Molecular phylogeny and forms of photosynthesis in tribe Salsoleae (Chenopodiaceae)

While many C₄ lineages have Kranz anatomy around individual veins, Salsoleae have evolved the Salsoloid Kranz anatomy where a continuous dual layer of chlorenchyma cells encloses the vascular and water-storage tissue. With the aim of elucidating the evolution of C₄ photosynthesis in Salsoleae, a broadly sampled molecular phylogeny and anatomical survey was conducted, together with biochemical, microscopic, and physiological analyses of selected photosynthetic types. From analyses of photosynthetic phenotypes, a model for evolution of this form of C₄ was compared with models for evolution of Kranz anatomy around individual veins. A functionally C₃ proto-Kranz phenotype (Proto-Kranz Sympegmoid) and intermediates with a photorespiratory pump (Kranz-like Sympegmoid and Kranz-like Salsoloid types) are considered crucial transitional steps towards C₄ development. The molecular phylogeny provides evidence for C₃ being the ancestral photosynthetic pathway but there is no phylogenetic evidence for the ancestry of C₃-C₄ intermediacy with respect to C₄ in Salsoleae. Traits considered advantageous in arid conditions, such as annual life form, central sclerenchyma in leaves, and reduction of surface area, evolved repeatedly in Salsoleae. The recurrent evolution of a green stem cortex taking over photosynthesis in C₄ clades of Salsoleae concurrent with leaf reduction was probably favoured by the higher productivity of the C₄ cycle.