RETENTION OF VACUOLE CONTENTS OF PLANT-CELLS DURING FIXATION

Changes in the semi-permeability of tonoplast during fixation were stu died using beetroot tissue. Using vacuole betacyanin as the indicator, the permeability of the tonoplast was assessed by the leakage of this pigment as determined by changes in the optical density of the soluti on bathing the tissue. Cryo-analytical scanning electron microscopy wa s used to monitor the changes in ion concentration in the cells during fixation. Fixatives were 3% glutaraldehyde or 4% formaldehyde in 0.02 5 M phosphate buffer at room temperature or on ice. Results showed tha t glutaraldehyde, especially at low temperature, takes as long as 30 h to disrupt the semi-permeability of the tonoplast of beetroot cells, while in formaldehyde on ice, these cells begin to lose their selectiv e permeability in 15 min. This study confirms that the semi-permeabili ty of the tonoplast may not be lost until long after the cytoplasm has been fixed and suggests that this explains why cold fixation in 3% gl utaraldehyde for about 12 h has become the most reliable standard proc edure for the successful preservation of vacuolated plant cells.