Nitrogen availability has a strong influence on developmental processes in plants. We show that the time of nitrogen supply regulates the course of leaf senescence in flag leaves of Hordeum vulgare. The senescence-specific decrease in chlorophyll content and photosystem II efficiency is clearly delayed when plants are fertilised with nitrate at the onset of leaf senescence. Concurrently, the additional supply of nitrate affects expression patterns of two marker genes of nitrogen metabolism. As shown by quantitative RT-PCR analyses, senescence-specific downregulation of plastidic glutamine synthetase (GS2) and senescence-specific upregulation of lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) are both clearly retarded. Depletion of nitrogen in experiments using hydroponic growth systems results in premature primary leaf senescence. The already started senescence processes can be even reversed by later nitrogen addition, as proved by a further increase in photosystem II efficiency and chlorophyll content, returning to the high values of controls which had not been deprived of nitrogen. Although both addition of nitrate or ammonium effectively reversed nitrogen depletion-induced primary leaf senescence, addition of urea did not. Additionally, effects of nitrogen supply on the course of leaf senescence were analysed in the model plant Arabidopsis thaliana. Leaves of A. thaliana show the same reversion of senescence processes after receiving additional nitrogen supply, indicating that the nitrogen response of leaf development is conserved in different plant species.