Environmental and physiological controls over oxygen and carbon isotope composition of Tasmanian blue gum, Eucalyptus globulus

Abstract

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.