Silver ions disrupt K ⁺ homeostasis and cellular integrity in intact barley (Hordeum vulgare L.) roots

The heavy metals silver, gold, and mercury can strongly inhibit aquaporin-mediated water flow across plant cell membranes, but critical examinations of their side effects are rare. Here, the short-lived radiotracer ⁴²K is used to demonstrate that these metals, especially silver, profoundly change potassium homeostasis in roots of intact barley (Hordeum vulgare L.) plants, by altering unidirectional K ⁺ fluxes. Doses as low as 5μM AgNO ₃ rapidly reduced K ⁺ influx to 5% that of controls, and brought about pronounced and immediate increases in K ⁺ efflux, while higher doses of Au ³⁺ and Hg ²⁺ were required to produce similar responses. Reduced influx and enhanced efflux of K ⁺ resulted in a net loss of >40% of root tissue K ⁺ during a 15min application of 500μM AgNO ₃, comprising the entire cytosolic potassium pool and about a third of the vacuolar pool. Silver also brought about major losses of UV-absorbing compounds, total electrolytes, and NH4 ⁺. Co-application, with silver, of the channel blockers Cs ⁺, TEA ⁺, or Ca ²⁺, did not affect the enhanced efflux, ruling out the involvement of outwardly rectifying ion channels. Taken together with an examination of propidium iodide staining under confocal microscopy, the results indicate that silver ions affect K ⁺ homeostasis by directly inhibiting K ⁺ influx at lower concentrations, and indirectly inhibiting K ⁺ influx and enhancing K ⁺ efflux, via membrane destruction, at higher concentrations. Ni ²⁺, Cd ²⁺, and Pb ²⁺, three heavy metals not generally known to affect aquaporins, did not enhance K ⁺ efflux or cause propidium iodide incorporation. The study reveals strong and previously unknown effects of major aquaporin inhibitors and recommends caution in their application.