Unique photosynthetic phenotypes in Portulaca (Portulacaceae): C(3)-C(4) intermediates and NAD-ME C(4) species with Pilosoid-type Kranz anatomy

Portulacaceae is a family that has considerable diversity in photosynthetic phenotypes. It is one of 19 families of terrestrial plants where species having C(4) photosynthesis have been found. Most species in Portulaca are in the alternate-leaved (AL) lineage, which includes one clade (Cryptopetala) with taxa lacking C(4) photosynthesis and three clades having C(4) species (Oleracea, Umbraticola and Pilosa). All three species in the Cryptopetala clade lack Kranz anatomy, the leaves have C(3)-like carbon isotope composition and they have low levels of C(4) cycle enzymes. Anatomical, biochemical and physiological analyses show they are all C(3)-C(4) intermediates. They have intermediate CO(2) compensation points, enrichment of organelles in the centripetal position in bundle sheath (BS) cells, with selective localization of glycine decarboxylase in BS mitochondria. In the three C(4) clades there are differences in Kranz anatomy types and form of malic enzyme (ME) reported to function in C(4) (NAD-ME versus NADP-ME): Oleracea (Atriplicoid, NAD-ME), Umbraticola (Atriplicoid, NADP-ME) and Pilosa (Pilosoid, NADP-ME). Structural and biochemical analyses were performed on Pilosa clade representatives having Pilosoid-type leaf anatomy with Kranz tissue enclosing individual peripheral vascular bundles and water storage in the center of the leaf. In this clade, all species except P. elatior are NADP-ME-type C(4) species with grana-deficient BS chloroplasts and grana-enriched M chloroplasts. Surprisingly, P. elatior has BS chloroplasts enriched in grana and NAD-ME-type photosynthesis. The results suggest photosynthetic phenotypes were probably derived from an ancestor with NADP-ME-type C(4), with two independent switches to NAD-ME type.