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

While many C-4 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 C-4 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 C-4 was compared with models for evolution of Kranz anatomy around individual veins. A functionally C-3 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 C-4 development. The molecular phylogeny provides evidence for C-3 being the ancestral photosynthetic pathway but there is no phylogenetic evidence for the ancestry of C-3-C-4 intermediacy with respect to C-4 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 C-4 clades of Salsoleae concurrent with leaf reduction was probably favoured by the higher productivity of the C-4 cycle.