The occurrence of C2 photosynthesis in Euphorbia subgenus Chamaesyce (Euphorbiaceae)

Abstract

This study investigated whether Euphorbia subgenus Chamaesyce subsection Acutae contains C₃–C₄ intermediate species utilizing C₂ photosynthesis, the process where photorespired CO₂ is concentrated into bundle sheath cells. Euphorbia species in subgenus Chamaesyce are generally C₄, but three species in subsection Acutae (E. acuta, E. angusta, and E. johnstonii) have C₃ isotopic ratios. Phylogenetically, subsection Acutae branches between basal C₃ clades within Euphorbia and the C₄ clade in subgenus Chamaesyce. Euphorbia angusta is C₃, as indicated by a photosynthetic CO₂ compensation point (Г) of 69 μmol mol⁻¹ at 30 °C, a lack of Kranz anatomy, and the occurrence of glycine decarboxylase in mesophyll tissues. Euphorbia acuta utilizes C₂ photosynthesis, as indicated by a Г of 33 μmol mol⁻¹ at 30 °C, Kranz-like anatomy with mitochondria restricted to the centripetal (inner) wall of the bundle sheath cells, and localization of glycine decarboxlyase to bundle sheath mitochondria. Low activities of PEP carboxylase, NADP malic enzyme, and NAD malic enzyme demonstrated no C₄ cycle activity occurs in E. acuta thereby classifying it as a Type I C₃–C₄ intermediate. Kranz-like anatomy in E. johnstonii indicates it also utilizes C₂ photosynthesis. Given the phylogenetically intermediate position of E. acuta and E. johnstonii, these results support the hypothesis that C₂ photosynthesis is an evolutionary intermediate condition between C₃ and C₄ photosynthesis.