The purifying tank containing Pontederia cordata was used to treat heavily polluted river water. The relationship between the diurnal variation of plant physiology and water quality was investigated. The study took place from 0800 to 1800 hours and in that period the physiological parameters of the plant and the water quality were analysed. Results indicated that the activity of peroxidase and catalase, the content of soluble protein and the rate of photosynthesis and transpiration were negatively correlated to the concentration of nitrogen and phosphorus in the river water, respectively. Higher sunlight intensity increased the activity of peroxidase and catalase, the content of soluble protein, rate of photosynthesis and transpiration. Sunlight also decreased the concentration of nitrogen and phosphorus, the cations (Al3+, Fe2+ and Ca2+), the anions (Cl−, NO3 − and SO42−) and the oxidation–reduction potential and electrical conductivity of the river water. During the treatment from 0800 to 1800 hours, soluble protein content, photosynthesis rate of the plant and dissolved oxygen concentration of the river water showed a bimodal distribution, with peaks at 1200 and 1400 hours and a decrease at 1300 hours due to decrease in photosynthesis at midday. Peroxidase and catalase activity, soluble protein content, photosynthesis and transpiration rate were highest between 1000 and 1400 hours, while nitrogen and phosphorus concentration of the polluted water decreased significantly during this time. The correlation between plant physiology and water quality provided valuable data on the role of this plant in the ecological restoration of polluted water.