Anatomical and enzymic studies of leaves of a C3 × C4Flaveria F1 hybrid exhibiting reduced photorespiration

Flaveria pringlei exhibits C₃ CO₂ compensation concentration (Г) values averaging 53 μl CO₂/l at 21% (v/v) O₂ and 25 ± 2°C. When this species is hybridized with the C₄ species, F. brownii (male) ($\text{Г = 6 μ}\text{l CO}{}_2\text{/}\text{l}$), the F₁ hybrid plants exhibit an average Г value of 31 μl CO₂/l at 21% O₂.Although light micrographs of leaf cross-sections show that the leaves of the hybrid plants possess the mesophyll arrangement characteristic of F. pringlei leaves, the hybrid plants have some bundle-sheath chloroplasts. However, the numbers of these organelles do not appear to be intermediate with respect to the numbers in the parents and are closest to the small number present in the bundle-sheath cells of F. pringlei leaves. The activities of key C₄ enzymes (in μmol · mg Chl^?1 · h^?1) are: phosphoenolpyruvate (PEP) carboxylase, 121; pyruvate, orthophosphate (P_i) dikinase, 26; NADP-malate dehydrogenase, 2529; and NADP-malic enzyme, 82. All of these activities are substantially higher than in F. pringlei, but are only 7–10% of those in F. brownii (with the exception of the NADP-malate dehydrogenase activity). These data suggest that a C₄ cycle might be operating to a limited extent in the hybrid plants resulting in reduced photorespiration.Whether or not C₄ photosynthesis occurs in these hybrid plants, they represent the first reported C₃ × C₄ F₁ hybrids to exhibit reduced Γ-values. This cross and its reciprocal should be useful models for studying the anatomical and biochemical factors determining the development of limited C₄ photosynthesis in C₃ species.