Genotypic differences in water use efficiency and (13)C discrimination in Eucalyptus globulus

Cuttings of three Eucalyptus globulus Labill. clones (called SM, VC, DG henceforward) were grown for 56 days in 10-l pots in a greenhouse. Every other day, eight pots of each clone were watered to field capacity (HW treatment), whereas the other eight pots of each clone received only 25% of the water needed to maintain the soil at field capacity (LW treatment). Transpirational water loss, biomass production, leaf gas exchange and water potential (at predawn and midday) were determined at different times during the experiment. Leaf tissue formed by the LW plants after the onset of the treatment was analyzed for (13)C/(12)C ratio against the PeeDee Belemnite standard (delta(13)C). The three clones differed significantly in growth capacity (SM > DG > VC) and in their response to water stress. Even though leaf water potentials were not significantly modified by withholding water, total biomass and plant leaf area were affected significantly by soil water deficits by the end of the experiment. The fastest growing clone (SM) was the most affected by water deficit. Long-term water use efficiency (i.e., biomass per unit of water transpired, WUE) was significantly increased by water deficit and was positively correlated with delta(13)C. Long-term WUE was well correlated with the quotient of the daily integrals of carbon assimilation rate and stomatal conductance. The value of delta(13)C was negatively correlated with the variables normally positively related with growth, such as specific leaf area (SLA) and the biomass/intercepted light quotient (epsilon), and it was positively related to the amount of carbon per unit leaf area, which is usually negatively correlated with relative growth rate.