ABSTRACT
Crassulacean acid metabolism (CAM) is apparently the most conspicuous performance of plants involving alternating production and consumption of hydrogen ions. The sweet taste of leaves at the end of the light period and their acid taste with immediated pheophytin production during homogenization at the end of the dark period provide direct evidence for “acid metabolism.” Thus, CAM is defined as massive diurnal fluctuation of titratable acidity [1]. However, it should be made clear at this point that there are many metabolic processes that also involve considerable or even higher rates of H+ production or H+ translocation than CAM. In such cases, H+ metabolism may be exceedingly high when related to shorter time periods, the volume of specialized cells, or the membrane area of specific organelles. Examples are apoplastic phloem loading, nitrate assimilation, and stomatal movement. This may be illustrated with one example: the typical rates of nocturnal CO2 fixation of CAM plants are in the range of 1 to 5 µmol CO2 m
−2 s−1. Assuming a 10-h night period and using the high rate of 2 µmol CO2 m
−2 s−1, it is calculated that about 72 mmol malic acid accumulates in leaves per 1 m2 of surface area. Simultaneously, 144 mmol of H+ are produced if starch is used for phosphoenolpyruvate (PEP) synthesis, protons being partly buffered but mainly pumped into the vacuole.
