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Nitrate Reductase Activities in Rice Genotypes in Irrigated Lowlands

Philippine Rice Research Institute, Maligaya, Muñoz, Nueva Ecija, 3119, Philippines
National Agriculture Research Center, 3-1-1 Kannondai, Tsukuba, Ibaraki, 305-0856, Japan
Faculty of Agriculture, Kagawa Univ., Miki, Kagawa, 761-0795, Japan

^* Corresponding author (pbd{at}mozcom.com).

Nitrate reductase (NR) is a key enzyme involved in nitrate assimilation in crops. In irrigated lowlands, specific activities of NADH- and NADPH-NR in rice (Oryza saliva L.) have not been investigated. The objectives of this study were to evaluate the nitrate reductase activity (NRA) patterns of rice genotypes with different NRA levels under irrigated lowland conditions, and to determine the effects of NR deficiency on nitrate use, grain yield, biomass, and other agronomic characteristics. Seven cultivars and seven NR mutants were grown under irrigated lowland conditions; bioassayed in vitro for NRA at different growth stages; and evaluated for grain yield, biomass, and other agronomic characters. NADH-NRA was present at minimal levels in all cultivars before transplanting. At 15 d after transplanting (DAT), NADH-NRA in all cultivars reached optimum levels and gradually declined until maturity. A similar pattern was observed in most of the NR mutants but with lower levels of activity. NADPHNRA in cultivars and mutants gradually increased from 15 to 45 DAT and then fluctuated between 5 to 10 nmol NO^–₂ min^–1 g^–1 fresh weight until maturity. Nitrate availability in the soil may significantly influence NRA and nitrate accumulation in rice in irrigated lowlands. Effects of low or deficient levels of NR on traits evaluated varied among mutants. A decrease of 27 to 73% in grain yield was observed in mutants relative to original cultivars. This study demonstrates that rice assimilates nitrate in anaerobic and flooded conditions, which may have implications for genetic improvement and nitrate or nitrogen management strategies in rice.

Received for publication September 23, 1997.