Low concentrations of phosphorous were added to the¯oodwater of submerged rice in an attempt to regulate algal growth, so as to increase¯oodwater O 2 concentration and plant survival during submergence for up to 12 days. Increasing the phosphorus concentration of thē oodwater by 1 ppm increased algal growth by at least 4-fold based on chlorophyll concentrations, and increased¯oodwater O 2 concentrations to over twice air saturation. However, additions of phosphorus reduced rice plant survival during submergence by up to 35%. The adverse eects of a high phosphorus concentration in the¯oodwater 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¯oodwater 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¯oodwater CO 2 concentrations at these pHs would have increased from 0.02 to 0.3 and 1.0 mol m 73 respectively. The results were used to question the importance of¯oodwater O 2 concentrations above anoxia for submergence tolerance of rice.

Leaf senescence in the rice (Oryza sativa) cultivars FR13A and IR42 under submergence was assessed in terms of changes in total chlorophyll, soluble amino acids and protein concentrations and peroxidase activity in crude extracts. The objective was to determine whether delay in leaf senescence was related to the submergence tolerance of a rice cultivar. Submergence induced senescence and its extent was notably dierent in the two cultivars. Results indicated a reduction in chlorophyll and protein concentrations in leaves during submergence but an increase in amino acid concentration and peroxidase activity. These relative changes were more pronounced in submergence intolerant IR42 than in tolerant FR13A even at four days after complete submergence. When plants were desubmerged and returned to standard irrigated conditions after 10 days of complete submergence the rate of recovery was dierent for the two cultivars. FR13A showed a greater protein and chlorophyll restoring ability compared with the intolerant IR42. The dierences observed between the responses of FR13A and IR42 to submergence were likely to be due to dierences in proteolysis. Senescence of leaves due to submergence was similar to the senescence of non-submerged excised leaves. Results demonstrate that, in the two cultivars studied, leaf senescence is an important biochemical mechanism in plants under submergence and its slower development in tolerant cultivars is, in part, responsible for submergence tolerance.