Genotypic Differences in Grain Yield and Nitrogen Uptake of Lowland Rice (Oryza sativa L.) under Irrigated and Rainfed Conditions

A field experiment was conducted in the 2005 wet season at the International Rice Research Institute (IRRI), Los Banos, Laguna, Philippines, to assess the differences in grain yield (GY) and nitrogen-use efficiency (NUE) of selected rice (Oryza sativa L.) genotypes, and to determine plant parameters that contribute to the improvement of NUE under irrigated and rainfed lowland conditions. Six upland, five rainfed, and eight irrigated rice genotypes were tested under two nitrogen (N) treatments (0 and 75 kg N ha(-1)) and two water regimes (irrigated and rainfed lowland conditions). Significant differences among the genotypes were observed in GY, yield components, total aboveground plant N uptake (TNU), harvest index (HI), and NUE parameters. Irrigated conditions resulted in 52% higher grain yield and 51% higher TNU compared with rainfed conditions. The average yield gain due to N application was 540 kg ha-1 under irrigated conditions, and 484 kg ha(-1) under rainfed conditions. The increase in GY was significantly correlated with TNU (R(2) = 0.91). The study identified GY, TNU, NUE, and HI as important plant parameters to identify N-efficient genotypes. Under irrigated conditions, N-efficient genotypes were PSBRc9, IR72, IR575114-PMI-5-B-1, CT6510-24-1-2, and PSB Rc80, whereas PSB Rc80, CT6510-24-1-2, IR72, IR575114-PMI-5-B-1, and PSB Rc14 performed well under rainfed conditions. These genotypes possess promising traits for improved N use and high grain yield. The most N-efficient genotypes selected in the experiment performed well in both water regimes, indicating that they can be used in a wide variety of lowland environments.