Malate Accumulation in Leaf Slices of Mesembryanthemum crystallinum in Relation to Osmotic Gradients Between the Cells and the Medium

Abstract

Leaf slices of two sets of M. crystallinum plants were used in the present study. The first set were plants grown in 400 mM NaCl and showing diurnal oscillations of malate levels typical of crassulacean acid metabolism (CAM). The second set were plants grown in non-saline media and exhibiting no CAM-like diurnal malate fluctuations. Both sets of leaf slices accumulated malate during a 12-h light or dark period, depending on the osmotic pressure of the incubation medium. Highest malate accumulations were obtained when media were isotonic or slightly hypertonic. These osmotic characteristics are similar to those of leaf slices of the CAM plant Kalanchoe daigremontiana as reported elsewhere. However, discrepancies are observed in light and temperature dependence. Unlike in K. daigremontiana leaf slices and in intact leaves with CAM (i.e. also in intact leaves of M. crystallinum grown on highly saline media), in both sets of M. crystallinum leaf slices used here light stimulated malate accumulation. Compared to 15°C, 25°C had either no effect on malate accumulation or stimulated malate accumulation. After leaf slices had accumulated malate in the dark in isotonic or slightly hypertonic media, malate accumulation continued in the light when the osmolarity of the medium remained unchanged. When the osmotic pressure of the medium was lowered considerably, however, malate accumulation in the light was much reduced or else there was a loss of malate from the tissue. Mechanisms different from CAM may be partially involved in the changes of malate levels in these experiments. The significance of the experimental results for the interpretation of the balance between net carbon gain via C₃ pathway and CAM in M. crystallinum is discussed.