Effects of Osmotic Gradients on Vacuolar Malic Acid Storage

Malate synthesis by CO(2) dark fixation and malate accumulation in the vacuoles of leaf slices of Kalanchoë daigremontiana Hamet et Perrier, a plant performing crassulacean acid metabolism, occurs only in external solutions where the osmotic pressure difference between the cells and the medium is low. Conversely, malate loss from the vacuoles depends on a high osmotic pressure difference between the cells and the medium and is observed in media of low osmotic pressure. This suggests that the diurnal oscillations of malate levels in crassulacean acid metabolism leaf cells are regulated by osmotic gradients. These findings support a model which is introduced to explain how the rhythm of crassulacean acid metabolism may function in the intact plant.