TANSLEY REVIEW No 1.

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Abstract
Summary: The term, photosynthetic acid metabolism (PAM), encompasses processes which involve four‐carbon (C4) acids as carbon‐carrying intermediates between the carboxylations catalysed by phosphoenolpyruvate (PEP) carboxylase and ribulose biphosphate (RuBP) carboxylase. These carboxylations may be separated either spatially, as in C4 photosynthesis which is not covered here, or temporally. Separation in time is referred to as diel PAM (DPAM) and the variations on this particular theme are the subject of this review. DPAM includes stomatal and astomatal types. The former types, usually described as crassulacean acid metabolism (CAM), are referred to here as stomatal CAM (SCAM). Astomatal PAM includes both aquatic and terrestrial forms; aquatic acid metabolism (AAM) and terrestrial astomatal acid metabolism (TAAM). Physiological, biochemical, structural, ecological and evolutionary aspects of these mechanisms of acquisition of carbon dioxide are reviewed. CONTENTS Summary 3 I. General Introduction to Photosynthetic Acid Metabolism (PAM) 4 II. Stomatal Crassulacean (Diel Photosynthetic) Acid Metabolism (SCAM) 5 III. Astomatal Photosynthetic Acid Metabolism 13 IV. Idling 18 V. Structural Variation in Plants Exhibiting Diel PAM (DPAM) 18 VI. The Mechanism of the Switch between C3 Photosynthesis and DPAM 19 VII. The Cost of DPAM 19 VIII. The Evolution of DPAM 20 IX. Concluding Comment 22 Acknowledgements 22 References 22