Changes in plasmalemma organization in cowpea radicle during imbibition in water and NaCl solutions

Abstract Freeze-fracture electron microscopy of the plasmalemma of dry cowpea radicle cells disclosed a normal-appearing membrane with a high area density of intramembraneous particles (IMPs). Seeds imbibed in either water or salt solution exhibited decreased area density of IMPs, but water-imbibed tissue showed the greatest decline. Mean particle size increased with hydration but not enough to suggest aggregation as the cause for this density decrease. Calculations of plasmalemma area expansion during imbibition show that such expansion can account for the decrease in IMPs per unit area in the cytoplasmic side (PF) of the membrane in water-imbibed, but not in salt-imbibed, tissue. During imbibition, there is a change in the ratio of IMPs per unit area of the PF versus EF (external) membrane faces, suggesting a relative increase in the number of EF particles. These changes in membrane structure are probably not related to any decrease in membrane permeability during the early phases of imbibition.