Membrane potentials and salt distribution in epidermal bladders and photosynthetic tissue of Mesembryanthemum crystallinum L.

Abstract Membrane potentials (pd's) of epidermal bladder cells and green leaf cells of Mesembryanthemum crystallinum L. are rather low (between ?10 and ?40 mV). During growth on 400 mM NaCl membrane potentials decrease further. As shown previously, plants grown on 400 mM NaCl show the diurnal changes of malate levels typical for plants having crassulacean acid metabolism. Therefore, in this study membrane potentials were measured at different times of the day, but no diurnal variations of pd were correlated with diurnal oscillations of malate levels. Resting potentials are similar in bladder cells and in green leaf cells and are similar in continuous light and darkness. Both bladder and leaf cells display light-triggered photosynthesis-dependent oscillations of the membrane potential although the bladder cells do not appear to be photosynthetically very active. This suggests electrical coupling between the bladder cells and the underlying green cells. Over a larger distance (2 mm) in the leaves, however, there is no direct evidence for electrical coupling. Cl^?, Na⁺, and K⁺ concentrations are similar in bladder cells and in the photosynthetically active tissue of leaves and stems. Bladder cells appear to contain high concentrations of free oxalate. The present findings corroborate earlier conclusions that the epidermal bladder cells of M. crystallinum function as peripheral water reservoirs providing protection from short term water stress.