ON THE DIFFICULTIES OF MEASURING OXYGEN RELEASE BY ROOT SYSTEMS OF WETLAND PLANTS

1 The suitability of bathing whole root systems in nutrient solutions for measuring root oxygen release was re-examined. The rate of oxygen release from the roots of the wetland plant Juncus ingens was measured in oxygen-depleted solution (oxygen concentration < 15 mumol l-1) and in a titanium (III) citrate buffer with a low redox potential similar to that of natural sediments (E(H) < - 200 mV). 2 When incubated in a solution containing 15 mumol O2 l-1 in a closed chamber, J ingens roots consumed oxygen until all the oxygen in the chamber was exhausted and there was zero net oxygen exchange between the roots and the solution. 3 When oxygen-depleted water was passed through the chamber, the rate of release ranged from 0 to 1.52 mumol h-1 g-1 root dry wt; oxygen release rates were lower when the chamber solution was stirred than when it was unstirred. 4 When titanium citrate was used in the closed chamber, the rate of root oxygen release was > 80 times higher than in the open-circuit system. The oxygen release in this buffer remained high until all the Ti3+ ions were oxidized and there was no external oxygen demand, after which it was undetectable. 5 The high rates of root oxygen release in titanium citrate buffer occur because it could scavenge the oxygen as it was released, preventing it from being reabsorbed by the respiring root tissue. 6 The results demonstrate how oxygen-depleted solutions can be unsuitable for measuring oxygen release by entire root systems. Explanations for this are discussed and it is concluded that recent doubts concerning the quantitative significance of oxygen release by roots, which were based on such methods, are without foundation.