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Growth and photosynthetic response of lowland rice following complete submergence is related to the concentration of CO2 dissolved in floodwater. Submergence of plants in stagnant solution at low CO2 concentration or solution gassed with air at 0.03 kPa CO2 (equilibrium of 0.01 mol m-3 dissolved CO2) decreased carbohydrates, and little or no growth occurred. Plants submerged in solutions gassed with 3-20 kPa CO2 in air (equilibrium of 0.9-6 mol m-3 CO2) showed at most small decreases in carbohydrates, and growth was up to 100% of the non-submerged plants. At pH 7.5, there was little net photosynthetic O2 evolution by detached submerged leaves even at high HCO3- concentrations, which suggests that these rice leaves could utilise only CO2 and not HCO3-. At pH 6.5, O2 evolution in solutions in equilibrium with 7.4 kPa CO2 was 3-4 fold higher than in solutions in equilibrium with 0.6 kPa CO2. Photorespiration was indicated by a decrease in the rate of net O2 evolution with increasing external O2. In stagnant solutions this reduction of O2 evolution was pronounced; at a CO2 concentration of 0.25 mol m-3 net O2 evolution ceased when the O2 concentration in the water had reached only 0.125 mol m-3. The requirement of photosynthesis for a combination of high CO2 concentrations and low external O2 was presumably due to slow diffusion of these gases in the unstirred layer of solution around the leaves.
Setter et al. (Thu,) studied this question.