Antioxidant protection during ageing and senescence in chloroplasts of tobacco with modulated life span.

We studied changes in antioxidant protection during ageing and senescence in chloroplasts of tobacco (Nicotiana tabacum L., cv. Wisconsin) with introduced SAG(12) promoter fused with ipt gene for cytokinin synthesis (transgenic plants with increased levels of cytokinins, SAG) or without it (control). Old leaves of SAG plants as well as their chloroplasts maintained higher physiological parameters compared to controls; accordingly, we concluded that their ageing was diverted due to increased cytokinin content. The chloroplast antioxidant protection did not decrease as well. Although antioxidant protection usually decreased in whole leaves of senescing control plants, ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) activity, which maintained the high redox state of ascorbate, increased in chloroplasts of old control leaves.