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 e ffusus 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 wid e range of external oxygen demands,,vith the ROL rate only depending on sin k strength at low demands, and no oxygen release into oxidized solutions. I n the titrimetric experiments, roots continued to release oxygen at constan t rates when provided with a constant external oxygen demand. ROL was highe r in J. effusus (9.5 +/- 1 x 10(-7) mol O-2 h(-1) root(-1)) than in J. infl exus (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 re ducing intensities, but that structural limitations to oxygen diffusion rat es prevent ROL from continuing to increase at higher external oxygen demand s.