MECHANISMS OF SOLUTE EFFLUX FROM SEED COATS - WHOLE-CELL K-FABA L( CURRENTS IN TRANSFER CELL PROTOPLASTS DERIVED FROM COATS OF DEVELOPING SEEDS OF VICIA)

In developing seed of Vicia faba L., solutes imported through the phlo em of the coats move symplastically from the sieve elements to a speci alized set of cells (the thin-walled parenchyma transfer cells) for re lease to the seed apoplast. Potassium (K+) is the predominant cation r eleased from the seed coats. To elucidate the mechanisms of K+ efflux from seed coat to seed apoplast, whole-cell currents across the plasma membranes of protoplasts of thin-walled parenchyma transfer cells wer e measured using the whole-cell patch-clamp technique. Membrane depola rization elicited a time-dependent and an instantaneous outward curren t. The reversal potential (E-R) of the time-dependent outward current was close to the potassium equilibrium potential (E-K) and it shifted in the same direction as E, upon changing the external K+ concentratio n, indicating that this current was largely carried by an efflux of K. The activation of the time-dependent outward K+ current could be wel l fitted by two exponential components plus a constant. The instantane ous outward current could also be carried by K+ efflux as suggested by ion substitution experiments. These K+ outward rectifier currents eli cited by membrane depolarization are probably too small to represent t he mechanism for the normal K+ efflux from seed coat cells. Membrane h yperpolarization more negative than -80 mV activated a time-dependent inward current. K+ influx was responsible for the inward current as th e current reversed at membrane voltage close to E-K and shifted in the same direction as E-K when external [K+] was varied. Activation of th is K+ inward rectifier current was well fitted with two exponential co mponents plus a constant. A regulating function for this current is su ggested.