Characterization of whole-cell K+ currents across the plasma membrane of pollen grain and tube protoplasts of Lilium longiflorum

Outward and inward currents, mainly carried by K+, were detected in protoplasts of pollen grains (PG) and pollen tubes (PT) of Lilium longiflorum Thunb. by using the whole-cell configuration of the patch-clamp technique. The outward K+ current (IK+out) was similar in both protoplast types, while the inward K+ current (IK+in) was higher in pollen tube protoplasts. In PT but not in PG protoplasts, inward K+ currents were already detectable at negative membrane voltages usually monitored in lily pollen. IK+in consisted of a slow and a fast current component, as revealed by fitting a sum of two exponential functions to the time-dependent current. The contribution of the fast component to the total inward current was higher in PT than in PG protoplasts, which was even more evident at acidic pH of the external medium. Therefore, based on the measured characteristics, the IK+in. of PT protoplasts may contribute to the endogenous K+ currents surrounding a growing pollen tube.