Is the leaf bundle sheath a "smart flux valve" for K⁺ nutrition?

Evidence has started to accumulate that the bundle sheath regulates the passage of water, minerals and metabolites between the mesophyll and the conducting vessels of xylem and phloem within the leaf veins which it envelops. Although potassium (K⁺) nutrition has been studied for several decades, and much is known about the uptake and recirculation of K⁺ within the plant, the potential regulatory role of bundle sheath with regard to K⁺ fluxes has just begun to be addressed. Here we have collected some facts and ideas about these processes.