Stoichiometric Nightmares: Studies of Photosynthetic O₂ and CO₂ Exchanges in CAM Plants

Abstract

The unusual gas-exchange stoichiometries of succulent plants which engage in crassulacean acid metabolism (CAM) have been recognized since de Saussure (1804), whose observations are best accessible today via Thomas (1949). In beautifully crafted sentences, sometimes exceeding 100 words, Thomas reviewed the history of gas-exchange experimentation in CAM plants. He left his indelible mark on the dark reactions of CAM with the words “The positive contribution made by the Leipzig workers was to draw attention to the possible cardinal importance (cf. Gustafson’s qualified statement) of carbon dioxide rather than oxygen in controlling diurnal fluctuations in the acidity of the green cells of plants showing crassulacean acid metabolism. The present writer felt that this possibility became a probability when he had considered the experimental results obtained at Leipzig and elsewhere in the light of Wood and Werkman’s discovery”. There is no clearer, and more generous, expression of the origins of the hypothesis that led us to an understanding of the mechanisms of dark CO₂ fixation and malic-acid synthesis in CAM plants. Appreciation of photosynthetic CO₂ fixation in CAM plants is less clear, historically. Bennet-Clark (1933) attributed the recognition of malate decarboxylation as an internal CO₂ source for photosynthesis to A. Mayer (1878, 1899) and to H.A. Spoehr (1913) (cited in Bennet-Clark 1933).