TRANSPORT, STORAGE AND MOBILIZATION OF NITROGEN BY TREES AND SHRUBS IN THE WET DRY TROPICS OF NORTHERN AUSTRALIA/

Xylem sap from woody species in the wet/dry tropics of northern Austra lia was analyzed for N compounds. At the peak of the dry season, argin ine was the main N compound in sap of most species of woodlands and de ciduous monsoon forest. In the wet season, a marked change occurred wi th amides becoming the main sap N constituents of most species. Specie s from an evergreen monsoon forest, with a permanent water source, tra nsported amides in the dry season. In the dry season, nitrate accounte d for 7 and 12% of total xylem sap N in species of deciduous and everg reen monsoon forests, respectively In the wet season, the proportion o f N present as nitrate increased to 22% in deciduous monsoon forest sp ecies. These results suggest that N is taken up and assimilated mainly in the wet season and that this newly assimilated N is mostly transpo rted as amide-N (woodland species, monsoon forest species) and nitrate (monsoon forest species). Arginine is the form in which stored N is r emobilized and transported by woodland and deciduous monsoon forest sp ecies in the dry season. Several proteins, which may represent bark st orage proteins, were detected in inner bark tissue from a range of tre es in the dry season, indicating that, although N uptake appears to be limited in the dry season, the many tree and shrub species that produ ce flowers, fruit or leaves in the dry season use stored N to support growth. Nitrogen characteristics of the studied species are discussed in relation to the tropical environment.