Abstract

This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999

An empirical modelling procedure was employed to follow uptake, transport and utilization of photo-assimilated carbon (C) and soil-derived nitrogen (N) over a 19-d period (November 1998) in 2-year-old plantation-grown trees of Eucalyptus globulus Labill. Models utilized data for gains and losses of C and N in dry matter (DM) of tree parts, CO₂ exchanges and transpiration of foliage, respiratory losses of stems and roots, C:N weight ratios of xylem and phloem sap collected at different sites within the system, and phloem sap sugar concentration gradients along trunks and branches to indicate directions of assimilate flow. The model for C depicted the fate of exported fixed C from four levels of branches on the shoot system, cycling of 16% of the C supplied from shoot to root back to the shoot in xylem, major involvement of xylem-derived C in nourishment of rapidly growing branches, and a net daily respiratory output per tree equivalent to 39% of its net daytime photosynthetic gain in C by foliage. The model for N showed that upper growing shoot parts gained more N mobilized from lower branches than was being acquired from soil. It also indicated high rates of cycling of N through mature foliage, effective retention of xylem-derived N by growing branches and apices, and feedback of substantial amounts of phloem-exported N from lower branches into xylem moving further up the trunk. Transpiration loss per tree was equivalent to 272 mL g^–1 DM accumulated. Data are discussed in relation to similarly executed C:N partitioning studies on herbaceous annual species.