VARIATION IN WATER-USE EFFICIENCY AND DELTA-C-13 LEVELS IN EUCALYPTUS-GRANDIS CLONES

This study aimed to determine whether the deltaC-13 levels in the foli age and twigs of four Eucalyptus grandis clones were related to their water use efficiency (WUE). This relationship has previously been demo nstrated in a number of herbaceous species but not in mature trees. Th e study involved accurate measurements of tree trunk growth and water use over a period of 4 months, with subsequent isotopic analysis of ma ture foliage from the north and south side of the canopy, and young le aves from the top of the canopy. The water use efficiencies were found to vary from 5.97 X 10(-3) to 12.3 x 10(-3) m3 m-3. Significant diffe rences were observed between clonal-mean water use efficiencies averag ed over six sampling periods. The average deltaC-13 of the mature and young foliage was found to be significantly correlated with WUE. Howev er, the correlation was weak, suggesting that the relationship between delta-C-13 and WUE is more complex in trees than suggested in the lit erature on crop plants. It is suggested that differences between sampl e trees in carbon allocation and leaf-to-air vapour pressure deficits may account for the poor correlation between delta-C-13 and WUE in the four E. grandis clones studied.