Plant Physiology - Musiyenko M. M. 2001

Photosynthesis: Physiological, Biochemical, and Ecological Aspects
Carbon Acid Metabolism in Succulent Plants (CAM Metabolism)

The history of studying Crassulacean Acid METABOLISM (CAM) began long before Photorespiration and the Hatch-Slack cycle were discovered. It was already known that succulents always accumulate organic acids at night, accompanied by the uptake of both CO₂ and O₂. All of this was well documented years before the direct link between dark CO₂ fixation and nighttime organic acid accumulation was revealed.

As is now understood, the accumulation of organic acids in the dark is accompanied by starch degradation, The formation of phosphoenolpyruvate via Glycolysis, and subsequent carboxylation mediated by PEP carboxylase (Fig. 75). The resulting OAA is reduced to malate and then stored in the vacuole. During the day (in light), malate is either decarboxylated by the action of malate dehydrogenase or converted into oxaloacetate and subsequently decarboxylated by PEP carboxykinase.

In essence, regarding the biochemical features of carbon dioxide assimilation, CAM (Crassulaceae Acid Metabolism) is similar to C₄ Photosynthesis: CO₂ is initially fixed via the carboxylation of phosphoenolpyruvate to yield C₄ dicarboxylic acids, which are subsequently decarboxylated, after which the newly released CO₂ is refixed through The Calvin Cycle. CAM plants differ from C₄ plants in that the latter separate primary CO₂ fixation from the Calvin cycle spatially (between mesophyll and bundle-sheath Cells), whereas succulent plants separate primary CO₂ fixation from the Calvin cycle temporally (night versus day). Naturally, There are also major differences in photosynthetic tissue anatomy and, to some extent, in the biochemistry of the pathway. During dark fixation in succulents, PEP carboxylase acts as the key enzyme. Succulents possess two CO₂-fixing systems: the light-driven Calvin cycle and the PEP carboxylase system (comprising malate dehydrogenase operating in both the dark and light).

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Fig. 75. Metabolic pathways of CAM in the dark and light. Enzymes are indicated by numbers:

1 — phosphoenolpyruvate carboxylase, 2 — malate dehydrogenase, 3 — NADP-dependent malate dehydrogenase, 4 — ribulose bisphosphate carboxylase, 5 — Pyruvate, orthophosphate dikinase. Reactions enclosed in dashed lines occur exclusively in plants belonging to the families Crassulaceae, Cactaceae, and Agavaceae.

Under certain conditions, some CAM plants can shift their metabolism and photosynthesize in a manner nearly identical to C₃ plants. Quite frequently, Gas Exchange in CAM plants is characterized by CO₂ uptake occurring during both day and night. It should be noted that this metabolic pathway is not restricted solely to the Crassulaceae family. Many succulents (such as halophytes) lack it, whereas numerous semi-succulents exhibit CAM metabolism. It has been identified in plants from a wide range of families: Agavaceae, Bromeliaceae, Cactaceae, Compositae, Crassulaceae, Cucurbitaceae, Euphorbiaceae, Liliaceae, Orchidaceae, Portulacaceae, and others.

Thus, the examined Water/58.html">Ecological features of various CO₂ fixation types lead to the Conclusion that the Calvin cycle plays a decisive role for all plants. Both the C₄ and CAM pathways of CO₂ fixation serve an auxiliary function by supplying additional carbon to be assimilated within the Calvin cycle.