Environmental and physiological controls over the oxygen and carbon isotope composition of Eucalyptus globulus.
Cernusak, Lucas*,1, Farquhar, Graham1, Pate, John1, 1 Environmental Biology Group, Canberra, ACT, Australia
ABSTRACT- We measured oxygen isotope ratios (18O) of xylem sap, phloem sap, leaves, wood, and bark of Eucalyptus globulus growing in southwestern Australia. Carbon isotope ratios (13C) were measured in dry matter of phloem sap, leaves, and wood. Results were used to test several aspects of a mechanistic model of oxygen isotope enrichment, and additionally provided insights into post-photosynthetic variations in dry matter 13C. Xylem water 18O varied little within different parts of the tree crown. Landscape-level variation in xylem water 18O was more pronounced, with plantations near the coast being enriched by up to 3‰ compared to plantations less than 100 km inland. Phloem water was significantly enriched in 18O compared to 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. Photosynthesis was 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 very 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. Nascent leaf tissues, on the other hand, were depleted in 13C compared to phloem sap sugars by 2‰. These 13C results suggest that in E. globulus, 13C enrichment of sink tissues compared to source leaves does not result from an enriching process within the sink tissue itself.
Key words: phloem, stable isotope, Tasmanian blue gum
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