Genotypic response and selection pressure on nitrogen‐use efficiency in rice under different nitrogen regimes

Eight rice genotypes including hybrid rice, semidwarf rice, and tall varieties were studied under greenhouse condition with three levels of nitrogen (N) applied at 0.50, 1.00, and 1.50 g N/plant. Grain dry weight, straw biomass (including root), grain, and straw N concentration (GN% and SN%) were measured at maturity. Harvest index (HI), physiological N‐use efficiency (PE), defined as a ration of grain yield to total N absorbed (Nt), uptake efficiency (UE), defined as a ratio of Nt to applied nitrogen (Na), and agronomical N‐use efficiency as a product of PE and UE were calculated based on the measurements. Chlorophyll content (Chl), soluble protein (SP), and photosynthetic N use efficiency (CER/N) of flag and the last second leaves were also measured. There was a significant variation among genotypes, N rates, and the interaction between genotype and N rate for all measured parameters, except GN% among genotypes. Similar selection pressure on PE and UE as the components of AE was observed at low N rate. At moderate N rate, the selection appeared favorable to PE but to UE at high N rate. The effects of measured parameters on PE and UE were different at different N rates, which may explain the difference in selection pressure on PE and UE under different N conditions.