Are carbon and oxygen isotope compositions of bulk leaf material reli-able predictors of water use efficiency in slow-growing drought-adapted species?

Abstract

Regulation of water use efficiency (WUE) is particularly important in Mediterranean ecosystems, where plants are periodically exposed to severe water stress. An important technique to measure WUE is carbon isotope discrimination (D 13C) in leaf material, which represents an integrative measure over the lifetime of a leaf. However, under conditions of limited resources, interpretation of D 13C may not be straightforward. During seasonal changes in environmental conditions regulation may occur through a coherent change in stomatal restriction of CO2 diffusion and down-regulation of photosynthetic capacity, which may result in unchanged WUE. Furthermore, in bulk leaf material of slow-growing evergreen species the influence of seasonal environmental constraints may be buffered by leaf longevity. Recently it has been suggested that the oxygen isotope ratio of bulk leaf material may reflect evaporative conditions and may be used to determine if variations in D 13C result from differences in photosynthetic capacity or from stomatal limitations. Seasonal changes in carbon and oxygen isotope composition of leaf dry matter and water potentials were studied in 11 species of a Mediterranean plant community. Within the ecosystem, there were pronounced differences in D 13C. Drought semi-deciduous species showed lower WUE and a seasonal variability in D 13C, ranging from 18.2¿ to 20.8¿, from autumn to spring, respectively. But even among the evergreen sclerophylls values ranged from 16.3¿ (Juniperus phoenicea) to 19.7¿ (Olea europaea), though with less seasonal variation. The usefulness of additional oxygen isotope analysis of the bulk leaf material to distinguish the above mentioned patterns will be tested.