Concentrations of CO₂ and O₂ in floodwater and in internodal lacunae of floating rice growing at 1–2 metre water depths

Abstract

Abstract. Environment and plant measurements were made to determine what factors may limit growth of deepwater and floating rice plants during partial or complete submergence. Field surveys included measurements of temperature, pH, light, O 2 and CO 2 in floodwater in Thailand. In addition, measurements were made of O 2 and CO 2 concentrations inside internodal lacunae of deepwater and floating rice growing at 0.5–2.0 m water depths. The bulk of measurements were taken during periods when the changes in water level were less than 50 mm d −1 . In the 0–0.02 m surface layer of floodwater at any location there were large changes in oxygen concentrations over diurnal cycles: there were decreases during the night down to 0.02–0.18 mol m −3 O 2 at 0600 h and increases during the day to 0.13–0.28 mol m −3 O 2 at 1500 h (0.28 mol m −3 being 120% of the O 2 concentration of air saturated water at 30°C). During the day oxygen concentrations decreased with increasing water depth; concentrations just above the soil surface were occasionally zero. Most of this gradient disappeared during the night, and at dawn the 0.6 m surface layer of water had uniform low O 2 concentrations. O 2 concentrations were also measured during flash floods in Thailand. In contrast to the conditions with only small increases in water level, the O 2 concentrations in the water during flash floods were more uniform with depth and changed little over a diurnal cycle, the O 2 ranging between 0.14–0.19 mol m −3 . In most locations floodwater contained 0.2–1.9 mol m −3 CO 2 and 0.7–1.6 mol m −3 bicarbonate; however, in a location with acid sulphate soil CO 2 was only 0.05–0.2 mol m −3 , and bicarbonate concentrations were several fold lower. Concentrations of CO 2 in floodwater increased with increasing water depth. O 2 and CO 2 concentrations inside internodal lacunae of rice were determined in the field when water depth were 1–2 m. Concentrations of O 2 in internodes at the water surface were 16–20%, and decreased to 10% and 5% at 0.8 and 1.8 m water depth respectively. There was no diurnal cycle in O 2 concentrations inside internodes. In contrast, CO 2 concentrations in the lacunae increased with water depth and ranged from 1–3% in internodes at the water surface to 5–10% in internodes at 1.8 m water depth. There was evidence for a diurnal cycle in CO 2 concentrations in the basal internode near the soil surface, CO 2 increased during the day and decreased during the night. The above data are used to show that there is little or no relationship between gas concentrations in floodwater and internodal lacunae of rice plants. Results are discussed in relation to O 2 supply to submerged portions of rice and metabolism of these tissues at low O 2 concentrations.