Expl Agric. (1999), volume 35, pp. 167±180 Printed in Great Britain Copyright # 1999 Cambridge University Press INFLUENCE OF PHOSPHORUS APPLICATION TO FLOODWATER ON OXYGEN CONCENTRATIONS AND SURVIVAL OF RICE DURING COMPLETE SUBMERGENCE By G. RAMA KRISHNAYYA, T. L. SETTER{{, R. K. SARKAR, P. KRISHNAN and I. RAVI Division of Plant Physiology, Central Rice Research Institute, Cuttack-753 006, India, and {Agriculture Western Australia, Baron-Hay Court, South Perth, WA 6151, Australia (Accepted 16 October 1997) SUMMARY 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 the ¯oodwater by 1 ppm increased algal growth by at least 4-fold based on chlorophyll concentra- tions, 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 eects 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. INTRODUCTION During the monsoon season in South and Southeast Asia, ¯ash-¯ooding may result in partial or complete submergence of rice crops, and subsequently either complete or partial loss of grain yields may occur. A total of 20 million hectares of rice-growing area is adversely aected by ¯ash-¯ooding (IRRI, 1975), with about half of this occurring in eastern India (Reddy and Sharma, 1992). The frequency of complete or partial submergence is not quanti®ed for any location, but it will depend on several factors including the maximum water depth, duration of ¯ooding, the rate of water increase and decrease, and the plant height and elongation during ¯ooding. Recent data produced by Ram et al. (1999) in eastern India suggest that partial and complete submergence occur at approximately equal frequencies. In this study complete submergence was evaluated since (i) it results in a consistent exposure of all tissues to the same {Author to whom correspondence should be addressed. Email: tsetter@agric.wa.gov.au