© 1998 Heron Publishing—Victoria, Canada
Transport, storage and mobilization of nitrogen by trees and shrubs in the wet/dry tropics of northern Australia
Susanne Schmidt (1) and George R. Stewart (1)
1. Department of Botany, The University of Queensland, Brisbane, Qld 4072, Australia / Received October 27, 1997
Summary
Xylem sap from woody species in the wet/dry tropics of northern Australia was analyzed for N compounds. At the peak of the
dry season, arginine was the main N compound in sap of most species of woodlands and deciduous monsoon forest. In the wet
season, a marked change occurred with amides becoming the main sap N constituents of most species. Species from an evergreen
monsoon forest, with a permanent water source, transported amides in the dry season. In the dry season, nitrate accounted
for 7 and 12% of total xylem sap N in species of deciduous and evergreen monsoon forests, respectively. In the wet season,
the proportion of N present as nitrate increased to 22% in deciduous monsoon forest species. These results suggest that N
is taken up and assimilated mainly in the wet season and that this newly assimilated N is mostly transported as amide-N (woodland
species, monsoon forest species) and nitrate (monsoon forest species). Arginine is the form in which stored N is remobilized
and transported by woodland and deciduous monsoon forest species in the dry season. Several proteins, which may represent
bark storage proteins, were detected in inner bark tissue from a range of trees in the dry season, indicating that, although
N uptake appears to be limited in the dry season, the many tree and shrub species that produce 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.
Keywords:
amides, monsoon forest, nitrate, nitrogen fixation, nitrogen transport, storage protein, tropical savanna woodland, ureides,
xylem sap.
