Uptake, partitioning and utilization of carbon and nitrogen in the phloem bleeding tree, Tasmanian blue gum (Eucalyptus globulus)

An empirical modelling procedure was employed to follow uptake, transport a nd utilization of photoassimilated 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 dir ections of assimilate flow. The model for C depicted the fate of exported f ixed 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 involve ment of xylem-derived C in nourishment of rapidly growing branches, and a n et daily respiratory output per tree equivalent to 39% of its net daytime p hotosynthetic gain in C by foliage. The model for N showed that upper growi ng shoot parts gained more N mobilized from lower branches than was being a cquired from soil. It also indicated high rates of cycling of N through mat ure foliage, effective retention of xylem-derived N by growing branches and apices, and feedback of substantial amounts of phloem-exported N from lowe r branches into xylem moving further up the trunk. Transpiration loss per t ree was equivalent to 272 mL g(-1) DM accumulated. Data are discussed in re lation to similarly executed C:N partitioning studies on herbaceous annual species.