Journal of Plant Physiology 162 (2005) 603—606 SHORT COMMUNICATION Nitrogen transport in the xylem sap of Quercus ilex: The role of ornithine Cristina Nabais a,Ã ,Ju¨rgenHagemeyer b , Helena Freitas a a Departamento de Botaˆnica, Faculdade de Cieˆncias e Tecnologia, Universidade de Coimbra, 3000 Coimbra, Portugal b Department of Ecology, Faculty of Biology, University of Bielefeld, 33615 Bielefeld, Germany Received 5 October 2004; accepted 15 November 2004 Summary The storage and remobilization of nitrogen in deciduous and evergreen species is a major source of N, supporting the seasonal growth of trees. In evergreens, in addition to wood and roots, older leaves are important reservoirs of N used in the growth of new foliage. Just before bud burst, when transpiration is inactive or low, and when uptake of nitrogen by the roots may be restricted due to low temperatures, levels of organic N in the xylem are high. Amino acids usually comprise the bulk of this organic N. Changes in amino acid concentrations in early spring are thought to result mainly from hydrolysis of N reserves, and not from current N uptake. The seasonal profiles of amino acids in the xylem sap of Quercus ilex, an evergreen Mediterranean tree, were investigated. The first amino acid detected in the xylem sap before spring was ornithine, which may result from the breakdown of arginine present in storage proteins. Arginine is one of the main amino acids present in storage proteins because each arginine molecule has four nitrogen atoms. When protein degradation increases the free arginine pool, the arginase activity is enhanced and, consequently, the conversion of arginine to ornithine. It seems that ornithine has an important role in N transport early in the growth season of Q. ilex. & 2005 Elsevier GmbH. All rights reserved. Introduction Evergreen species tend to dominate sites where nutrient resources are scarce, employing a con- servation strategy (Monk, 1966). The internal remobilization of nitrogen can contribute a large proportion of the annual supply required to support growth of new shoots (Millard, 1996; Bollmark et al., 1999). Internal remobilization includes both reserves re-circulated after storage, and recycling from senescing tissues (Cherbuy et al., 2001). In evergreen species, old leaves are assumed to play the main role in supplying carbon and nutrients for newly developing shoots (Chapin et al., 1990). ARTICLE IN PRESS www.elsevier.de/jplph KEYWORDS Amino acids; Nitrogen; Ornithine; Quercus ilex 0176-1617/$ - see front matter & 2005 Elsevier GmbH. All rights reserved. doi:10.1016/j.jplph.2004.11.004 Ã Corresponding author. Tel.: +351 239 855 244; fax: +351 239 855 211. E-mail address: crnabais@bot.uc.pt (C. Nabais).