Thermodynamics of Malate Transport across the Tonoplast of Leaf Cells of CAM Plants

The electrochemical potential difference for each dissociationstate of malic acid across the tonoplast of leaf cells was examinedin two CAM plants, Graptopetalum paraguayense and Kalanchoëdaigremontiana. The concentration of malic acid in each dissociationstate was estimated from an analysis of pH and concentrationsof ionic species that included calcium, malate and isocitrate.The vacuoles contained 30–40 mM isocitrate and 50–70mM calcium in G. paraguayense, and 20–30 mM isocitrateand 70–100 mM calcium in K. daigremontiana. For the calculationof the pattern of dissociation of malic acid, the formationof chelates of calcium with malate and isocitrate, which havedifferent stability constants depending on the dissociationof the acids, were also taken into consideration. The vacuolarconcentrations of the divalently dissociated form of malic acid(mal^2– were 4–7 mM and 1-3 mM in G. paraguayenseand in K. daigremontiana, respectively. To obtain informationabout the cytoplasmic concentration of malate, the apparentinhibition constant for malate of phosphoenolpyruvate carboxylasewas measured. It was about 330 µM in the dark period and60 µM in the light period. Considering an inside-positivemembrane potential, we conclude that mal^2– can be takenup passively into the vacuole during the dark period and canbe released passively from the vacuole during the light period.Two types of channel (the "SV-type" channel and a novel "MU-type"channel) which we found recently in G. paraguayense Iwasakiet al. (1992) Plant Physiol. 98: 1494] are probably involvedin the uptake and the release of malate in the diurnal CAM rhythm.The existence of a large pH-buffering capacity due to isocitricacid in the vacuole allows the accumulation of a large amountof malic acid during the diurnal CAM rhythm. (Received February 12, 1992; Accepted July 10, 1992)