Knowledge on relationship between the rate of root respiration and dissolved oxygen (DO) concentration is essential for understanding the oxygen balance around roots in hydroponics. An airtight hydroponic system for the measurement of root respiration of lettuce was constructed. The initial concentrations of DO were 26 μmol⋅liter^-1 (“treatment one-tenth”), 130 μmol⋅liter^-1 (“treatment one-half”), 260 μmol⋅liter^-1 (“treatment saturation”), and 380 μmol⋅liter^-1 (“treatment over-saturation”) . From the patterns of decreasing DO concentration during a 48 hr measurement period, the minimum DO concentration for root respiration and the rate of root respiration were calculated. The minimum DO concentration for the root respiration of lettuce 26 days after transplanting was around 78.2 μmol⋅liter^-1. The changes of DO concentration in treatments with plants four and ten days after transplanting were small.
The rate of root respiration was presented as a function of DO concentration, and increased as the DO concentration of nutrient solution increased. For DO concentrations in the range from 190 to 230 μmol⋅liter^-1 which is usually observed in well aerated hydroponic beds, the rate of root respiration of lettuce 26 days after transplanting ranged from 2.9 to 6.7 μmol⋅hr^-1⋅ (ml roots) ^-1 at 23°C. In “treatment saturation, ” and larger root growth and greater amount of transpiration were attained in this treatment than in other treatments.