Vacuolar acidity, protein profile, and crystal composition of epidermal bladder cells of the halophyte Mesembryanthemum crystallinum

The halophyte Mesembryanthemum crytallinum L. (ice plant) is marked by giant epidermal bladder cells (EBC). The differentiation of pavement cells into EBC occurs at an early developmental stage. EBC occupy most of the surface area in the aerial parts of salt-stressed mature ice plants. A large vacuolar reservoir for ion and water storage plays an important role in salinity adaptation. To monitor the acidity of the vacuole at different developmental stages of EBC, peels from the abaxial surface were stained with a pH-sensitive dye, neutral red (NR). Presence of both NR-stained (acidic) and NR-unstained (neutral) EBC were found at the juvenile stage in ice plants. Continuous exposure to illumination decreased the acidity of the NR-stained cells. The EBC protein profile illustrated the prominent co-existence of highly acidic and basic proteins in these specialised cells. Major proteins that accumulate in EBC are involved in photosynthesis, sodium compartmentalisation, and defence. Numerous raphide crystals were found in well fertilised ice plants. Salt-stressed cells exhibited changes in the surface charge and element composition of raphide crystals. A disappearance of potassium in the high-salt grown crystals suggests that these crystals might serve as a potassium reservoir to maintain the Na⁺/K⁺ homeostasis in this halophyte.