Influence of phosphorus application to floodwater on oxygen concentrationsand survival of rice during complete submergence

Low concentrations of phosphorous were added to the floodwater of submerged rice in an attempt to regulate algal growth, so as to increase floodwater O-2 concentration and plant survival during submergence for up to 12 days. Increasing the phosphorus concentration of the floodwater by 1 ppm increase d algal growth by at least 4-fold based on chlorophyll concentrations, and increased floodwater O-2 concentrations to over twice air saturation. Howev er, additions of phosphorus reduced rice plant survival during submergence by up to 35%. The adverse effects of a high phosphorus concentration in the floodwater were mainly attributed to competition between the algae and the submerged rice for CO2 and, perhaps, light. The importance of photosynthes is during the submergence of rice was supported ly the results of experimen ts in which floodwater CO2 concentration was manipulated by altering pH. Th e survival of an intolerant rice cultivar during submergence increased from 0 to 17 and 62% at pH 8, 7 and 5 respectively, while floodwater CO2 concen trations at these pHs would have increased from 0.02 to 0.3 and 1.0 mol m(- 3) respectively. The results were used to question the importance of floodw ater O-2 concentrations above anoxia for submergence tolerance of rice.