Abstract: Solution culture experiments were conducted to investigate the effect of nitrate/ammonium ratio (50:50, 75:25 and 100:0) on nitrate content, the activities of key enzymes for nitrogen metabolism in pakchoi, and the expression and subcellular localization of nitrate transporter gene in two pakchoi varieties (low-nitrate variety ‘JN4’ and high-nitrate variety ‘HKTX’). The results showed that nitrate contents in leaves of ‘HKTX’ and JN4 subjected to treatment at NO3 - / NH4 + ratio of 100:0 increased respectively by 13.2% and 30.4% compared with treatment at NO3 - / NH4 + ratio of 150:50; nitrate contents in petioles of ‘HKTX’ and ‘JN4’ were increased respectively by 14.3% and 4.9% compared with treatment at NO3 - / NH4 + ratio of 50:50. With the increase of NO3 - / NH4 + ratio, the activities of nitrate reductase (NR) and glutamate synthase (GOGAT) in pakchoi decreased, the activity of nitrite reductase (NiR) increased, and the activity of glutamate dehydrogenase (GDH) initially increased and then decreased. Significant differences in NR, GOGAT, NiR, and GDH were observed between the two varieties. Significant expression of low affinity transporter gene (NRT1) was detected in both varieties, and a higher expression level was observed in ‘JN4’ than in ‘HKTX’. NRT1 was a low affinity nitrate transporter located in the plant cell membrane.

Key words: nitrate/ammonium ratio, nitrate content, key enzymes involved in nitrogen metabolism, nitrate transporter gene, subcellular localization, pakchoi