Ionic Balance during Malic Acid Accumulation in Vacuoles of a CAM Plant, Graptopetalum paraguayense

We studied the ionic balance during diurnal changes in the levelsof accumulated malic acid and hydrogen ion in the vacuoles ofGraptopetalum paraguayense, a crassulacean acid metabolism (CAM)plant. There was a clear diurnal rhythm of the pH and the totalmalic acid content, but the amount of negative charges due tothe unprotonated carboxyl groups of malic acid remained approximatelyconstant. The negative charges were balanced by the positivecharges of cations, which were also constant throughout thediurnal CAM rhythm. The results gave evidence for the electroneutralityof the translocation of the malic acid and protons across thetonoplast membrane. (Received July 30, 1987; Accepted March 17, 1988)