Carbon dioxide and ferricyanide parallel each other to inhibit Commelina stomatal opening in a putative Ca2+-independent fashion

It has been suggested that both CO2 and the non-permeating electron acceptor ferricyanide (FeCN) inhibit stomatal opening through interfering with guard cell proton extrusion. Whether the CO2 and FeCN inhibitory effects result from a common process was studied by comparing these effects in epidermal peels of Commelina communis (L.). Phenylarsine oxide (PAO) inhibited stomatal opening in response to fusicoccin (FC) and the auxin, indolyl-3-butyric acid (IBA), but not in response to the membrane-permeable derivative of the auxin putative second messenger, 8-bromoguanosine 3′,5′-cyclic monophosphate (8-Br-cGMP). Since PAO has been previously shown to inhibit FC-induced allosteric activation of the plant plasma membrane H+-ATPase from the low-activity state to the high-activity state, the H+-ATPase activity state could be high in response to FC or IBA, but not in response to 8-Br-cGMP. CO2 and FeCN paralleled each other through extent and kinetics of their inhibition of stomatal opening. When added 1 h before applying either FC, IBA or 8-Br-cGMP, the H+-ATPase activator, butyric acid, removed the CO2 and FeCN inhibitory effects. These CO2 and FeCN effects were higher on the 8-Br-cGMP response and, only in this case, pretreating the peels with FC removed them. Together, these results show a close parallel between CO2 and FeCN in inhibiting stomatal opening. They were discussed on the basis of a possible competition for reducing power between the functioning of an electrogenic plasma membrane H+-ATPase and CO2 fixation or FeCN reduction. Such a competition would agree with a Ca2+-independent inhibition of stomatal opening, which was pharmacologically supported by the fact that the CO2 and FeCN inhibitory effects were not modified by the potent Ca2+ buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N ′,N ′-tetraacetic acid.