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Quantitative Leaf Anatomy of C3 and C4 Cyperaceae and Comparisons with the Poaceae

Relative amounts of photosynthetic tissues were quantified in C3 and C4 Cyperaceae species to evaluate generalizations about C4 leaf anatomy on the basis of the well-characterized Poaceae. Three of four C4 anatomical types of the Cyperaceae have an unusual version of Kranz anatomy: one or two cell layers intervene between primary carbon assimilation (PCA), equivalent to mesophyll and photosynthetic carbon reduction (PCR), equivalent to chlorenchymatous inner-border parenchyma of the vascular tissue, increasing the number of cell diameters along the diffusion pathway for C4 metabolites. The absolute distances between PCA and PCR tissues are approximately equivalent in the two families, indicating that the extra layers do not increase the mean length of the diffusion pathway required for efficient metabolite transfer in C4 photosynthesis (although minimum distances are greater in C4 Cyperaceae). The proportion of intercellular space in contact with PCA cells (including that of adjacent aerenchyma) is greater in the Cyperaceae than in the Poaceae, and this feature may have played a role in the evolution of PCR tissue in the position of inner-border parenchyma. The "extra" layers present in C4 Cyperaceae may also contribute to the barrier for diffusion of CO2 from PCR to intercellular space. The ratio of PCA:PCR tissue area is much greater in the Cyperaceae, because of the combined effect of increased PCA (predominantly intercellular space) tissue and reduced PCR tissue compared with the Poaceae. The reduced PCR tissue may reflect both developmental and functional constraints as a consequence of evolutionary selection for PCR cells in a position that isolates them from intercellular space but preserves the essential elements of C4 Kranz anatomy.