The effects of elevated CO 2 and water stress on whole plant CO 2 exchange, carbon allocation and osmoregulation in oak seedlings

Abstract

Seedlings of Quercus robur L grown under present (350 mu-mol mol-1) or twice the present (700 mu-mol mol-1) atmospheric CO-2 concentrations, were either maintained well-watered or subjected to a drought constraint late in the growing season (25 August 1993). Despite an initial stimulation of biomass growth (+44%) by elevated CO-2, there was no significant difference in plant dry weight at the end of the growing season (15 October 1993) between the two CO-2 treatments, irrespective of watering regime. Under drought conditions, although there was no growth increase in response to elevated CO-2 concentration, there was a stimulation in net photosynthesis. In addition, the respiration rate of the root + soil system (root dry matter basis) was slightly lower in the elevated than in the ambient CO-2 concentration. These results, together with the results from short-term 13C labelling, suggest enhanced plant carbon losses through processes not assessed here (aerial respiration, root exudation, etc) under elevated CO-2 concentration. In the droughted conditions, new carbon relative specific allocation values (RSA) were greater under elevated CO-2 than under ambient CO-2 concentration in both leaf and root compartments. Osmotic potentials at full turgor (pi-o) were lowered in response to water stress in leaves by 0.4 MPa for the elevated CO-2 treatment only. In roots, osmotic adjustment (0.3 MPa) occurred in both the CO-2 treatments.