Differential responses of abaxial and adaxial guard cells of broad bean toabscisic acid and calcium

Regulation by abscisic acid (ABA) and Ca2+ of broad bean (Vicia faba) abaxi al and adaxial guard cell movements and inward K+ currents were compared. O ne millimolar Ca2+ in the bathing medium inhibited abaxial stomatal opening by 60% but only inhibited adaxial stomatal opening by 15%. The addition of 1 mu M ABA in the bathing medium resulted in 80% inhibition of abaxial but only 45% inhibition of adaxial stomatal opening. Similarly, ABA and Ca2+ e ach stimulated greater abaxial stomatal closure than adaxial stomatal closu re. Whole-cell patch-clamp results showed that the inward K+ currents of ab axial guard cells were inhibited by 60% (-180 mV) in the presence of 1.5 mu M Ca2+ in the cytoplasm, whereas the inward K+ currents of adaxial guard c ells were not affected at all by the same treatment. Although 1 mu M ABA in the cytoplasm inhibited the inward K+ currents to a similar extent for bot h abaxial and adaxial guard cells, the former were more sensitive to ABA ap plied externally. These results suggest that the abaxial stomata are more s ensitive to Ca2+ and ABA than adaxial stomata in regard to stomatal opening and closing processes and that the regulation of the inward K+ currents by ABA may not proceed via a Ca2+-signaling pathway in adaxial guard cells. T herefore, there may be different pathways for ABA- and Ca2+-mediated signal transduction in abaxial and adaxial guard cells.