Quantification of Visible Structural Changes of the V 0 V 1 ‐ATPase in the Leaf‐tonoplast of Mesembryanthemum crystallinum by Freeze‐fracture Replicas Prepared During the C 3 ‐Photosynthesis to CAM Transition*

Abstract

Plants of the annual facultative halophyte and facultative CAM-plant, Mesembryanthemum crystallinum L., were irrigated with a solution containing NaCl when they had developed 3 leaf pairs. This treatment induced CAM and the plants were then watered with 400 mM NaCl until the end of the experiment of 37 days. A separate set of plants was simultaneously maintained as non-salt treated controls. Tonoplast vesicles were prepared from the leaves at regular intervals during the time-course of the experiment. Three samples of each preparation were freezed fractured, and carbon/platinum-replicas taken. On a total of 1400 fracture faces the diameters and densities per unit area of intramembraneous particles were measured. The results show an increase in the average diameter of particles from 6.5 nm to 8.5 nm and an increase of the relative amount of fracture faces with high particle densities related to the total of fracture faces obtained; both of which kinetically correlated to CAM induction. This increase in size and density of particles, which are known to belong to the H+-transporting ATPase of the tonoplast. shows independently of and in addition to protein analyses, that an increased amount of ATPase-protein is incorporated into the membrane during CAM induction. Some possible explanations for the increase in ATPase particle size are discussed.