Effect of external oxygen demand on radial oxygen loss by Juncus roots in titanium citrate solutions

Radial oxygen loss (ROL) from the roots of two semiaquatic rushes, Juncus effusus L. and Juncus inflexus L., was studied in reducing titanium citrate buffer, using both closed incubations and a dow-through, titrimetric system, In dosed experiments, roots released oxygen at a constant rate over a wide range of external oxygen demands,,vith the ROL rate only depending on sink strength at low demands, and no oxygen release into oxidized solutions. In the titrimetric experiments, roots continued to release oxygen at constant rates when provided with a constant external oxygen demand. ROL was higher in J. effusus (9.5 +/- 1 x 10(-7) mol O-2 h(-1) root(-1)) than in J. inflexus (4.5 +/- 0.5 x 10(-7) mol O-2 h-(1) root(-1)). Light and dark changes around the shoots did not affect the ROL rate in J, inflexus, whereas in J, effusus ROL was approximate to 1.75 times higher in the light than in the dark, presumably due to changes in stomatal aperture. These results suggest that ROL is controlled by the external oxygen demand at low to moderate reducing intensities, but that structural limitations to oxygen diffusion rates prevent ROL from continuing to increase at higher external oxygen demands.