Effects of glucose on the uptake and metabolism of glycine in pakchoi (Brassica chinensis L.) exposed to various nitrogen sources

BACKGROUND: Plants can absorb amino acids as a nitrogen (N) source, and glucose is an important part of root rhizodeposition and the soil sugar pool, which participates in the regulation of plant growth and uptake. In pakchoi, the effect of glucose concentration on the glycine N uptake from a nutrient mixture composed of glycine, ammonium, and nitrate, or from a single N solution of glycine alone was studied using specific substrate (15)N-labeling and (15)N-gas chromatography mass spectrometry. RESULTS: The optimal glucose concentration for plant growth was 4.5 μM or 25 μM when supplied with glycine alone or the N mixture, respectively, and resulted in a >25% increase in seedling biomass. The addition of glucose affected the relative contribution from organic or inorganic sources to overall N uptake. When glucose was added at optimal concentrations, glycine was preferentially used as an N source, while the relative contribution from nitrate was reduced. The limiting step for glycine N contribution was active uptake in the roots in high glucose and single-N-source conditions; however, root metabolism of glycine to serine was limiting in high-glucose and mixed-N-source conditions. CONCLUSIONS: The addition of low concentrations of glucose increased the relative uptake of organic nitrogen and reduced the uptake of nitrate, suggesting a feasible way to decrease nitrate content and increase the edible quality of vegetables.