Metabolomic analysis of night-released soybean root exudates under high- and low-K conditions

Root exudates of soybean grown under low-, normal-, and high-K+ conditions were evaluated using metabolomics. Two soybean cultivars, ‘Satonohohoemi’ (low-K-tolerant) and ‘Tachinagaha’ (low-K sensitive), were hydroponically grown with 6 (K6), 60 (K60), and 120 (K120) mg K L−1 for 7 and 14 days after transplanting (DAT). Root exudates were collected, and metabolites were evaluated using capillary electrophoresis time-of-flight mass spectrometry. Rroot K concentration was lower in K6 and higher in K120 compared to K60. Number of metabolites detected in ‘Satonohohoemi’ and ‘Tachinagaha’ was 43 and 39, respectively. Score plot of the principal component analysis (PCA) showed a clear grouping of the root exudate metabolites based one three K concentrations and two sampling times, with the first PCA accounting for 65.7% and 71.5% of the variance for the ‘Satonohohoemi’ and ‘Tachinagaha’, respectively. High-K conditions enhanced release of root exudate metabolites to rhizosphere in both soybean cultivars from 18% to 42%. However, low-K conditions resulted in a 1.2-fold increase and a 2.5-fold decrease in the release of root exudate metabolites in ‘Tachinagaha’ and ‘Satonohohoemi’, respectively, at 7 and 14 DAT. These results reveal that soybean roots exude many metabolites in response to high- and low-K conditions. The number of increased metabolites was higher after long-term (14 DAT) compared to that of the short-term (7 DAT) of exposure of high- and low-K conditions. Soybean cultivars differ in the capacity to release metabolites by altering the exudation of specific metabolites for a better adaptation to the high- and low-K conditions.