C3-C 4 Intermediate species in the genus Flaveria: leaf anatomy, ultrastructure, and the effect of O2 on the CO 2 compensation concentration.

Leaf anatomical, ultrastructural, and CO2-exchange analyses of three closely related species of Flaveria indicate that they are C3-C4 intermediate plants. The leaf mesophyll of F. floridana J.R. Johnston, F. linearis Lag., and F. chloraefolia A. Gray is typical of that in dicotyledonous C3 plants, but the bundle sheath cells contain granal, starch-containing chloroplasts. In F. floridana and F. chloraefolia, the chloroplasts and numerous associated mitochondria are arranged largely centripetally, as in the closely related C4 species, F. brownii A.M. Powell. In F. linearis, fewer mitochondria are present and the chloroplasts are more evenly distributed throughout the bundle sheath cytosol. There is no correlation between the bundle sheath ultrastructure and CO2 compensation concentration. (Γ) values of these C3-C4 intermediate Flaveria species. At 21% O2 and 25°C, Γ for F. chloraefolia, F. linearis, and F. floridana is 23-26, 14-19, and 8-10 μl CO2 l(-1), respectively. The O2 dependence of Γ is the greatest for F. chloraefolia and F. linearis (similar to that for C3-C4 intermediate Panicum and Moricandia species) and the least for F. floridana, whose O2 response is identical to that for F. brownii from 1.5 to 21% O2, but greater at higher pO2. The variation in leaf anatomy, bundle sheath ultrastructure, and O2 dependence of Γ among these Flaveria species may indicate an active evolution in the pathway of photosynthetic carbon metabolism within this genus.