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

Regulation by abscisic acid (ABA) and Ca²⁺ of broad bean (Vicia faba) abaxial and adaxial guard cell movements and inward K⁺ currents were compared. One millimolar Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ 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 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. Therefore, there may be different pathways for ABA- and Ca²⁺-mediated signal transduction in abaxial and adaxial guard cells.