Characterization of C4 Type Leaf Anatomy in Grasses (Poaceae). Mesophyll: Bundle Sheath Area Ratios

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Abstract
The cross-sectional area of ‘primary carbon assimilation’ (PCA) (or mesophyll) tissue and of ‘photosynthetic carbon reduction’ (PCR) (or parenchymatous bundle sheath, PBS) tissue associated with each vein has been measured in transverse sections of leaf blades of 124 grass species (Poaceae). The species sample is representative of all major grass taxa, and of all photosynthetic types found in this family, viz. C3, C3/C4 intermediate, C4 NADP-malic enzyme type (NADP-ME), C4 NAD-malic enzyme type (NAD-ME) and PEP carboxykinase type (PCK). Mean PCA (or mesophyll) area per vein varies between photosynthetic types in the order C3 > NAD-ME > PCK = NADP-ME, mean PCR (or PBS) area per vein in the order NAD-ME > PCK = C3 > NADP-ME, and mean PCA/PCR (or mesophyll/PBS) area ratio in the order C3 > NADP-ME > NAD-ME > PCK. Since grass leaves have parallel venation, tissue areas and area ratios are directly proportional to tissue volumes and volume ratios. Regression analyses of plots of PCA (or mesophyll) area per vein against PCR (or PBS) area per vein yield characteristic slopes for photosynthetic types. Differences between types in all these parameters are nearly always statistically significant, even within high level taxonomic groups (Eupanicoids and Chloridoids). However, differences between major taxa (Eupanicoids, Andropogonoids, Chloridoids), within a photosynthetic type, are frequently not significant. This histometric characterization of photosynthetic types is discussed in relation to the co-operation of PCA and PCR tissues in C4 photosynthesis, to possible differences between C4 types in PCR spatial requirements and to the developmental origin of PCR tissue.