Biochemical and cytological relationships in C4 plants.

C4 plants can be divided into three groups based on differences in activities of three decarboxylating enzymes: NADP-malic enzyme, NAD-malic enzyme, and phosphopyruvate carboxykinase.In the Gramineae the three C4 groups are distinguished by anatomical and ultrastructural characteristics of bundle-sheath chloroplasts. NADP-malic enzyme species lack well-developed grana in bundle-sheath chloroplasts (grana reduced) and the bundle-sheath chloroplasts are in the centrifugal position. NAD-malic enzyme species have bundle-sheath chloroplasts in the centripetal position and contain grana. Phosphopyruvate carboxykinase species have bundle-sheath chloroplasts in the centrifugal position and they contain grana. NADP-malic enzyme species of the Gramineae have only been found in the subfamilies Aristidoideae and Panicoideae. With the exception of the genera Panicum, and Urochloa, NAD-malic enzyme species and phosphopyruvate carboxykinase species have only been found in the subfamily Eragrostoideae. C4 species of the genus Panicum are found among all three of the C4 groups.The dicotyledonous C4 species examined fall into two groups: those having high NADP-malic enzyme and those having high NAD-malic enzyme. No phosphopyruvate carboxykinase C4 species have been found among the dicotyledons. The NADP-malic enzyme C4 species of the dicotyledons like NADP-malic enzyme species of the Gramineae have bundle-sheath chloroplasts with reduced grana but in contrast to NADP-malic enzyme species of the Gramineae the bundle-sheath chloroplasts are in the centripetal position. The NAD-malic enzyme species of the dicotyledons like the NAD-malic enzyme species of the Gramineae have bundlesheath chloroplasts in the centripetal position with well developed grana.The results are discussed in terms of evolutionary and functional diversification of C4 plants.