Rapid, Futile K⁺ Cycling and Pool-Size Dynamics Define Low-Affinity Potassium Transport in Barley

Using the short-lived radiotracer ⁴²K⁺, we present a comprehensive subcellular flux analysis of low-affinity K⁺ transport in plants. We overturn the paradigm of cytosolic K⁺ pool-size homeostasis and demonstrate that low-affinity K⁺ transport is characterized by futile cycling of K⁺ at the plasma membrane. Using two methods of compartmental analysis in intact seedlings of barley (Hordeum vulgare L. cv Klondike), we present data for steady-state unidirectional influx, efflux, net flux, cytosolic pool size, and exchange kinetics, and show that, with increasing external [K⁺] ([K⁺]ext), both influx and efflux increase dramatically, and that the ratio of efflux to influx exceeds 70% at [K⁺]ext >= 20 mM. Increasing [K⁺]ext, furthermore, leads to a shortening of the half-time for cytosolic K⁺ exchange, to values 2 to 3 times lower than are characteristic of high-affinity transport. Cytosolic K⁺ concentrations are shown to vary between 40 and 200 mM, depending on [K⁺]ext, on nitrogen treatment (NO₃⁻ or NH₄⁺), and on the dominant mode of transport (high- or low-affinity transport), illustrating the dynamic nature of the cytosolic K⁺ pool, rather than its homeostatic maintenance. Based on measurements of trans-plasma membrane electrical potential, estimates of cytosolic K⁺ pool size, and the magnitude of unidirectional K⁺ fluxes, we describe efflux as the most energetically demanding of the cellular K⁺ fluxes that constitute low-affinity transport.