Age‐dependence of the antioxidative system in tobacco with enhanced glutathione reductase activity or senescence‐induced production of cytokinins

Tobacco leaves of plants with enhanced glutathione reductase activity (GR46‐27, Nicotiana tabacum L. cv. Samsun) or with autoregulated senescence‐induced production of cytokinins (P SAG12 ‐IPT, N. tabacum L. cv. Wisconsin) were studied during the course of leaf development and senescence by measuring photosynthesis, chlorophyll and protein content, the antioxidants ascorbate, glutathione and α ‐tocopherol as well as the antioxidative enzymes ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2) and superoxide dismutase (SOD, EC 1.15.1.1). The photosynthetic rate, as well as the chlorophyll and protein content, dropped with increasing leaf age after having reached a maximum at the end of the exponential growth phase. The concentrations of the water‐soluble antioxidants ascorbate and glutathione fell continuously with age, whereas the concentration of the lipophilic α ‐tocopherol increased. The activities of the antioxidative enzymes APX, GR and SOD reached their maximum at the beginning of leaf development, but were reduced in senescing leaves. The age‐dependent course of the measured leaf parameters in GR46‐27 leaves was similar to the one in wild‐type leaves, with the exception of an overall enhanced GR activity. In contrast, in old leaves of P SAG12 ‐IPT plants, which possess a much higher life span, the chlorophyll and protein content, the photosynthetic rate, the antioxidant concentrations of ascorbate and glutathione as well as the activities of the antioxidative enzymes were higher than in wild‐type leaves. The results show that the capacity of the antioxidative system to scavenge radicals is sufficiently balanced with the plant metabolism, and its decline with increasing age is not the cause, but a consequence of senescence and ageing in plants.