Reduced Apparent Photorespiration by the C3-C4 Intermediate Species, Moricandia arvensis and Panicum milioides

Abstract

The CO2/O2 specificity factor of sucrose gradient purified ribulose 1,5-bisphosphate carboxylase/oxygenase from the C3-C4 intermediate plants M. arvensis (79 .+-. 1) and P. milioides (89 .+-. 2) was similar to the respective values of the enzyme from the closely related C3 species, M. foetida (80 .+-. 5) and P. laxum (86 .+-. 2). The kinetic properties of this bifunctional enzyme do not explain the reduced rates of photorespiration exhibited by either of these intermediate species. Dark/light ratios for aminoacetonitrile-sensitive 14CO2 evolution during decarboxylation of exogenous [1-14C]glycine by leaf discs had values of 9.0 with M. arvensis and 11.8 with P. milioides. Equivalent ratios with M. foetida and P. laxum were 2.5 and 3.2, respectively. Similar results were obtained using [1-14C]glycolate as the exogenous photorespiratory substrate, with dark/light 14CO2 evolution ratios for the C3-C4 and C3 leaf discs averaging 6.6 and 2.0, respectively. Stimulating photosynthetic CO2 fixation by progressively increasing photon flux density from 0-1900 .mu.mol m-2 s-1 caused a concomitant reduction in 14CO2 evolution from leaf discs of M. arvensis and P. milioides supplied with [1-14C]glycine. Inhibition of photosynthesis by DCMU [3-(3,4-dichlorophenyl)-1,1-dimethyl urea] or the Calvin cycle inhibitor DL-glyceraldehyde increased 14CO2 evolution in the light to rates comparable to those in the dark. P. miliodes and M. arvensis apparently are capable of a more efficient internal recycling of photorespiratory CO2 via ribulose 1,5-bisphosphate carboxylase/oxygenase than closely related C3 plants, and this may partially account for the reduced rates of apparent photorespiration by these intermediate species.