Abstract:Based on the previous investigations of the dissociation constant and m~Mmum binding capacity of ABA binding proteins per guard cell protoplast(GCP) in Viciafaba L., the authors have furthered their study in the effects of pH and light/dark on the activity of ABA binding proteins. The results showed that pH did not change the dissociation constant but at different pH the number of the hotrod ABA molecules per GCP was different. After illumination for 90 min the dissociation constant Df the ABA binding proteins was increased and the ABA binding capacity per GCP was decreased. Both effects were reversed when GCPs were kept in fully dark for 90 min. The results indicated that the change of stomata sensitivity of ABA by light/dark could be regulated through the change of dissociation contant to the ABA binding proteins.