The diverse diaspora of CAM: a pole-to-pole sketch

Abstract

Background CAM photosynthesis is a successful adaptation that has evolved often in angiosperms, gymnosperms, ferns and lycophytes. Present in ca. 5 % of vascular plants, the CAM diaspora includes all continents barring Antarctica. Species with CAM inhabit most landscapes colonized by vascular plants, from the Arctic Circle to Tierra del Fuego, from below sea-level to 4,800 m, from rainforests to deserts. They have colonised terrestrial, epiphytic, lithophytic, palustrine and aquatic systems developing perennial, annual or geophyte strategies that may be structurally arborescent, shrub, forb, cladode, epiphyte, vine or leafless with photosynthetic roots. CAM may enhance survival by conserving water, trapping carbon, reducing carbon loss and/or via photoprotection. Scope This review assesses the phylogenetic diversity and historical biogeography of selected lineages with CAM viz. ferns, gymnosperms and eumagnoliids, Orchidaceae, Bromeliaceae, Crassulaceae, Euphorbiaceae, Aizoaceae, Portulacineae (Montiaceae, Basellaceae, Halophytaceae, Didiereaceae, Talinaceae, Portulacaceae, Anacampserotaceae, Cactaceae) and aquatics. Conclusions Most extant CAM lineages diversified since the Oligocene/Miocene as the planet dried and CO2 concentrations dropped. Radiations exploited changing ecological landscapes including Andean emergence, Panamanian Isthmus closure, Sundaland emergence and submergence, changing climates and desertification. Evidence remains sparse for or against theories that CAM-biochemistry tends to evolve prior to pronounced changes in anatomy, and that CAM tends to be a culminating xerophytic trait. In perennial taxa, any form of CAM may occur depending upon the lineage and the habitat, although facultative CAM appears uncommon in epiphytes. CAM annuals lack strong CAM. In CAM annuals, C3+CAM predominates and inducible- or facultative- CAM are common.

Low internal air space in plants with crassulacean acid metabolism may be an anatomical spandrel

“…2 . There are two primary benefits conferred by CAM ( Holtum, 2023 ). First, the diurnal decarboxylation of malic acid during phase III allows photosynthesis to occur without reliance on atmospheric CO 2 , meaning CAM plants can keep their stomata shut during the hotter, low-humidity portions of the day ( Borland et al ., 2015 ).…”

Additional notes on the taxonomy of Parakeelya Hershk. (Montiaceae)

“…The use of Calandrinia unqualified refers to "fake Calandrinia" and not "real Calandrinia," hence is conceptually illegitimate. Holtum (2023) discussed Montiaceae taxonomy and phylogeny, but did not cite Hershkovitz' (2019) revision of these, which recognized Rumicastrum and criticized Thiele et al (2018) and the continued use of Calandrinia for the Australian species. And he did not cite Hershkovitz (2020a), which also criticized Thiele et al (2018) and recombined in Rumicastrum essentially all Australian species named in Calandrinia.…”

“…26 Presumably they also knew also that the genomic data proved that "fake Calandrinia s. l." was not monophyletic. Notably, subsequent publications by these collaborators (Hancock et al, 2018;Holtum et al, 2021;West & Albrecht, 2022;Albrecht & West, 2023;Holtum, 2023) acknowledged relations of Rumicastrum and polyphyly of "fake Calandrinia s. l.," but they now justified continued use of Calandrinia for a different reason, viz. the pending Thiele et al (2018) conservation proposal.…”