FIELD COMPARISON OF TRANSPIRATION, STOMATAL CONDUCTANCE AND VULNERABILITY TO CAVITATION OF QUERCUS-PETRAEA AND QUERCUS-ROBUR UNDER WATER-STRESS

Water relations were analysed in adult oaks (Quercus petraea and Q robur) during a period of water shortage in a simplified lysimeter. Sap flux densities and stomatal conductance were reduced by almost-equal-to 70% at maximal drought intensity. Predawn leaf water potential then ranged from -1.7 to -2.0 MPa. The slightly lower transpiration observed in pedunculate oaks could be ascribed to their smaller crown development. Nevertheless, no significant difference in stomatal conductance could be observed between the two species. They also had the same percent loss of conductivity (almost-equal-to 80%) in petioles at maximal drought intensity when midday leaf water potential had dropped to almost-equal-to -3.0 MPa. Finally, good agreement was found between observed losses of hydraulic conductivity during in situ dehydration and the vulnerability curves obtained under laboratory conditions. The shifts in maximal conductivity observed in some droughted trees probably accentuated discrepancies between field and laboratory data. However, a correction procedure helped overcome these artifacts.