Effects of nitrate supply on plant growth, nitrate accumulation, metabolic nitrate concentration and nitrate reductase activity in three leafy vegetables

Three leafy vegetables, rape (Brassica campestris L.), Chinese cabbage (Brassica chinensis var. Oleifera Makino et Nenoto) and spinach (Spinacia oleracea L.), were grown in plastic pots with 5 kg soil per pot at five nitrate supply rates, 0.00 (N-1), 0.15 (N-2), 0.30 (N-3) 0.45 (N-4), and 0.60 (N-5) g N kg(-1) soil to investigate the effects of nitrate supply on plant growth, nitrate accumulation and nitrate reductase activity (NRA) 9 weeks after sowing. The optimum yield appeared at N-3, while above N-4, a strong decrease in plant growth occurred. The nitrate concentration increased with nitrate supply in the whole plant and the different organs except in roots where nitrate concentration at N-5 decreased compared with N-4. The nitrate concentration in both the metabolic pool (MP) and the storage pool (SP) of the leaf blades increased with nitrate supply. From N-1 to W NRA increased most rapidly. The highest NRA occurred at N-4. However, nitrate reductase (NR) activities were not significantly different between N-3, N-4 and N-5, which imply that there is a threshold of nitrate concentration in MP (NMP) to induce NRA. The parameters of NR for nitrate were measured by the in vivo method. The K-m values we obtained were similar to the reported values by the in vitro method. which confirms the feasibility of the anaerobic method for determining NRA and NMP. Finally, the effects of the posttranslational regulation of NR were discussed. (C) 2004 Elsevier Ireland Ltd. All rights reserved.