There Is No “typical” CAM Leaf

Crassulacean acid metabolism (CAM) is a common feature of plants growing in arid, saline, or drought-prone environments, and can be either obligate or inducible when used in concert with C₃ or C₄ photosynthesis. In CAM plants, carbon dioxide is taken up during the night and converted to malate. Malic acid accumulates through the night and is decarboxylated during the day. Conventional botanical wisdom tells us that plants using CAM have a standard set of accompanying anatomical features, including thick leaves with large mesophyll cells, which contain large vacuoles for the containment of malic acid produced during the night. To date, however, no study has evaluated the link between these features and obligate or inducible CAM mode.

In a study recently published in Annals of Botany, Ana Herrera set out to test these assumptions. She compared published values of the carbon isotopic ratio (δ¹³C), an indicator of CAM mode, to measurements of leaf thickness, cell area and density, the proportion of intercellular space in the mesophyll, and the length of cell wall facing the intercellular air spaces. Herrera’s analysis covered 81 species in 15 relatively phylogenetically distant families across nine orders.

Herrera found that neither leaf thickness nor various measures of mesophyll structure were predictive of CAM mode or degree of expression across all taxa. Associations held within some families: for example, a strong correlation between leaf thickness and δ¹³C in some Crassulaceae species, as well as one between leaf thickness and nighttime malic acid accumulation in some species of the Bromeliaceae that carry out obligate CAM, but nothing could be generalized across distantly related plants. As Herrera puts it, “it should be accepted that there are no ‘typical CAM’ values of traits.”

“A systematic, quantitative comparison of vacuole area in CAM vs. C₃ plants would be highly desirable,” Herrera writes. “Although time-consuming, expensive and requiring specialized equipment and techniques not usually available in most laboratories, it should help elucidate the relationship between mesophyll succulence and CAM mode.”