© 2005 Heron Publishing—Victoria, Canada
Environmental and physiological controls over oxygen and carbon isotope composition of Tasmanian blue gum, Eucalyptus globulus
Lucas A. Cernusak (1, 2, 3), Graham D. Farquhar (1) and John S. Pate (4)
1. Environmental Biology Group and Cooperative Research Center for Greenhouse Accounting, Research School of Biological Sciences,
Institute of Advanced Studies, Australian National University, GPO Box 475, Canberra, ACT 2601, Australia / 2. School of Science, Faculty of Education, Health and Science, Charles Darwin University, Darwin, NT 0909, Australia / 3. Corresponding author (lucas.cernusak@cdu.edu.au) / 4. School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Nedlands, WA 6907,
Australia / Received February 20, 2004; accepted July 19, 2004; published online December 1, 2004
Summary
We measured oxygen isotope ratios (δ18O) of xylem sap, phloem sap, leaves, wood and bark of Eucalyptus globulus Labill. growing in southwestern Australia. Carbon isotope ratios (δ13C) were measured in the dry matter of phloem sap, leaves and wood. Results were used to test several aspects of a mechanistic
model of 18O enrichment and provided insights into post-photosynthetic variations in dry matter δ13C. Xylem water δ18O varied little within the tree crown, whereas variation at the landscape-level was more pronounced, with plantations near
the coast being enriched by up to 3‰ compared with plantations less than 100 km inland. Phloem water was significantly enriched
in 18O compared with xylem water in two of three sampling campaigns; mean enrichments were 0.5 and 0.8‰. Phloem sap sugars exported
from E. globulus leaves closely reflected observed leaf water enrichment when diurnal variation in photosynthesis was taken into account.
Photosynthetic rates were higher in the morning than in the afternoon, whereas leaf water 18O enrichment increased to maximum values in the afternoon. A non-steady-state model of leaf water 18O enrichment accurately predicted observed values through a full diel cycle. Mean estimates of the proportion of organic oxygen
effectively exchanging with xylem water during cellulose synthesis were close to 0.40 for both leaves and wood. Carbon isotope
ratios of nascent xylem tissues did not differ from those of phloem sap sugars collected concurrently, whereas nascent leaf
tissues were depleted in 13C by 2‰ compared with phloem sap sugars, suggesting that, in E. globulus, 13C enrichment of sink tissues compared with source leaves does not result from an enriching process within the sink tissue.
Keywords:
isotope signal, isotopic fractionation, leaf water enrichment, translocation.
