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 stress-induced modulation of signal transduction in guard cells. Plants were irrigated with a non-saline medium to promote C3 photosynthesis or, alternatively, a medium containing NaCl in order to induce Crassulacean acid metabolism (CAM). For plants grown on a non-saline medium, blue light was more effective than red in causing stomatal apertures to widen. This differential response to light quality by stomata, which is typically attributed to separate photoreceptor systems in C3 plants, was not observed for stomata in epidermal tissue isolated from CAM plants. 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 large opening response by stomata from C3 plants. Despite a lack of opening 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-induced opening response by stomata following induction of CAM in M. crystallinum may be a result of inactivation of both guard-cell photoreceptors, which are functional in unstressed (C3) plants.