Operating at the very low end of the crassulacean acid metabolism spectrum: Sesuvium portulacastrum (Aizoaceae).

Klaus Winter,Joseph A M Holtum,Milton Garcia,Aurelio Virgo,Christine Raines

doi:10.1093/jxb/ery431

Abstract

Demonstration of crassulacean acid metabolism (CAM) in species with low usage of this system relative to C3-photosynthetic CO2 assimilation can be challenging experimentally but provides crucial information on the early steps of CAM evolution. Here, weakly expressed CAM was detected in the well-known pantropical coastal, leaf-succulent herb Sesuvium portulacastrum, demonstrating that CAM is present in the Sesuvioideae, the only sub-family of the Aizoaceae in which it had not yet been shown conclusively. In outdoor plots in Panama, leaves and stems of S. portulacastrum consistently exhibited a small degree of nocturnal acidification which, in leaves, increased during the dry season. In potted plants, nocturnal acidification was mainly facultative, as levels of acidification increased in a reversible manner following the imposition of short-term water-stress. In drought-stressed plants, nocturnal net CO2 exchange approached the CO2-compensation point, consistent with low rates of CO2 dark fixation sufficient to eliminate respiratory carbon loss. Detection of low-level CAM in S. portulacastrum adds to the growing number of species that cannot be considered C3 plants sensu stricto, although they obtain CO2 principally via the C3 pathway. Knowledge about the presence/absence of low-level CAM is critical when assessing trajectories of CAM evolution in lineages. The genus Sesuvium is of particular interest because it also contains C4 species.

Highlights

The Aizoaceae, a family of about 1600 species in the order Caryophyllales (Hartmann, 2017), contains more species with succulent leaves than any other eudicot family (HernándezLedesma et al, 2015)
Weakly expressed crassulacean acid metabolism (CAM) was detected in the well-known pantropical coastal, leaf-succulent herb Sesuvium portulacastrum, demonstrating that CAM is present in the Sesuvioideae, the only sub-family of the Aizoaceae in which it had not yet been shown conclusively
The Sesuvioideae, the earliest diverging sub-family (Klak et al, 2003; Bohley et al, 2015), is unique in the Aizoaceae in that it is the only sub-family within which CAM has not yet been demonstrated unequivocally (Bohley et al, 2015) and it is the only sub-family that contains species with C4 photosynthesis, which is present in the genera Sesuvium (Bittrich, 1990; Bohley et al, 2015), Trianthema, and Zaleya (Carolin et al, 1978).Within Sesuvium, two clades have been recognized, one composed of species with C4-type anatomy, C4 biochemistry

Summary

Introduction

The Aizoaceae, a family of about 1600 species in the order Caryophyllales (Hartmann, 2017), contains more species with succulent leaves than any other eudicot family (HernándezLedesma et al, 2015). S. edmonstonei, native to the Galapagos, a reported isotope value of −21.5‰ is intermediate between those characteristic of C3 plants and those of plants with C4 photosynthesis or full CAM (Winter and Holtum, 2002; Bohley et al, 2015; Alonso-Cantabrana and von Caemmerer, 2016), for plants of a northern Venezuelan population the δ13C value is –24‰ (Medina et al, 2008). We were further encouraged to do so by a single observation of small nocturnal acidification in field-growing S. maritimum (Walter) Britton, Sterns & Poggenb. that exhibited a C3-type isotope value of –26‰ (Martin et al, 1982)

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