WHOLE-PLANT CO2 EXCHANGE, CARBON PARTITIONING AND GROWTH IN QUERCUS-ROBUR SEEDLINGS EXPOSED TO ELEVATED CO2

Pedunculate oak acorns (Quercus robur L.) were germinated and grown un der nonlimiting nutritional and water conditions in controlled-environ ment greenhouses with ambient (350 mu mol mol(-1)) or elevated (700 mu mol mol(-1)) CO2 concentrations. A semiclosed gas exchange measuremen ts, and (CO2)-C-13 labelling, system (1.5% (CO2)-C-13) was used to sim ultaneously assess (a) the CO2 exchange of both aerial and below-groun d (roots plus soil) compartments of the soil-plant system and (b) the partitioning of the recently photo-assimilated carbon. Measurements we re made during the fast aerial growth phase (July 30) and at the end o f the growing season (October 15). On July 30, whole-plant dry mass ha d been increased by 44% since the beginning of the growing season in t he elevated CO2 treatment, whereas at the end of the growing season th e enhancing effect was only 17%. Elevated CO2 stimulated net CO2 assim ilation rate per unit leaf area (A) in July (+40%), whereas in October this stimulation had disappeared. The respiratory CO2 evolution of th e root-soil compartment (individual plant basis) was stimulated by 35% under the elevated CO2 conditions on July 30, but not on October 15. In July, relative specific allocation (RSA), a parameter expressing th e sink strength, was higher in all compartments under 700 mu mol mol(- 1) compared to 350 mu mol mol(-1). Moreover in root tips, the RSA valu es determined 4 h after the labelling were particularly high (7.8%)wit h elevated CO2, whereas under ambient CO2 RSA values were close to zer o.