C3?C4 Intermediate species in the genus Flaveria: leaf anatomy, ultrastructure, and the effect of O2 on the CO2 compensation concentration

Abstract

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.