Water relations of the epidermal bladder cells of the halophytic species Mesembryanthemum crystallinum: Direct measurements of hydrostatic pressure and hydraulic conductivity.

Water exchange between the bladder cells of stems of the halophytic species Mesembryanthemum crystallinum and the subepidermal cortical cell layers was determined by direct cell turgor pressure measurements using a pressure probe. Treating the system bladder cell/subepidermal cortex as two homogeneous elastic compartments the hydraulic conductivity, L p, of the barrier separating both compartments was found to be L p=2·10(-6) cm ·s(-1)·bar(-1). As discussed, this value essentially reflects the hydraulic conductivity of the bladder cell membrane, which is unusually high compared with the values expected for higher plant cells. The L p-values did not show a dependence on cell turgor pressure over a large pressure range (0.1 to 6 bar) nor on the salinity of the bladder sap (osmolarity range: 500 to 1700 mOsmol). Furthermore, the stationary pressure-values obtained in the pressure-flow experiments point to unusually high permeabilities of the bladder membrane to electrolytes (KCl and NaCl). The unique transport properties of the bladder cell membrane for salt and water suggest a special physiological function of the bladders in the water economy of the plant. It is assumed that the bladders form a reservoir for salts and water in the plant which upon osmotic stress acts as a buffering system to protect the photosynthetic tissue from osmotic injury.