Leaf water relations and stomatal behavior of four allopatric Eucalyptus species planted in Mediterranean southwestern Australia

In 1986, four allopatric Eucalyptus species (E. camaldulensis Dehnh, E. saligna Smith, E. leucoxylon F. Muell and E. platypus Hook.) were planted together in a 480-mm rainfall zone, in 8-m wide contour belts as part of a plan to minimize waterlogging and secondary salinization. Throughout 1997, 1998 and 1999, there was significant inter-specific variation in predawn leaf water potential (Ψ(pd)); however, maximum stomatal conductance (g(sm)) only differed significantly between species in mid to late summer. Relationships between g(sm) and Ψ(pd) were significant and showed that stomata of E. camaldulensis were significantly more sensitive to Ψ(pd), and presumably soil water potential, than stomata of E. leucoxylon or E. platypus. When applied to the Ψ(pd) data, these relationships predicted that g(sm), and by inference transpiration, varied much less between species than Ψ(pd). Diurnal measurements throughout the season confirmed this prediction, and showed that E. camaldulensis and E. saligna avoided drought by gaining access to deeper water, whereas E. leucoxylon and E. platypus maintained greater g(sm) at a given water stress than E. camaldulensis or E. saligna. Osmotic potentials measured after rehydration and water release curves of the leaves indicated that different mechanisms accounted for the apparent drought tolerance of E. leucoxylon and E. platypus. In summer, E. leucoxylon reduced osmotic potential at full and zero turgot by similar amounts compared with winter. In summer, E. platypus had a significantly lower bulk elastic modulus and relative water content at turgor loss point than E. camaldulensis, E. saligna or E. leucoxylon. This elastic adjustment resulted in a larger difference between osmotic potential at full and zero turgor in summer than in winter. The inherently low osmotic potential in E. leucoxylon and elastic adjustment in E. platypus resulted in turgor loss at a similar and significantly lower water potential than in E. camaldulensis or E. saligna. These results have implications for species selection for planting to manage groundwater recharge in areas prone to water-logging and secondary salinization.