Senescence and nitrogen metabolism in annual plants

This review covers the changes in the nitrogen metabolism during senescence of annual plants. The remobilization processes in senescing tissues are important for the N-economy in higher plants and allow the reutilization of nitrogen in growing plant parts (e.g. developing leaves, fruits and seeds). The nitrogen fluxes and the enzyme pattern change considerably during leaf senescence. The activity of enzymes involved in C-, N- and S-assimilation decreases, while catabolic enzymes reach their highest activities. Various metabolic activities, enzymes and structures are lost sequentially, indicating that senescence is a highly organized process. Iso- enzymes, located in various subcellular compartments, differ often in their time courses. For glutamine synthetase it is suggested that there are two forms differing in their suscepti- bilities to proteolytic inactivation. Chloroplast degradation is of special interest, since a high percentage of leaf nitrogen is present in these organelles. Recent investigations suggest that at least the initial steps in the breakdown of chlorophyll and chloroplast proteins occur in the intact organelles and not, as assumed earlier, in the vacuole. The question of how these cata- bolic processes may be regulated is raised again