ELECTRICAL-PROPERTIES OF INTACT POLLEN GRAINS OF LILIUM-LONGIFLORUM - CHARACTERISTICS OF THE NON-GERMINATING POLLEN GRAIN

As a first step to characterizing membrane function during the process of germination in pollen, the transport characteristics of Lilium longiflorum Thunb, pollen were examined in the quiescent grains. Membrane voltages were recorded, and conventional voltage-clamp measurements were carried out with double-barrelled microelectrodes, The resting membrane voltage (V-m) was found to depend on the external K+ concentration and generally followed the equilibrium potential for K+ (I-k,I-in). In some cells a more negative V-m was measured indicating the presence of an electrogenic pump. The presence of a pump current was also detected by a depolarization of the plasma membrane after inhibition of ATP synthesis by addition of CN- and SHAM. Besides this pump, two other current components were found in the plasma membrane of ungerminated pollen grains: an outward potassium current (I-k,I-out) and an inward K+ current (I-k,I-in). Outward K+ currents were detected at membrane voltages more positive than the resting voltage and were blocked by externally applied TEA(+) and Ba2+. The voltage at which I-k,I-out was first detected shifted in parallel with the equilibrium potential for K+. By contrast, activation of I-k,I-in was less affected by the external K+ concentration, except that the magnitude of the inward current increased with K+ concentration. The detected current components may be involved in initiation of osmotic water influx for germination by allowing a K+ influx after the membrane voltage has been driven more negative than E(k) by an electrogenic pump.