Submergence of Rice. I. Growth and Photosynthetic Response to CO₂ Enrichment of Floodwater

Abstract

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.