Outward Current Conducting ion Channels in Tonoplasts of Vigna unguiculata

Patch clamp studies were done on tonoplasts of V unguiculata. Apart from the usually present inward rectifying K+ conducting channels (IRC), vacuoles showed outward rectifying channels (ORC), although less frequently. The ORC are activated at physiological tonoplast potentials, in contrast to IRC, and have lower single channel conductances. Another difference is that ORC have a higher cation anion selectivity (P(K+)/P(Cl-) = 17-22) and a different Ca++ dependence. Addition of BaCl2 to the cytoplasmic side of the tonoplast caused a 50 % decrease in the IRC conductance but did not affect ORC conductance.Outward rectifying channels were observed in root, stem and storage tissue of several other plant species and may form an integral part of higher plant tonoplasts. However, the frequency of appearance in all species was low.