Abstract
Seasonal changes in tissue water relations of Erica arborea L., Myrtus communis L. and Juniperus communis L., grown in a Mediterranean environment, were analysed under field conditions over a 12 month period by comparing plants grown in the proximity of a natural CO 2 spring (about 700 μmol mol -1 atmospheric CO 2 concentration, [CO 2]) with plants in ambient conditions. Tissue water relations varied in response to changes in water availability, but the seasonal course of tissue water relations parameters was also related to ontogeny. Tissue water relations of these co-occurring shrubs were not alike. Osmotic potentials and saturated mass/dry mass ratio were lowest during peak drought stress periods. Diurnal changes in osmotic potential at the point of turgor loss were least early in the season, maximal in mid-season, and decreased again in autumn. Turgor potentials decreased as drought progressed and were highest in late fall and mid-winter. Symplastic water fraction was highest in mid-spring for E. arborea and M. communis and decreased during the summer, while the opposite was observed for J. communis. Common to all species, under elevated [CO 2], was an increase of turgor pressure, particularly during the summer months. Other parameters showed species-specific responses to long-term elevated [CO 2]. In particular, exposure to elevated [CO 2] increased osmotic potentials in E. arborea under drought, while the opposite was the case for J. communis. Site differences in predawn to midday shifts were not strong in any of the species. Differences in tissue water relations suggest that the coexistence of these shrubs in the same environment with similar water availability are partially based on differential water relations strategies and water use patterns. Regardless of the mechanisms, growth of these shrubs in elevated [CO 2] may be either less, similarly or more affected by drought stress than plants in ambient [CO 2] depending on the species and season.