The evolution of photosynthetic anatomy in Viburnum (Adoxaceae)

Premise of research: Leaf mesophyll is often differentiated into a palisade layer with tightly packed, elongated cells (I-cells) and a spongy layer with loosely packed, complex shaped cells. An alternative palisade type, composed of branched H-cells, has evolved in a number of plant lineages. Viburnum (Adoxaceae) possesses both types of palisade, providing an opportunity to assess the significance of evolutionary switches between these forms. Methodology: An anatomical survey of 80 species spanning the Viburnum phylogeny permitted an analysis of palisade differences in relation to other characters. A geometric model of leaf mesophyll surface area for CO2 absorption correlated well with measured photosynthetic capacity in a subset of species, allowing us to infer shifts in photosynthetic function. Pivotal results: Ancestrally, viburnums probably produced a palisade with one layer of H-cells. Multiple transitions to two layers of H-cells (H2) and to one or two layers of I-cells (I1, I2) o...