Long-distance signaling within Coleus x hybridus leaves; mediated by changes in intra-leaf CO2?

Rapid long-distance signaling in plants can occur via several mechanisms, i ncluding symplastic electric coupling and pressure waves. We show here in v ariegated Coleus leaves a rapid propagation of electrical signals that appe ars to be caused by changes in intra-leaf CO2 concentrations. Green leaf ce lls, when illuminated. undergo a rapid depolarization of their membrane pot ential (V-m) and an increase in their apoplastic pH (pH(a)) by a process th at requires photosynthesis. This is followed by a slower hyperpolarization of V-m and apoplastic acidification, which do not require photosynthesis. W hite (chlorophyll-lacking) leaf cells, when in isolated white leaf segments , show only the slow response, but when in mixed (i.e. green and white) seg ments, the rapid V-m depolarization and increase in pH(a) propagate over mo re than 10 mm from the green to the white cells. Similarly, these responses propagate 12-20 mm from illuminated to unilluminated green cells. The fact that the propagation of these responses is eliminated when the leaf air sp aces are infiltrated with solution indicates that the signal moves in the a poplast rather than the symplast. A depolarization of the mesophyll cells i s induced in the dark by a decrease in apoplastic CO2 but not by an increas e in pH(a). These results support the hypothesis that the propagating signa l for the depolarization of the white mesophyll cells is a photosynthetical ly induced decrease in the CO2 level of the air spaces throughout the leaf.