The electrochemical potential difference for each dissociation state of malic acid across the tonoplast of leaf cells was examined in two CAM plants, Graptopetalum paraguayense and Kalanchoë daigremontiana. The concentration of malic acid in each dissociation state was estimated from an analysis of pH and concentrations of ionic species that included calcium, malate and isocitrate. The vacuoles contained 30-40 mM isocitrate and 50-70 mM calcium in G. paraguayense, and 20-30 mM isocitrate and 70-100 mM calcium in K. daigremontiana. For the calculation of the pattern of dissociation of malic acid, the formation of chelates of calcium with malate and isocitrate, which have different stability constants depending on the dissociation of the acids, were also taken into consideration. The vacuolar concentrations of the divalently dissociated form of malic acid (mal2- were 4-7 mM and 1-3 mM in G. paraguayense and in K. daigremontiana, respectively. To obtain information about the cytoplasmic concentration of malate, the apparent inhibition constant for malate of phosphoenolpyruvate carboxylase was measured. It was about 330 μM in the dark period and 60 μM in the light period. Considering an inside-positive membrane potential, we conclude that mal2- can be taken up passively into the vacuole during the dark period and can be 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 [Iwasaki et al. (1992) Plant Physiol. 98: 1494] are probably involved in the uptake and the release of malate in the diurnal CAM rhythm. The existence of a large pH-buffering capacity due to isocitric acid in the vacuole allows the accumulation of a large amount of malic acid during the diurnal CAM rhythm.
|Number of pages||8|
|Journal||Plant and Cell Physiology|
|Publication status||Published - Oct 1992|
All Science Journal Classification (ASJC) codes
- Plant Science
- Cell Biology