CSIRO PUBLISHING www.publish.csiro.au/journals/fpb Functional Plant Biology, 2007, 34, 1137–1149 Canopy development and hydraulic function in Eucalyptus tereticornis grown in drought in CO 2 -enriched atmospheres Brian J. Atwell A,E , Martin L. Henery A,B , Gordon S. Rogers C , Saman P. Seneweera D , Marie Treadwell A and Jann P. Conroy D A Department of Biological Sciences, Macquarie University, NSW 2109, Australia. B School of Botany and Zoology, The Australian National University, Canberra, ACT 0200, Australia. C AHR Consulting, PO Box 552, Sutherland, NSW 2232, Australia. D Centre for Plant and Food Sciences, University of Western Sydney, Hawkesbury, Locked Bag No. 1, Richmond, NSW 2753, Australia. E Corresponding author. Email: batwell@rna.bio.mq.edu.au Abstract. We report on the relationship between growth, partitioning of shoot biomass and hydraulic development of Eucalyptus tereticornis Sm. grown in glasshouses for six months. Close coordination of stem vascular capacity and shoot architecture is vital for survival of eucalypts, especially as developing trees are increasingly subjected to spasmodic droughts and rising atmospheric CO 2 levels. Trees were exposed to constant soil moisture deficits in 45L pots (30–50% below field capacity), while atmospheric CO 2 was raised to 700 μL CO 2 L -1 in matched glasshouses using a hierarchical, multi-factorial design. Enrichment with CO 2 stimulated shoot growth rates for 12–15 weeks in well-watered trees but after six months of CO 2 enrichment, shoot biomasses were not significantly heavier (30% stimulation) in ambient conditions. By contrast, constant drought arrested shoot growth after 20 weeks under ambient conditions, whereas elevated CO 2 sustained growth in drought and ultimately doubled the shoot biomass relative to ambient conditions. These growth responses were achieved through an enhancement of lateral branching up to 8-fold due to CO 2 enrichment. In spite of larger transpiring canopies, CO 2 enrichment also improved the daytime water status of leaves of droughted trees. Stem xylem development was highly regulated, with vessels per unit area and cross sectional area of xylem vessels in stems correlated inversely across all treatments. Furthermore, vessel numbers related to the numbers of leaves on lateral branches, broadly supporting predictions arising from Pipe Model Theory that the area of conducting tissue should correlate with leaf area. Diminished water use of trees in drought coincided with a population of narrower xylem vessels, constraining hydraulic capacity of stems. Commensurate with the positive effects of elevated CO 2 on growth, development and leaf water relations of droughted trees, the capacity for long-distance water transport also increased. Additional keywords: biomass allocation, carbon dioxide, drought, xylem. Introduction Terrestrial vegetation makes a significant contribution to the global carbon cycle, exchanging ∼15% of the atmospheric carbon pool annually (Amthor 1995), with trees accounting for up to 70% of terrestrial net primary production (Melillo et al. 1993). Typically, in the mature eucalypt forests of Australia, as much as 90% of aboveground carbon is stored as wood (Hopmans et al. 1993). Hence, woody organs are a particularly important fate for long-term carbon storage under rising atmospheric CO 2 levels because of the slow turnover of this pool. However, there is limited information on the impact of abiotic factors on the early development of hardwood species in high CO 2 atmospheres (Conroy et al. 1992; Wong et al. 1992) and particularly little is known about the coordination of development when drought and atmospheric CO 2 enrichment coincide (Roden and Ball 1996), as will become increasingly common in southern Australia. Models of plant productivity predict important interactions between drought, nutrient supply and carbon assimilation (Kirschbaum et al. 1997; Nowak et al. 2004) but the relative response of leaf canopies and xylem transport that sustains water supply to these canopies is less thoroughly explored. Positive growth responses to elevated CO 2 atmospheres ([CO 2 ]) in woody species (largely softwoods) have been widely reported when other resources were adequately supplied (e.g. Idso et al. 1991; Norby et al. 1995; Whitehead et al. 1995; Roden and Ball 1996; H¨ attenschwiler and K¨ orner 1997; Heath and Kerstiens 1997; Jach and Ceulemans 1999; Atwell et al. 2003; Liberloo et al. 2006). Deciduous trees might respond more to CO 2 enrichment than conifers (Ceulemans and Mousseau 1994) but the evidence is inconclusive (Curtis and Wang 1998). Limited data from eucalypts indicate that they might be among the most responsive of all woody plants, with 2–3 fold growth stimulation when [CO 2 ] was doubled for up to © CSIRO 2007 10.1071/FP06338 1445-4408/07/121137