Characterization of the sink/source transition in tobacco (Nicotiana tabacum L.) shoots in relation to nitrogen management and leaf senescence

The metabolic, biochemical and molecular events occurring during tobacco (N icotiana tabacum) leaf ageing are presented, with a particular emphasis on nitrogen metabolism. An integrated model describing the source/sink relatio nship existing between leaves of different developmental stages along the m ain plant axis is proposed. The results of our study show that a tobacco pl ant can be divided into two main sections with regards to sink/source relat ionships. Sink-to-source transition occurs at a particular leaf stage in wh ich a breakpoint corresponding to an accumulation of carbohydrates and a de pletion of both organic and inorganic nitrogen is observed. The sink/source transition is also marked by the appearence of endoproteolytic activities and the induction of both cytosolic glutamine synthetase and NAD(H)-depende nt glutamate dehydrogenase transcripts. proteins and activities. The role o f the newly induced enzymes and the nature of the potential metabolic and d evelopmental signals involved in the regulation of their expression during leaf senescence are discussed.