Abstract:The drought tolerant (Heinong 44) and sensitive (Heinong 65) genotypes of soybean were chosen as materials to investigate effects of drought stress and consequent re-watering on the key enzyme activities related to nitrogen metabolism, including nitrate reductase(NR), glutamine synthetase(GS) and glutamate synthase(GOGAT) at different growth stages. The relationship between drought resistance and differences among the key enzyme was studied, providing a theoretical basis for the selection of soybean varieties and cultivation of drought-resistant.The results showed that nitrate reductase of the leaves in both varieties presented unimodal curve, whereas glutamine synthetase and glutamate synthase presented bimodal curve in the whole growth period of well-watered plants. There was no significant difference of NR between light and middle drought stress at seeding stage, but there was a significant decrease under the serious drought condition. The GS activity increased with the increasing drought stress, but GOGAT activity decreased with the increasing drought stress. NR activity of drought tolerant genotypes of soybean (Heinong 44) was significantly higher than the well-watered soybean, but there was an adverse result in sensitive (Heinong 65) genotypes of soybean after re-watering treatment at different stages. However, the rusults of GS were in contrast with that of NR. After re-watering, GS activities exhibited an increase but still lower than the control treatment. All of these enzymes activities in the leaves of tolerant drought genotypes were higher than that in the sensitive one.Therefore, the damage of drought stress to soybean was mitigated by increasing NR, GS and GOGAT activities.