Root exudates are derived from plant metabolites and their composition is affected by the plant nutritional status. The purpose of this study was to examine soybean root exudates under phosphorus (P) deficiency using a metabolite profiling technique. Glycine max L. (cv. Suzuyutaka) was grown in a culture solution at P concentrations of 0 (P0) and 8 (P8) mg P L −1 for 1, 5, 10 and 15 days after transplanting. Shoot extract, root extract and root exudates were collected and their metabolites were determined by capillary electrophoresis/ time-of-flight mass spectrometry (CE-TOF MS). The shoot P concentration and dry weight of soybean plants grown at P0 were lower than those at P8. One hundred and eight, 116 and 79 metabolites were identified in the shoot extract, root extract and root exudates, respectively. The concentrations of several metabolites including amino acids and organic acids in root exudates were higher at P0 than at P8, irrespective of the P concentration in the shoot or root extract. These findings suggest that soybean roots actively release metabolites in response to P deficiency.

Root exudates improve the nutrient acquisition of plants and affect rhizosphere microbial communities. The plant nutrient status affects the composition of root exudates. The purpose of this study was to examine common bean (Phaseolus vulgaris L.) root exudates under phosphorus (P) deficiency using a metabolite profiling technique. Common bean plants were grown in a culture solution at P concentrations of 0 (P0), 1 (P1) and 8 (P8) mg P L−1 for 1, 10 and 20 days after transplanting (DAT). Root exudates were collected, and their metabolites were determined by capillary electrophoresis time-of-flight mass spectrometry (CE-TOF MS). The shoot P concentration and dry weight of common bean plants grown at P0 were lower than those grown at P8. One hundred and fifty-nine, 203 and 212 metabolites were identified in the root exudates, and 16% (26/159), 13% (26/203) and 9% (20/212) of metabolites showed a P0/P8 ratio higher than 2.0 at 1, 10 and 20 DAT, respectively. The relative peak areas of several metabolites, including organic acids and amino acids, in root exudates were higher at P0 than at P8. These results suggest that more than 10% of primary and secondary metabolites are induced to exude from roots of common bean by P deficiency.

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Environmental samples such as soil solutions contain inorganic ions such as NH4(+), K(+), Na(+), NO3(-), and PO4(3-) in high concentrations, which must be removed prior to capillary electrophoresis-mass spectrometry analysis to obtain accurate results. However, the separation of these inorganic ions from ionic metabolites, which are the target compounds in capillary electrophoresis-mass spectrometry analysis, is difficult because the physicochemical properties of the inorganic ions are similar to those of the ionic metabolites. In this study, we used various solid-phase extraction (SPE) columns for the purification of the samples containing inorganic ions in high concentrations. We found that cation-exchange SPE columns successfully filtered out the inorganic ions while retaining most of the organic compounds, which were easily collected with high recovery rates. In addition, 17 cationic metabolites in the soil solution were quantified by CE-MS analysis following the SPE purification process. The results suggest that our method can be used to analyze other environmental samples containing inorganic ions in high concentrations.