Differential Responses of Abaxial and Adaxial Guard Cells of Broad Bean to Abscisic Acid and Calcium

Regulation by abscisic acid (ABA) and Ca(2+) of broad bean (Vicia faba) abaxial and adaxial guard cell movements and inward K(+) currents were compared. One millimolar Ca(2+) in the bathing medium inhibited abaxial stomatal opening by 60% but only inhibited adaxial stomatal opening by 15%. The addition of 1 μm ABA in the bathing medium resulted in 80% inhibition of abaxial but only 45% inhibition of adaxial stomatal opening. Similarly, ABA and Ca(2+) each stimulated greater abaxial stomatal closure than adaxial stomatal closure. Whole-cell patch-clamp results showed that the inward K(+) currents of abaxial guard cells were inhibited by 60% (−180 mV) in the presence of 1.5 μm Ca(2+) in the cytoplasm, whereas the inward K(+) currents of adaxial guard cells were not affected at all by the same treatment. Although 1 μm ABA in the cytoplasm inhibited the inward K(+) currents to a similar extent for both abaxial and adaxial guard cells, the former were more sensitive to ABA applied externally. These results suggest that the abaxial stomata are more sensitive to Ca(2+) 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 Ca(2+)-signaling pathway in adaxial guard cells. Therefore, there may be different pathways for ABA- and Ca(2+)-mediated signal transduction in abaxial and adaxial guard cells.