THE EFFECTS OF ELEVATED CO2 AND WATER-STRESS ON WHOLE-PLANT CO2 EXCHANGE, CARBON ALLOCATION AND OSMOREGULATION IN OAK SEEDLINGS

Seedlings of Quercus robur L grown under present (350 mu mol mol(-1)) or twice the present (700 mu mol mol(-1)) atmospheric CO2 concentratio ns, were either maintained well-watered or subjected to a drought cons traint late in the growing season (25 August 1993). Despite an initial stimulation of biomass growth (+44%) by elevated CO2, there was no si gnificant difference in plant dry weight at the end of the growing sea son (15 October 1993) between the two CO2 treatments, irrespective of watering regime. Under drought conditions, although there was no growt h increase in response to elevated CO2 concentration, there was a stim ulation in net photosynthesis. In addition, the respiration rate of th e root + soil system (root dry matter basis) was slightly lower in the elevated than in the ambient CO2 concentration. These results, togeth er with the results from short-term C-13 labelling, suggest enhanced p lant carbon losses through processes not assessed here (aerial respira tion, root exudation, etc) under elevated CO2 concentration. In the dr oughted conditions, new carbon relative specific allocation values (RS A) were greater under elevated CO2 than under ambient CO2 concentratio n in both leaf and root compartments. Osmotic potentials at full turgo r (pi(o)) were lowered in response to water stress in leaves by 0.4 MP a for the elevated CO2 treatment only. In roots, osmotic adjustment (0 .3 MPa) occurred in both the CO2 treatments.