Effects of light intensity and water temperature on oxygen release from roots into water lettuce rhizosphere

The oxygen release rate into the rhizosphere by a floating aquatic plant—water lettuce—was determined under various light intensities (0.0–1.2×105 lx) and water temperatures (10–35 °C). The net specific oxygen release rate was expressed by a model equation comprising the gross oxygen release rate and the rhizosphere respiration terms. Experimental and simulated results show that the net specific oxygen release rate increased with light intensity up to the optimal value, but slight inhibition by higher light intensities was observed at 10–20 °C. With increased water temperature, the respiration rate became larger than the gross oxygen release rate. The maximum net specific oxygen release rate of 11.0–12.5 mg-O2 kg-wet−1 h−1 was obtained at the optimal condition of about 25 °C and 9.0×104–1.1×105 lx. The net oxygen release rate was negligible at 35 °C at any light intensity because the respiration rate was much greater than the gross oxygen release rate into the rhizosphere.