Potassium transport in bundle sheath cells of maize and its role for plant nutrition

Abstract

Studies with laser microprobe mass analysis (LAMMA) on minor veins of maize leaves revealed that transport of rubidium (as a tracer for potassium) from the xylem vessel to the symplasts occurs via the plasmalemma of the adjacent bundle sheath cells. Because of the suberin layer surrounding the bundle sheath cells, mesophyll cells and bundle sheath cells face different apoplasts and have to adjust their transport facilities to this situation. Whole-cell patch-clamp studies on isolated bundle sheath cells and mesophyll cells revealed three different kinds of potassium channel which were distinguished by their temporal behaviour, their pH dependence and their permeability to rubidium. The channels were attributed to mesophyll cells (MM-2), bundle sheath cells with contact to the xylem vessels (MM-2) and bundle sheath cells without contact to the xylem vessels (MM-1), respectively. Those in bundle sheath cells with contact to the xylem were stimulated by an acidic xylem fluid indicating a special role of these potassium channels for charge balance during uptake of anions by cotransport whereas those in the mesophyll cells were suppressed by acid medium. Gluconate instead of chloride revealed a considerable conductance for chloride which under normal conditions masked a hyperpolarisation of membrane potential as caused by an obviously strong electrogenic proton pump.