Abstract: The discharge of wastewater from livestock farms with high-concentration nitrogen and phosphorus causes huge environmental pollution. Spirodela polyrrhiza has a strong ability to purify water pollutants. However, studies on the absorption and purification of nitrogen and phosphorus by different geographic populations of S. polyrrhiza are still lacking. In this study, 15 S. polyrrhiza populations were collected from different regions of southern China to examine the differences in physiological response and absorption capacity of nitrogen and phosphorus in waters of different geographic populations of S. polyrrhiza. The results showed that the growth phenotypes and physiological indicators (number of thallus, fresh weight, dry weight, root length, and protein content) of S. polyrrhiza significantly differed among the geographic populations (P<0.001). The physiological response and absorption ability of different geographic populations of S. polyrrhiza to nitrogen and phosphorus in water were quite different. Different geographic populations of S. polyrrhiza had different tolerance ability to high nitrogen concentration, with the nitrogen tolerance index ranging from 64.47% to 119.42%. Among them, Kunming and Leshan populations had the highest nitrogen absorption capacity (0.12 and 0.11 mg·pot^-1·d^-1, respectively). The tolerance of different geographic populations to high phosphorus was quite different, with the phosphorus tolerance index ranging from 81.22% to 157.73%. The phosphorus absorption ability of Dali and Wuhan populations was the highest (0.06 and 0.05 mg·pot^-1·d^-1, respectively). The number of thallus, biomass, and tolerance were significantly positively correlated to purifying ability of nitrogen and phosphorus across different geographic populations, indicating that those factors play an important role in purifying nitrogen and phosphorus. This study revealed the physiological response of different geographic populations of S. polyrrhiza to high concentrations of nitrogen and phosphorus and the differences in the ability to purify nitrogen and phosphorus in water at the growth phenotype and physiological level. Meanwhile, we obtained S. polyrrhiza populations with ideal purification effects on nitrogen and phosphorus pollution in waters. The results will provide a scientific basis for the rational use of S. polyrrhiza resources for water purification.

Key words: Spirodela polyrrhiza, geographical population, nitrogen and phosphorus purification, number of thallus, physiological response.