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

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Abstract
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, Lp, of the barrier separating both compartments was found to be Lp=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 Lp-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.