The molecular mechanisms by which higher plant cells take up K+ across the plasma membrane (plasmalemma) remain unknown. Physiological transport studies in a large number of higher plant cell types, including guard cells, have suggested that at least two distinct types of K(+)-uptake mechanisms exist, permitting low-affinity and high-affinity K+ accumulation, respectively. Recent patch clamp studies have revealed the presence of inward-conducting (inward-rectifying) K+ channels in the plasma membrane of higher plant cells. Research on guard cells has suggested that these K+ channels provide a major pathway for proton pump-driven K+ uptake during stomatal opening. In the present study the contribution of inward-rectifying K+ channels to higher plant cell K+ uptake was investigated by examining kinetic properties of guard cell K+ channels in Vicia faba in response to changes in the extracellular K+ concentration. Increasing the extracellular K+ concentration in the range from 0.3 mM to 11.25 mM led to enhancement of inward K+ currents and changes in current-voltage characteristics of K+ channels. The increase in K+ conductance as a function of the extracellular K+ concentration revealed a K(+)-equilibrium dissociation constant (Km) of approximately 3.5 mM, which suggests that inward-rectifying K+ channels can function as a molecular mechanism for low-affinity K+ uptake. Lowering the extracellular K+ concentration in the range from 11 mM to 1 mM induced negative shifts in the activation potential of K+ channels, such that these channels function as a K+ sensor, permitting only K+ uptake. At low extracellular K+ concentrations of 0.3 mM K+, inward-rectifying K+ channels induce hyperpolarization. Results from the present study suggest that inward-rectifying K+ channels constitute an essential molecular mechanism for plant nutrition and growth control by providing a K(+)-sensing and voltage-dependent pathway for low-affinity K+ uptake into higher plant cells and additionally by contributing to plasma membrane potential regulation.