Nitrogen Metabolism in Senescing Leaves

Leaf senescence is a highly organized process and not a passive decay. Photosynthesizing mesophyll cells lose their functions in an early phase, while the epidermal layer with the stomates and the phloem remains functional throughout senescence. The subcellular compartmentation is maintained and allows the cooperation of different organelles in the remobilization of constituents. Nitrogen metabolism changes at the onset of senescence from assimilation to remobilization. Enzymes involved in nitrate reduction are lost, while some enzymes of intermediary nitrogen metabolism are maintained longer, and some catabolic enzymes reach highest activities during senescence. Chloroplasts are dismantled early, but mitochondria remain active and may fuel remobilization processes. Chloroplast proteins are degraded, and this nitrogen fraction can be translocated via the phloem from senescing leaves to sinks within the same plant. In contrast, chlorophyll is degraded, fragments produced reach the vacuole, and catabolites accumulate there. Nuclear DNA is maintained until a very late phase. The export of nitrogen from senescing plant parts is important for the economic use of this macronutrient. The regulation of senescence at the whole plant level as well as at the molecular level is only rudimentarily known, although interesting new aspects have been presented recently.