Abstract
Annual variations in the water relations and stomatal response of Erica arborea, Myrtus communis and Juniperus communis occurring at a natural CO2 vent were analysed under Mediterranean field conditions. A distinct gradient of CO2 concentration ([CO2]) exists between two sites near a natural CO2-emitting vent, with higher [CO2] (700 μmol mol-1) in the proximity of the CO2 spring. Plants at the CO2 spring site have been growing for generations at elevated [CO2]. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits during the summer drought were severe. Leaf conductance and water potential recovered after major rainfalls in September to predrought values. Strong relationships between leaf conductance, predawn water potential, and leaf-specific hydraulic resistance are consistent with the role of stomata in regulating plant water status. Considerable between-species variation in sensitivity of water potentials and stomatal characters to elevated [CO2] were observed. Common to all the shrubs were a reduction in leaf conductance and an increase in water potentials in response to elevated [CO2]. Elevated [CO2] decreased the sensitivity of leaf conductance to vapour pressure deficit. Morphological characters (including stomatal density and degree of sclerophylly) showed site-dependent variations, but degree and sign of such changes varied with the species and/or the season. Measurements of discrimination against 13C provided evidence for long-term decreases of water use efficiency in CO2 spring plants. Analysis of C isotope composition suggested that a downward adjustment of photosynthetic capacity may have occurred under elevated [CO2]. Elevated [CO2] effects on water relations and leaf morphology persisted in the long term, but the three shrubs growing in the same environment showed species-specific responses.