EFFECT OF APOPLASTIC AND CYTOPLASMIC PH ON INWARD AND OUTWARD RECTIFYING PLASMA-MEMBRANE K-THALIANA CULTURED-CELLS( CHANNELS OF ARABIDOPSIS)

The possible regulation of plasmamembrane K+ channels by apoplastic an d cytoplasmic pi-Is was investigated by means of the patch-clamp techn ique in the whole-cell configuration in protoplasts isolated from cell suspension cultures of Arabidopsis thaliana (L.) Heynh. The increase of the apoplastic pH from 5.2 to 8.2 stimulated the activity of inward rectifying K+ channels (IRCs) by increasing the amplitude and the act ivation rate of the currents, whereas it did not influence the activit y of outward rectifying K+ channels (ORCs). These results are in contr ast with those reported for guard cells, where the alkalinization of t he external pH inhibited (Blatt, 1992) or did not influence (Schwartz et al., 1994) IRC activity and stimulated ORC activity (Ilan et al., 1 994). In agreement with the findings obtained in guard cells (Blatt, 1 992) the decrease of cytoplasmic pH; induced by means of a permeant we ak acid, increased both the amplitude and the activation rate of the c urrents flowing through IRCs and it decreased the activity of ORCs by reducing the current amplitude without affecting the activation rate. The results provide evidence for a pH effect:on plasma membrane K+ cha nnels in ground tissues as already reported for guard cells, although some peculiar characteristics are observed in the different types of c ells.