Osmotic water permeability of cell membranes from <i>Mesembryanthemum crystallinum</i> leaves: effects of age and salinity

Abstract

The osmotic water permeability ( P os ) of cell membranes isolated from leaves of 40‐, 50‐ and 60‐day‐old Mesembryanthemum crystallinum plants was estimated by measuring light‐scattering kinetics using stopped‐flow spectrophotometry. The measurements were performed on the plasma membrane (PM), purified tonoplast (TP), and TP‐enriched vesicles. The PM and TP‐enriched vesicles were obtained by partitioning the microsomal fraction in an aqueous polymer two‐phase system, whereas the purified TP vesicles were prepared by microsomal vesicle flotation on a sucrose cushion. The P os of isolated membranes declined with plant age. The kinetic experiments showed that there was no difference between the P os of the PM and TP isolated from plants of all ages. A 24‐h exposure of plants to 400 m M NaCl caused a decline in the P os as well. These findings suggest that, during M. crystallinum transition to CAM, which was induced by plant ageing or salinity, plant osmoregulatory responses included changes in the P os of the leaf‐cell membranes. These variations in the P os are discussed in the context of adaptive mechanisms responsible for the maintenance of the water balance in the common ice plant.