Seasonal patterns of tissue water relations in three Mediterranean shrubs co-occurring at a natural CO2 spring

Seasonal changes in tissue water relations of Erica arborea L., Myrtus comm unis L. and Juniperus communis L., grown in a Mediterranean environment, we re analysed under field conditions over a 12 month period by comparing plan ts grown in the proximity of a natural CO2 spring (about 700 mu mol mol(-1) atmospheric CO2 concentration, [CO2]) with plants in ambient conditions. T issue water relations varied in response to changes in water availability, but the seasonal course of tissue water relations parameters was also relat ed to ontogeny. Tissue water relations of these co-occurring shrubs were no t alike. Osmotic potentials and saturated mass/dry mass ratio were lowest d uring peak drought stress periods. Diurnal changes in osmotic potential at the point of turgor loss were least early in the season, maximal in mid-sea son, and decreased again in autumn. Turgor potentials decreased as drought progressed and were highest in late fall and mid-winter. Symplastic water f raction was highest in mid-spring for E. arborea and M. communis and decrea sed during the summer, while the opposite was observed for J. communis. Com mon to all species, under elevated [CO2], was an increase of turgor pressur e, particularly during the summer months. Other parameters showed species-s pecific responses to long-term elevated [CO2]. In particular, exposure to e levated [CO2] increased osmotic potentials in E. arborea under drought, whi le the opposite was the case for J. communis. Site differences in predawn t o midday shifts were not strong in any of the species. Differences in tissu e water relations suggest that the coexistence of these shrubs in the same environment with similar water availability are partially based on differen tial water relations strategies and water use patterns. Regardless of the m echanisms, growth of these shrubs in elevated [CO2] may be either less, sim ilarly or more affected by drought stress than plants in ambient [CO2] depe nding on the species and season.