Diurnal changes in the malic acid content of vacuoles isolated from leaves of the crassulacean acid metabolism plant, Sedum telephium

Abstract

The accumulation of malic acid at night in green cells followed by malic acid depletion the next day is characteristic of Crassulacean acid metabolism (CAM) plants. The diurnal changes in malic acid content are accompanied by reciprocal changes in starch content with starch providing the carbon skeleton for phosphoenolpyruvate, the CO* acceptor at night. Malic acid is decarboxylated during the day to provide COZ for photosynthesis [ 11. In contrast, other organic acids such as isocitric do not exhibit a daily fluctuation in levels [2]. With the realization that massive amounts of malic acid accumulate at night [ 1,2], it was widely postulated that malic acid is stored in the large vacuoles of green CAM cells. Indeed, calculations based on Bryophyllum leaf cell volumes and the amount of acid accumulated at night showed that ‘if malic acid were confined to the cytoplasm, its concentration would be 7-8 M, but if it were in the vacuole also, its average concentration would be 0.25 M’ [3]. Thus, to prevent a variety of deleterious effects of acidic conditions on cytoplasmic components, a green CAM cell would in theory temporarily sequester malic acid in its vacuole at night. However, data to test this hypothesis are not available. Recently, techniques have been developed for isolating large quantities of vacuoles from plant tissues [4-71. Isolated Bvophyllum leaf vacuoles [6] contained 0.002-0.02 nmol malic acid/vacuole which comprised the bulk of the malic acid present. However