Effects of hypoxia and elevated carbon dioxide concentration on water flux through Populus roots

Restricted gas exchange between the rhizosphere and aerial environment reduces the concentration of oxygen (O(2)) and elevates the concentration of carbon dioxide (CO(2)) in the root zone, thereby leading to increased resistance to root water uptake. In this study, the effects of hypoxia and 20% CO(2) on water flux (J(v)) through roots of hybrid poplar (Populus trichocarpa Torr. & A. Gray x P. deltoides Bartr. ex Marsh) were measured in detached root systems under pressure in solution culture. Because stomata closed and there was no change in foliar water potential in hypoxic plants, root resistance was measured in detached systems as opposed to using whole plant measurements. However, under aerated conditions root resistance values were similar in intact plants and excised roots. Water fluxes through pressurized root systems treated with nitrogen and low oxygen (< 2% O(2)), elevated CO(2) (20% CO(2)), and low O(2) with elevated CO(2) concentrations were reduced to 40, 51 and 58%, respectively, of J(v) of plants aerated with ambient air. Reductions in J(v) occurred more rapidly in response to elevated CO(2) than to low O(2) concentrations. The effects of low O(2) and elevated CO(2) were not additive. Changes in pH that resulted from elevated CO(2) concentrations did not account for the reduction in J(v). When root systems of intact plants were pretreated for 24 or 48 h with low O(2) concentration, J(v) measured on pressurization was reduced by 33 and 48%, respectively, compared to aerated roots. Stomatal conductance was also reduced, however, so leaf water potential of plants with hypoxic roots were similar to those of aerated controls.