WATER RELATIONS OF OAK SPECIES GROWING IN THE NATURAL CO2 SPRING OF RAPOLANO (CENTRAL ITALY)

The effect of elevated atmospheric carbon dioxide on water relations w as examined on downy oak (Quercus pubescens) and helm oak (Q ilex) tre es. The study was conducted on trees growing in a naturally enriched C O2 spring. Sap velocity and sap flow were measured by the heat pulse t echnique. On the same trees, daily courses of xylem water potential, l eaf conductance and transpiration were monitored. Plant water relation s were evaluated by pressure-volume analysis method on shoots; on the same branches, relative conductivity of xylem was measured. Both speci es exhibited increased osmotic potential and decreased symplasmic frac tion of water in trees adapted to increased CO2. Downy oak showed lowe r stomatal conductance under elevated CO2, but helm oak did not. Both species displayed higher sap flow in control trees. In both species, i ncreased carbon dioxide did not influence xylem embolism formation.