Flood water conditions significantly influenced the photosyn- thesis, yield and nitrogen concentration of rice submerged at maximum tillering stage. The main reason for reduced yield and photosynthesis is the reduced under water light intensity at crop canopy level, light intensity declined sharply with increasing water depth from 5 to 30 and further to 80 cm, resulting in negligible light penetration at canopy level. Panda et al. (2006) reported that submergence under darkness (due to turbidity) decreased the survival percentage in rice due to the degradation of chloroplast, structural and functional integrity and severe inhibition of photosynthesis. Dissolved oxygen in the flood water is another reason for lower photosynthesis and yield in turbid water, DO declined markedly in turbid water, on an average DO in turbid water was 103.1% lower during flooding period as compared to clear water (Appendix 2). This might be due to the consumption of DO at night through respiration by submerged rice and other micro-organisms of flood water depletes oxygen overnight. In contrast, oxygenation of water during the day could be caused by underwater photosynthesis of rice plants (Das et al., 2009; Singh et al., 2009) but in turbid water light is the limiting factor. Silt deposition might be another reason for decreased photosynthesis in turbid water, as silt particle on the leaf surface clogged the stomata and hampered the stomatal conductance reflecting in lower Pn rate. Apart from underwater PAR and DO, leaf N concentration also strongly affected Pn rate after desubmergence through affecting effective tillers, strong positive correlation between Pn rate, Gs and leaf N content supported the above hypothesis (Fig. 4a).