Increased cytokinin levels in transgenic P-SAG12-IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plan ts growing under conditions of nitrogen remobilization. Interactions betwee n cytokinin metabolism, Rubisco and protein levels, photosynthesis and plan t nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabac um L.) plants showing delayed leaf senescence through a novel type of enhan ced cytokinin syn-thesis, i.e. targeted to senescing leaves and negatively auto-regulated (P-SAG12-IPT), thus preventing developmental abnormalities. Plants were grown with growth-limiting nitrogen supply. Compared to the wil d-type, endogenous levels of free zeatin (Z)- and Z riboside (ZR)-type cyto kinins were increased up to 15-fold (total ZR up to 100-fold) in senescing leaves, and twofold in younger leaves of P-SAG12-IPT. In these plants, the senescence-associated declines in N, protein and Rubisco levels and photosy nthesis rates were delayed. Senescing leaves accumulated more (N-15-labelle d) N than younger leaves, associated with reduced shoot N accumulation (-60 %) and a partially inverted canopy N profile in P-SAG12-IPT plants. While r oot N accumulation was not affected, N translocation to non-senescing leave s was progressively reduced. We discuss potential consequences of these mod ified sink-source relations, associated with delayed leaf senescence, for p lant productivity and the efficiency of utilization of light and minerals.