THE STOMATAL RESPONSE TO CO2 IS LINKED TO CHANGES IN GUARD-CELL ZEAXANTHIN

The mechanisms mediating CO2 sensing and light-CO2 interactions in gua rd cells are unknown, In growth chamber-grown Vicia faba leaves kept u nder constant light (500 mu mol m(-2) s(-1)) and temperature, guard ce ll zeaxanthin content tracked ambient [CO2] and stomatal apertures, In creases in [CO2] from 400 to 1200 cm(3) m(-3) decreased zeaxanthin con tent from 180 to 80 mmol mol-l Chi and decreased stomatal apertures by 7.0 mu m. Changes in zeaxanthin and aperture were reversed when [CO2] was lowered, Guard cell zeaxanthin content was linearly correlated wi th stomatal apertures, In the dark, the CO2-induced changes in stomata l aperture were much smaller, and guard cell zeaxanthin content did no t change with chamber [CO2]. Guard cell zeaxanthin also tracked [CO2] and stomatal aperture in illuminated stomata from epidermal peels. Dit hiothreitol (DTT), an inhibitor of zeaxanthin formation, eliminated CO 2-induced zeaxanthin changes in guard cells from illuminated epidermal peels and reduced the stomatal CO2 response to the level observed in the dark. These data suggest that CO2-dependent changes in the zeaxant hin content of guard cells could modulate CO2-dependent changes of sto matal apertures in the light while a zeaxanthin-independent CO2 sensin g mechanism would modulate the CO2 response in the dark.