Leaf water use efficiency differs between Eucalyptus seedlings from contrasting rainfall environments

This study investigates the putative role of thicker leaves in enhancing photosynthetic capacity and water-use efficiency (WUE) of Eucalyptus species native to xeric environments. Three Eucalyptus species, Eucalyptus grandis Hill. (ex Maiden), E. sideroxylon Cunn. (ex Woolls) and E. occidentalis (Endl.), were grown under well-watered or water-limited conditions in a single compartment of a temperature-controlled glasshouse. Eucalyptus grandis is native to a mesic environment while E. sideroxylon and E. occidentalis are native to xeric environments. Leaves of E. sideroxylon and E. occidentalis were thicker and contained more nitrogen (N) on a leaf-area basis than E. grandis. Leaf gas-exchange measurements indicated that the photosynthetic capacity of E. sideroxylon and E.occidentalis was greater than E. grandis and that stomatal conductance and WUE were negatively correlated. Whole-plant, gas-exchange and carbon-isotope measurements showed that E. sideroxylon and E. occidentalis had lower WUE than E. grandis under both well-watered and water-limited conditions. However, there was no difference in N-use efficiency between species. We suggest that stomatal conductance and leaf N content are functionally linked in these seedlings and conclude that thick leaves can, in some conditions, result in low WUE.