Application of ³⁹K NMR spectroscopy to potassium transport in mung bean root tips

The intracellular K⁺ concentration and its change in mung bean [Vigna mungo (L.) Hepper] root tips were investigated non-invasively with ³⁹K nuclear magnetic resonance spectroscopy using a membrane impermeable shift reagent, dysprosium (III) tripolyphosphate [Dy(PPP_i)^7-₂]. The K⁺ resonance was shifted to higher magnetic field in proportion to the concentration of the shift reagent. In addition to a reference capillary peak for measuring the K⁺ concentration, two well-resolved peaks (intra- and extracellular K⁺ resonances) were observed in the 39K NMR spectra of mung bean root tips. The intracellular K⁺ concentration was determined to be 41 mM, which was similar to the value obtained by flame photometry. When 20 mM KCl was added to the external medium, the intensity of the intracellular K⁺ resonance gradually increased and the net K⁺ uptake rate was calculated to be 4.1 micromoles per gram fresh weight per hour. After removal of KCl from the perfusion medium, the intracellular K⁺ concentration considerably decreased. With ³¹P NMR method, 2.5 mM Dy(PPP_j)^7-1₂ and 20 mM KCl had little effect on the ATP level in the cells. We have indicated that the ³⁹K NMR method can be used to determine the K⁺ levels and net fluxes of the K⁺ transport in perfused root tips successively.