Leaf Anatomy of Orcuttieae (poaceae: Chloridoideae): More Evidence of C4 Photosynthesis without Kranz Anatomy

C4 photosynthesis without Kranz anatomy (single-cell C4 photosynthesis) occurs in only 0.003% of known species of C4 flowering plants. To add insight into the evolution of C4 photosynthesis, we studied the tribe Orcuttieae (Poaceae: Chloridoideae), which has species that can grow under both aquatic and terrestrial conditions, and utilize single-cell C4 photosynthesis when growing submerged. Carbon isotope ratios from aquatic, floating, and terrestrial leaves were in the range 212.25 to 214.31, suggesting that all species carry out C4 photosynthesis. Using light microscopy, we examined the anatomy of aquatic, floating and terrestrial leaves from eight of the nine species in the tribe to assess the pattern of evolution of C4 photosynthesis and Kranz anatomy among these vernal pool grasses. Kranz anatomy was present in all floating and terrestrial leaves of Orcuttia californica, O. inaequalis, O. pilosa, O. tenuis, O. viscida,Tuctoria greenei, T. mucronata, and Neostapfia colusana. Although carbon isotope data indicated C4 photosynthesis, aquatic leaves of all members of Orcuttia lacked Kranz anatomy, while aquatic leaves of Tuctoria and Neostapfia possessed Kranz anatomy. When considered in a phylogenetic context, these findings support previously proposed hypotheses suggesting that Orcuttieae are derived from a terrestrial ancestor and are now becoming more specialized to an aquatic environment.