Abstract: | In developing seed ofVicia faba L., solutes imported throughthe phloem of the coats move symplastically from the sieve elementsto a specialized set of cells (the thin-walled parenchyma transfercells) for release to the seed apoplast. Potassium (K+) is thepredominant cation released from the seed coats. To elucidatethe mechanisms of K+ efflux from seed coat to seed apoplast,whole-cell currents across the plasma membranes of protoplastsof thin-walled parenchyma transfer cells were measured usingthe whole-cell patch-clamp technique. Membrane depolarizationelicited a time-dependent and an instantaneous outward current.The reversal potential (ER of the time-dependent outward currentwas close to the potassium equilibrium potential (EK and itshifted in the same direction as EK upon changing the externalK+ concentration, indicating that this current was largely carriedby an efflux of K+. The activation of the time-dependent outwardK+ current could be well fitted by two exponential componentsplus a constant. The instantaneous outward current could alsobe carried by K+ efflux as suggested by ion substitution experiments.These K+ outward rectifier currents elicited by membrane depolarizationare probably too small to represent the mechanism for the normalK+ efflux from seed coat cells. Membrane hyperpolarization morenegative than 80 mV activated a time-dependent inwardcurrent. K+ influx was responsible for the inward current asthe current reversed at membrane voltage close to EK and shiftedin the same direction as EK when external K+] was varied. Activationof this K+inward rectifier current was well fitted with twoexponential components plus a constant. A regulating functionfor this current is suggested. Key words: Potassium outward rectifier, potassium inward rectifier, transfer cell protoplast, seed coat, Vicia faba L |