Effects of water deficits on 13C discrimination and transpiration efficiency of Eucalyptus globulus clones

Potted cuttings of three Eucalyptus globulus Labill. clones (AR3, CN44, MP11) were grown over 6 months in a greenhouse under three watering regimes: well watered (HW), moderate soil water deficit (MS) and severe soil water deficit (SS). Transpiration efficiency (W = total dry matter/water transpired) and leaf intrinsic gas exchange efficiency (A/gs = carbon assimilation rate/stomatal conductance) increased under water stress and were positively correlated with the stable carbon isotope composition of leaf tissue (δ13C). The clones did not vary significantly with respect to A/gs and W. However, statistically significant differences were detected among clones in δ13C, A and biomass. W did not differ between the MS and SS regimes, probably due to plant acclimation to increasing soil water deficits. The increase in W with soil water deficits relative to the well watered control was primarily associated with stomatal closure, but was also influenced by differences in respiratory carbon losses (?c) and variation in the leaf-to-air water vapour difference (v). Variance in ?c and v may explain partially why the two levels of soil water deficit were different in regard to δ13C but not in terms of W.