Degree of C(4) Photosynthesis in C(4) and C(3)-C(4)Flaveria Species and Their Hybrids : II. Inhibition of Apparent Photosynthesis by a Phosphoenolpyruvate Carboxylase Inhibitor.

Hybrids have been made between species of Flaveria exhibiting varying levels of C(4) photosynthesis. The degree of C(4) photosynthesis expressed in four interspecific hybrids (Flaveria trinervia [C(4)] x F. linearis [C(3)-C(4)], F. brownii [C(4)-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 CO(2) and at 20 and 210 mL L(-1) of O(2). Photosynthesis at 210 mL L(-1) of O(2) 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 O(2) was increased when DCDP was used, except in the C(4) species, F. trinervia, in which photosynthesis was insensitive to O(2). Except for F. trinervia, control plants with less O(2) sensitivity (more C(4)-like) exhibited a progressively greater change in O(2) inhibition of photosynthesis when treated with DCDP. This increased O(2) inhibition probably resulted from decreased CO(2) concentrations in bundle sheath cells due to inhibition of phosphoenolpyruvate carboxylase. The inhibition of photosynthesis by DCDP is concluded to underestimate the degree of C(4) photosynthesis in the interspecific hybrids because increased direct assimilation of atmospheric CO(2) by ribulose bisphosphate carboxylase may compensate for inhibition of phosphoenolpyruvate carboxylase.