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 (Nicotiana tabacum) leaf ageing are presented, with a particular emphasis on nitrogen metabolism. An integrated model describing the source/sink relationship existing between leaves of different developmental stages along the main plant axis is proposed. The results of our study show that a tobacco plant can be divided into two main sections with regards to sink/source relationships. Sink-to-source transition occurs at a particular leaf stage in which a breakpoint corresponding to an accumulation of carbohydrates and a depletion 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)-dependent glutamate dehydrogenase transcripts, proteins and activities. The role of the newly induced enzymes and the nature of the potential metabolic and developmental signals involved in the regulation of their expression during leaf senescence are discussed.