© 2005 Heron Publishing—Victoria, Canada
Why does phosphorus limitation increase wood density in Eucalyptus grandis seedlings?
D. S. Thomas (1, 2, 3), K. D. Montagu (4, 5) and J. P. Conroy (1)
1. Centre for Horticulture and Plant Sciences, University of Western Sydney, Locked Bag 1797, South Penrith, NSW 1797, Australia / 2. Forests NSW, P.O. Box J19, Coffs Harbour Jetty, NSW 2450, Australia / 3. Corresponding author (danet@sf.nsw.gov.au) / 4. Science and Research, New South Wales Department of Primary Industries, P.O. Box 100, Beecroft, NSW 2119, Australia / 5. Cooperative Research Centre for Irrigation Futures, c/- School of Environment and Agriculture, University of Western Sydney,
Locked Bag 1797, South Penrith, NSW 1797, Australia / Received February 7, 2005; accepted May 7, 2005; published online October 3, 2005
Summary
Wood density influences both the physiological function and economic value of tree stems. We examined the relationship between
phosphorus (P) supply and stem wood density of Eucalyptus grandis Hill ex Maiden seedlings grown with varying soil P additions and determined how changes in wood anatomy and biomass partitioning
affect the relationship. Plant height, stem diameter and total biomass increased by 400–500% with increasing P supply. Stem
wood density decreased sharply from 520 to 380 kg m–3 as P supply increased to 70 mg P kgsoil-1. Further increases in P supply to 1000 mg P kgsoil-1 had no effect on wood density. The increase in wood density at low soil P supply arose principally from enhanced secondary
wall thickening of stem fiber cells. Cell wall thickness increased from 3.6 to 4.5 µm as soil P supply decreased. Because
fiber cell diameter was independent of soil P (12 µm ± 0.3), the proportion of the stem occupied by cell wall material increased
as P supply declined. The enhanced secondary wall thickening of stem fiber cells at low P supply was not associated with changes
in whole-plant biomass partitioning. Instead, low P supply appeared to alter biomass partitioning within the stem in favor
of secondary wall thickening. Thus, increased wood density in E. grandis seedlings grown at low P soil supply was associated with inhibited stem cambial activity, resulting in an increased proportion
of photoassimilates available for secondary wall thickening of fiber cells.
Keywords:
biomass partitioning, eucalypt, fiber cell, nutrition, xylem vessel.
