Carbon isotope composition of plant photosynthetic tissues reflects a Crassulacean Acid Metabolism (CAM) continuum in the majority of CAM lineages

Messerschmid, Thibaud F. E. and Wehling, Judith and Bobon, Nadine and Kahmen, Ansgar and Klak, Cornelia and Los, Jessica A. and Nelson, Daniel B. and dos Santos, Patricia and de Vos, Jurriaan M. and Kadereit, Gudrun. (2021) Carbon isotope composition of plant photosynthetic tissues reflects a Crassulacean Acid Metabolism (CAM) continuum in the majority of CAM lineages. PERSPECTIVES IN PLANT ECOLOGY EVOLUTION AND SYSTEMATICS, 51. ARTN 125619.

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The stable carbon isotope composition of plant tissues, commonly expressed as delta C-13, holds a wealth of information about photosynthetic pathway, water relations and stress physiology. Crassulacean acid metabolism (CAM) is a derived form of photosynthesis that allows plants to fix carbon at a higher water-use efficiency compared to the ancestral C-3 photosynthesis. While the central carbon-fixing enzyme of C-3 plants, Rubisco, strongly discriminates against the heavy C-13 isotope, CAM is characterized by a dual use of Rubisco and the much less discriminating PEP carboxylase as carbon-fixing enzymes, causing the delta C-13 values of CAM plant tissues to be generally less negative than those found in C-3 plants. Past studies of delta C-13 variation in CAM plant lineages have repeatedly found a bimodal distribution with very few samples representative of the range around -20 parts per thousand that is intermediate between C-3- and CAM-like values. Although delta C-13 values of facultative CAM plants have long been known to extend well into the range below -20 parts per thousand, this value is often tentatively used as threshold for character coding to distinguish C-3 from CAM species in studies of CAM evolution. Compiling 6623 delta C-13 values reported in the literature for CAM/C-3 vascular plant lineages and presenting new data for 581 accessions mainly of the succulent Mesembryanthemoideae (Aizoaceae) and Aeonieae (Crassulaceae), we here investigate the diverse patterns of delta C-13 distribution in different plant families and sub-familial taxa and demonstrate that a bimodal distribution is not universally present in all lineages. Moreover, we show by means of mixture modelling that the bimodal distribution of delta C-13 values in the full dataset as well as in the very well-sampled Bromeliaceae is best described by a combination of three rather than two Gaussian distributions with one intermediary cluster between the more evident clusters of C-3- and CAM-like values. In view of these results and the furthermore emerging unimodal distribution of delta C-13 values in Mesembryanthemoideae with mean close to -20 parts per thousand, we conclude that the evident continuum between CAM and C-3 photosynthesis cautions against the usage of a delta C-13 threshold in macroevolutionary studies. Finally, the observed diversity of delta C-13 distribution patterns between monophyletic lineages urges for lineage-specific reconstructions rather than a unifying model of CAM evolution.
Faculties and Departments:05 Faculty of Science > Departement Umweltwissenschaften > Integrative Biologie > Physiological Plant Ecology (Kahmen)
UniBasel Contributors:Kahmen, Ansgar and Nelson, Daniel and de Vos, Jurriaan M. and Pinheiro Dos Santos, Patricia Alexandra
Item Type:Article, refereed
Article Subtype:Research Article
Note:Publication type according to Uni Basel Research Database: Journal article
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Last Modified:03 Feb 2022 07:08
Deposited On:03 Feb 2022 07:08

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