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Metabolism and Enzymology

Degree of C₄ Photosynthesis in C₄ and C₃-C₄Flaveria Species and Their Hybrids ¹

II. Inhibition of Apparent Photosynthesis by a Phosphoenolpyruvate Carboxylase Inhibitor

Department of Agronomy, University of Georgia, Athens, Georgia 30602, Department of Biochemistry, University of Georgia, Athens, Georgia 30602

Hybrids have been made between species of Flaveria exhibiting varying levels of C₄ photosynthesis. The degree of C₄ photosynthesis expressed in four interspecific hybrids (Flaveria trinervia [C₄] x F. linearis [C₃-C₄], F. brownii [C₄-like] x F. linearis, and two three-species hybrids from F. trinervia x [F. brownii x F. linearis]) was estimated by inhibiting phosphoenolpyruvate carboxylase in vivo with 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate (DCDP). The inhibitor was fed to detached leaves at a concentration of 4 mM, and apparent photosynthesis was measured at atmospheric levels of CO₂ and at 20 and 210 mL L^–1 of O₂. Photosynthesis at 210 mL L^–1 of O₂ was inhibited 32% by DCDP in F. linearis, by 60% in F. brownii, and by 87% in F. trinervia. Inhibition in the hybrids ranged from 38 to 52%. The inhibition of photosynthesis by 210 mL L^–1 of O₂ was increased when DCDP was used, except in the C₄ species, F. trinervia, in which photosynthesis was insensitive to O₂. Except for F. trinervia, control plants with less O₂ sensitivity (more C₄-like) exhibited a progressively greater change in O₂ inhibition of photosynthesis when treated with DCDP. This increased O₂ inhibition probably resulted from decreased CO₂ concentrations in bundle sheath cells due to inhibition of phosphoenolpyruvate carboxylase. The inhibition of photosynthesis by DCDP is concluded to underestimate the degree of C₄ photosynthesis in the interspecific hybrids because increased direct assimilation of atmospheric CO₂ by ribulose bisphosphate carboxylase may compensate for inhibition of phosphoenolpyruvate carboxylase.