The stable carbon isotope composition of plant tissues, commonly expressed as δ¹³C, 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 C₃ photosynthesis. While the central carbon-fixing enzyme of C₃ plants, Rubisco, strongly discriminates against the heavy ¹³C isotope, CAM is characterized by a dual use of Rubisco and the much less discriminating PEP carboxylase as carbon-fixing enzymes, causing the δ¹³C values of CAM plant tissues to be generally less negative than those found in C₃ plants. Past studies of δ¹³C variation in CAM plant lineages have repeatedly found a bimodal distribution with very few samples representative of the range around -20‰ that is intermediate between C₃- and CAM-like values. Although δ¹³C values of facultative CAM plants have long been known to extend well into the range below -20‰, this value is often tentatively used as threshold for character coding to distinguish C₃ from CAM species in studies of CAM evolution. Compiling 6623 δ¹³C values reported in the literature for CAM/C₃ 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 δ¹³C 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 δ¹³C 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 C₃- and CAM-like values. In view of these results and the furthermore emerging unimodal distribution of δ¹³C values in Mesembryanthemoideae with mean close to -20‰, we conclude that the evident continuum between CAM and C₃ photosynthesis cautions against the usage of a δ¹³C threshold in macroevolutionary studies. Finally, the observed diversity of δ¹³C distribution patterns between monophyletic lineages urges for lineage-specific reconstructions rather than a unifying model of CAM evolution.

植物组织的稳定碳同位素组成，通常表示为 δ ¹³ C，包含关于光合作用途径、水分关系和胁迫生理学的丰富信息。景天酸代谢 (CAM) 是光合作用的衍生形式，与祖先的 C ₃光合作用相比，它使植物能够以更高的水分利用效率固定碳。虽然 C ₃植物的中心固碳酶Rubisco 强烈区分重¹³ C 同位素，但 CAM 的特点是双重使用 Rubisco 和不太区分的 PEP 羧化酶作为固碳酶，导致 δ ¹³ C CAM 植物组织的值通常比 C _{3 中}发现的值更小植物。过去对CAM 植物谱系中 δ ¹³ C 变异的研究反复发现双峰分布，很少有样本代表 -20‰ 左右的范围，即介于 C ₃ - 和 CAM 样值之间的中间值。尽管长期以来已知兼性 CAM 植物的δ ¹³ C 值可以很好地扩展到低于 -20‰ 的范围，但在 CAM 进化研究中，该值通常暂时用作字符编码的阈值，以区分 C ₃和 CAM 物种。为 CAM/C ₃编制文献中报告的6623 δ ¹³ C 值维管植物谱系，并提供主要为多肉植物 Mesembryanthemoideae (Aizoaceae) 和 Aeonieae (Crassulaceae) 的 581 种材料的新数据，我们在这里研究了不同植物科和亚科分类群中 δ ¹³ C 分布的不同模式，并证明了双峰分布并非普遍存在于所有谱系中。此外，我们通过混合建模表明，完整数据集以及采样良好的凤梨科植物中 δ ¹³ C 值的双峰分布最好通过三个而不是两个高斯分布的组合来描述更明显的 C ₃簇- 和类似 CAM 的值。鉴于这些结果以及在 Mesembryanthemoideae中 δ ¹³ C 值的进一步出现的单峰分布，平均值接近 -20‰，我们得出结论，CAM 和 C ₃光合作用之间的明显连续性警告不要在宏观进化中使用 δ ¹³ C 阈值。学习。最后，观察到的单系谱系之间δ ¹³ C 分布模式的多样性需要谱系特异性重建，而不是 CAM 进化的统一模型。