Nitrogen accumulation and mobilization in Citrus leaves throughout the annual cycle

The changes of protein and amino acid contents of young and old leaves from field citrus trees [Citrus sinensis (L.) Osbeck cv. Washington Navel] were studied throughout the year. the total protein content of old leaves decreased during the spring and summer growths, whereas in young developing leaves it increased rapidly. The end of the spring and summer flushes was followed by a quick recovery of the initial protein content due to a process of reversible senescence. The evolution of SDS‐PAGE proteinograms indicated that most of the foliar proteins contributed to the nitrogen mobilization during the spring and summer growth periods. A protein band of molecular weight ca 55,000, probably containing the large subunit of ribulose‐1,5‐bisphosphate carboxylase/oxygenase, decreased relatively more than the others. The amino acid content of the leaves increased during winter time due to the accumulation of proline, which was mobilized during the spring flush. The possible nitrogen storage function of proline is discussed.