Functional Anatomical Traits of the Photosynthetic Organs of Plants with Crassulacean Acid Metabolism

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation to water and/or CO2 limited environments that has evolved in 400 genera from 36 families of higher plants. Despite the taxonomic and ecological diversity of CAM, plants with this photosynthetic specialization share a number of common anatomical traits that impinge on the physiological processes underpinning photosynthetic CO2 assimilation and water use. Thick, succulent leaves and/or stems are typical for terrestrial CAM plants. The large cells within these succulent tissues serve to accommodate the overnight vacuolar accumulation of malic acid that defines CAM and also increase water storage capacity. Significant morphological and anatomical diversity exists among leaf and stem succulents that impact on water-use strategies and thus the predisposition towards CAM. We provide an overview of CAM diversity in terms of leaf and stem anatomy, leaf venation and stomatal patterning. We consider the physiological implications of these anatomical traits in terms of water use and leaf hydraulic properties as well as the impacts on CO2 uptake and carbon gain. We also discuss which anatomical traits are likely to be important determinants for the mode and level of CAM that might be engineered into non-CAM species as a means of improving plant water use efficiency.