Energetics of malate accumulation in the vacuoles of Kalanchoë tubiflora cells

Abstract

1. Introduction The diurnal rhythm of crassulacean acid metabo- lism is characterized by dark fixation of CO2 and accumulation of malic acid during the night. Photo- synthetic fixation of COZ derived from decarboxyla- tion of this malate takes place while the stomata are closed during the following light period. Photosyn- thetic fixation of atmospheric CO2 occurs during the latter part of the day if the stomata open. The quan- tum requirements for long-term growth of CAM plants [ 1,2] and for photosynthesis during the light period [3] have been determined. Measurements of respiratory 02 uptake during the dark period have been discussed in relation to a possible competition of respiration and malate synthesis by PEP-C for sub- strate (i.e., PEP) [4]. Until now the process of malic-acid transport into the vacuole has not been included in the assessment of the energy budget of CAM. This malic-acid trans- port across the tonoplast is active, probably driven by a H’-ATPase [5,6]. Here we provide the basis for cal- culation of energy turnover and balance during malate accumulation. The results put some restraints on