INDUCTION OF CAM IN MESEMBRYANTHEMUM-CRYSTALLINUM ABOLISHES THE STOMATAL RESPONSE TO BLUE-LIGHT AND LIGHT-DEPENDENT ZEAXANTHIN FORMATION INGUARD-CELL CHLOROPLASTS

Facultative CAM plants such as Mesembryanthemum crystallinum (ice plan t) possess C3 metabolism when unstressed but develop CAM under water o r salt stress, When ice plants shift from C3 metabolism to CAM, their stomata remain closed during the day and open at night, Recent studies have shown that the stomatal response of ice plants in the C3 mode de pends solely on the guard cell response to blue light. Recent evidence for a possible role of the xanthophyll, zeaxanthin in blue light phot oreception of guard cells led to the question of whether changes in th e regulation of the xanthophyll cycle in guard cells parallel the shif t from diurnal to nocturnal stomatal opening associated with CAM induc tion, In the present study, light-dependent stomatal opening and the o peration of the xanthophyll cycle were characterized in guard cells is olated from ice plants shifting from C3 metabolism to CAM, Stomata in epidermis detached from leaves with C3 metabolism opened in response t o white light and blue light, but they did not open in response to red light, Guard cells from these leaves showed light-dependent conversio n of violaxanthin to zeaxanthin, Induction of CAM by NaCl abolished bo th white light- and blue light-stimulated stomatal opening and light-d ependent zeaxanthin formation, When guard cells isolated from leaves w ith CAM were treated with 100 mM ascorbate, pH 5.0 for 1 h in darkness , guard cell zeaxanthin content increased at rates equal to or higher than those stimulated by light in guard cells from leaves in the C3 mo de, The ascorbate effect indicates that chloroplasts in guard cells fr om leaves with CAM retain their competence to operate the xanthophyll cycle, but that zeaxanthin formation does not take place in the light, The data suggest that inhibition of light-dependent zeaxanthin format ion in guard cells might be one of the regulatory steps mediating the shift from diurnal to nocturnal stomatal opening typical of plants wit h CAM.