Selection for mutants with low nitrate uptake ability in rice (Oryza sativa)

Screening for mutants deficient in the high affinity system of nitrate uptake was performed using mutagenized M2 population of rice (Oryza sativa, cv. Nipponbare or Kinmaze). For selecting mutants, M2 seedlings were transferred individually to 10 ml solution containing 250 μM potassium nitrate and 500 μM calcium sulphate at 20 or 28°C. After 6 or 24 h, nitrate concentration of the solution was determined with a nitrate selective electrode and the seedlings showing impaired nitrate uptake were selected as nitrate uptake deficient variants. Of 74 variants, three were confirmed to be mutants with low nitrate uptake ability in the M3 generation. Potassium uptake ability also decreased in the mutants. Three mutants were divided into two groups based on the analysis of nitrate reductase (NR, EC 1.6.6.1) activity and chlorate resistance. Two, NUE13 and NUE36, had a lower level of NR activity than the original cultivar and were not resistant to chlorate, while the seedlings of NUE50 had the same level of NR activity as the original cultivar and were more resistant to chlorate than the original cultivar. All mutants were resistant to cesium, a toxic ion analogue for potassium, suggesting that the decreased levels of both nitrate and potassium uptake were coupled to the change of plasma membrane H+‐ATPase activity.