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 rob ur) during a period of water shortage in a simplified lysimeter. Sap f lux densities and stomatal conductance were reduced by almost-equal-to 70% at maximal drought intensity. Predawn leaf water potential then r anged from -1.7 to -2.0 MPa. The slightly lower transpiration observed in pedunculate oaks could be ascribed to their smaller crown developm ent. Nevertheless, no significant difference in stomatal conductance c ould be observed between the two species. They also had the same perce nt loss of conductivity (almost-equal-to 80%) in petioles at maximal d rought intensity when midday leaf water potential had dropped to almos t-equal-to -3.0 MPa. Finally, good agreement was found between observe d 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 acce ntuated discrepancies between field and laboratory data. However, a co rrection procedure helped overcome these artifacts.