To investigate the possibility of simultaneously reducing nitrate and oxalate by amino acid application just before harvesting in spinach, plants were cultivated hydroponically until 7 days before harvesting, and then were treated by adding glutamine (1, 2, 3, 4 and 5 mM), glycine, glycolate, and glyoxylate (1, 3 and 5 mM) to nutrient solution. The treated plants were harvested after 7 days of treatment, and analyzed regarding their nitrate and oxalate concentrations of leaf blades, and nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate-glyoxylate aminotransferase (GGAT) activities of leaf blades. The nutrient solution on treatment of glutamine, glycolate and glyoxylate became cloudy, especially at high concentrations. These phenomena led to decreases in the fresh weight of leaf blades and petioles. However, the fresh weights on treatment with glycine were comparable to those of the control. The application of glycine at 5 mM simultaneously decreased nitrate and oxalate concentrations of leaf blades. The NR activity of glycine treatment at 5 mM was markedly decreased, possibly caused by the depletion of nitrate as the substrate for NR. These results indicate that the simultaneous reduction of nitrate and oxalate by glycine application occurred by the deactivation of NR with a decrease in nitrate, diminishing the role of oxalate as a neutralizer for the reduction reaction of nitrate.
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