Abstract: Using Viciafaba L. as the experimental material, the authors had demonstrated a kind of binding protein at the outside of plasmalemma which was highly affined to ABA. The optimum pH for the binding was 6.5; the binding activity at 25 ℃ was relatively higher than that at 0 ℃ and reaching its maximum after 30 minutes incubation. The dissociation constant of the ABA binding to the protein was 2.0 Ï 10-8 mol/L. There were about 3.2 Ï 106 binding sites per guard cell protoplast(GCP). There was functionally essential disulfide bond in the protein and the function of the protein was related to the existence of Ca2 + in the medium.

Key words: Vicia faba, Guard cell protoplasts, ABA binding proteins