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An analysis was carried out of the mechanism of malic-acid efflux from vacuoles of mesophyll cells of the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana. Following its accumulation in the vacuole as a result of nocturnal CO2 fixation, the malic acid is passively transported back across the tonoplast in the subsequent light period and is decarboxylated in the cytoplasm. Malic-acid efflux was studied using leaf slices in solution or by following malic-acid utilization (deacidification) in leaves of intact plants. Samples of leaf-cell sap were taken at different times during the day-night rhythm to establish the relation between cell-sap pH and malate content. From the empirically determined pK values for malic acid in the cell sap, it was then possible to calculate the proportion of malate existing as the undissociated acid (H2mal0) and in the anionic forms (Hmal1- and mal2-) for all times during the CAM rhythm. In leaf-slice experiments it has been found that the rate of malic-acid efflux increases exponentially with the malic-acid content of the tissue. This is shown to be related to the increasing amounts of H2mal0 present at high malic-acid contents. At low malic-acid contents (<65 mol m-3), when H2mal0 is not present in significant amounts, efflux must be in the form of Hmal-1 and/or mal2-. At high malic-acid contents it is suggested that efflux occurs predominantly in the form of passive, noncatalyzed diffusion of H2mal0 across the tonoplast by a 'lipid-solution' mechanism. This is supported by the fact that the slope of the curve relating efflux to H2mal0 concentration, when corrected for the presumed contributions from Hmal1- and mal2- transport and plotted on a log-log basis, approaches 1.0 at the highest malic-acid contents. Moreover, the permeability coefficient required to be consistent with such a mechanism {Mathematical expression} is similar to that estimated from a Collander plot, using the partition coefficient of malic acid between ether and water. We suggest that {Mathematical expression} may be important in determining the maximum amounts of malic acid that can be accumulated during the CAM rhythm. © 1984 Springer-Verlag New York Inc.

Original publication

DOI

10.1007/BF01868979

Type

Journal article

Journal

The Journal of Membrane Biology

Publication Date

01/06/1984

Volume

81

Pages

149 - 158