Vacuolar pH Measurement in Higher Plant Cells

[(14)C]Methylamine is rapidly accumulated by Acer pseudoplatanus cells cultivated in liquid medium. The accumulation ratio of intracellular concentration to the extracellular one reaches, within 60 minutes, values as high as 3,000. This lipophilic amine appears to enter the cells through a diffusion process and is probably mainly accumulated as a cation inside the large acidic vacuolar compartment.A large discrepancy has been observed between the vacuolar pH calculated from the distribution of methylamine molecules between the cells and their culture medium and the pH of the sap extracted from frozen-thawed cells, the pH of the sap being higher than the calculated values.THE PH ESTIMATED THROUGH THE USE OF THE METHYLAMINE METHOD MUST BE CONSIDERED WITH SOME CARE AS EVIDENCE HAS BEEN OBTAINED SHOWING THAT THE INTRACELLULAR ACCUMULATION OF THIS BASE DEPENDS NOT ONLY ON THE VACUOLAR PH BUT ALSO ON: (a) the transplasmalemma and transtonoplast potential differences; (b) the relative permeabilities of these membranes to the nonionized and ionized form of the base; and (c) the adsorption of methylamine on cellular constituents.