2 TYPES OF CHANNELS INVOLVED IN THE MALATE ION-TRANSPORT ACROSS THE TONOPLAST OF A CRASSULACEAN ACID METABOLISM PLANT

Ion channels in tonoplast of leaf cells of a Crassulacean acid metabolism plant, Graptopetalum paraguayense, using the patch clamp technique were investigated. Results showed the existence of two types of channels involved in the malate ion transport across the tonoplast. One type corresponded to the slow-activating vacuolar-type (R Hedrich, E Neher [1987] Nature 329: 833-836), probably taking part in the malate efflux from vacuoles. Another showed the membrane potential-dependent channel current of malate flux over a wide range of cytoplasmic free Ca2+ concentration (10(-8)-10(-5) molar), a property favoring the malate uptake. This type seems to be different from the fast-activating vacuolar-type.