, the leaves of nodulated plants had more protein carbonyl groups and greater lipid peroxidation than the nitrate-fed controls. These results demonstrate that the leaves of nodulated plants undergo an earlier inhibition of photosynthesis and suffer enhanced oxidation during the senescence phase than those from nitrate-fed plants.