MODULATION OF LIGHT-DEPENDENT STOMATAL OPENING IN ISOLATED EPIDERMIS FOLLOWING INDUCTION OF CRASSULACEAN ACID METABOLISM IN MESEMBRYANTHEMUM-CRYSTALLINUM L

Light quality-dependent stomatal opening in epidermal tissue isolated from Mesembryanthemum crystallinum was investigated to determine stres s-induced modulation of signal transduction in guard cells. Plants wer e irrigated with a non-saline medium to promote C-3 photosynthesis or, alternatively, a medium containing NaCl in order to induce Crassulace an acid metabolism (CAM). For plants grown on a non-saline medium, blu e light was more effective than red in causing stomatal apertures to w iden. This differential response to light quality by stomata, which is typically attributed to separate photoreceptor systems in C-3 plants, was not observed for stomata in epidermal tissue isolated from CAM pl ants. Regardless of color, stomata prepared from saline-treated plants showed no opening response to light, but rather displayed a degree of closing (approximately 12%) compared to aperture widths measured from dark-adapted epidermal tissue. Stomata from CAM plants also failed to show further opening when treated with fusicoccin, in contrast to a l arge opening response by stomata from C-3 plants. Despite a lack of op ening by stomata isolated from CAM plants in response to either light or fusicoccin, apertures retained the capacity to close in response to (S)-abscisic acid. The results suggest that the lack of a photo-induc ed opening response by stomata following induction of CAM in M. crysta llinum may be a result of inactivation of both guard-cell photorecepto rs, which are functional in unstressed (C-3) plants.