Influence of Phosphorus Application to Floodwater on Oxygen Concentrations and 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 increased algal growth by at least 4-fold based on chlorophyll concentrations, and increased floodwater O 2 concentrations to over twice air saturation. However, 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 CO 2 and, perhaps, light. The importance of photosynthesis during the submergence of rice was supported by the results of experiments in which floodwater CO 2 concentration was manipulated by altering pH. The survival of an intolerant rice cultivar during submergence increased from 0 to 17 and 62% at pH 8, 7 and 5 respectively, while floodwater CO 2 concentrations 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 floodwater O 2 concentrations above anoxia for submergence tolerance of rice.