NET OXYGEN AND CARBON-DIOXIDE BALANCES IN SOLUTIONS BATHING ROOTS OF WETLAND PLANTS

(1) A procedure and experimental apparatus are described that permit determination of oxygen and carbon-dioxide generation in nutrient cultures bathing roots of mature wetland plants. Roots ot five different species were isolated from the atmosphere so that the only significant route of gaseous transfer was through the plant itself. (2) At the level of the whole plant, and on a time scale of weeks, the net effect of wetland plants on the root environment was generally not to oxygenate it. Although plants transported large amounts of oxygen between shoots and roots, most of that oxygen was consumed before it reached the bulk solution. Oxygen content of solutions bathing the root systems typically remained at or below 31-mu-mol l-1. However. solutions were greatly enriched in carbon dioxide, containing c. 1-5 mmol l-1. The data are consistent with the hypothesis that the plants transported large amounts of O2 to the roots but that most of the O2 was consumed by respiration within the plant or by microbes supported by carbon compounds derived from the plant. (3) Rates of oxygen generation within root zone solutions generally were well below 35-mu-mol O2 plant-1 day-1 for most experimental plants. Only Scirpus acutus generated oxygen in solution at rates as high as 58-mu-mol O2 plant-1 day-1 (4) Microscopic analysis of roots revealed a high proportion of cortical aerenchyma (40-48%) in 86-99% of the sampled lengths of primary roots.