This study was conducted to investigate the physiological characteristics of nitrogen efficiency(NUE) of semi-winter and spring wheat varieties. The activities of key enzymes involved in nitrogen metabolism were determined and the correlations between the activities of these enzymes and NUE were analyzed. At grain filling stage,the activities of nitrate reductase( NR),glutamate synthase( GOGAT) and glutamine synthetase( GS) in the flag leaves and the grains of semi-winter wheat varieties were higher than th...