Hypoxia-induced changes in net H+, K+and O2fluxes acrossthe plasma membrane (PM) of epidermal root cells weremeasured using the non-invasive microelectrode ion fluxmeasurement (MIFE) system in elongation, meristem andmature root zones of two barley (Hordeum vulgareL.)varieties contrasting in their waterlogging (WL) tolerance.The ultimate goal of this study was to shed light on themechanisms underlying effects of WL on plant nutrientacquisition and mechanisms of WL tolerance in barley. Ourmeasurements revealed that functionally different barleyroot zones have rather different O2requirements, with thehighest O2influx being in the elongation zone of the rootat about 1 mm from the tip. Oxygen deprivation has qualitativelydifferent effects on the activity of PM ion transportersin mature and elongation zones. In the mature zone,hypoxic treatment caused a very sharp decline in K+uptakein the WL sensitive variety Naso Nijo, but did not reduceK+influx in the WL tolerant TX9425 variety. In the elongationzone, onset of hypoxia enhanced K+uptake from rootsof both cultivars. Pharmacological experiments suggestedthat hypoxia-induced K+flux responses are likely to bemediated by both K+-inward- (KIR) and non-selective cationchannels (NSCC) in the elongation zone, while in themature zone K+-outward- (KOR) channels are the keycontributors. Overall, our results suggest that oxygen deprivationhas an immediate and substantial effect on root ionflux patterns, and that this effect is different in WL-sensitiveand WL-tolerant cultivars. To what extent this differencein ion flux response to hypoxia is a factor conferring WLtolerance in barley remains to be answered in future studies.