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Plant and Cell Physiology, 1982, Vol. 23, No. 7 1185-1195
© 1982

Article

Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

Gerald E. Edwards1, Maurice S.B. Ku1 and Marshall D. Hatch2

1Department of Botany, Washington State University Pullman, Washington 99164, U.S.A.
2Division of Plant Industry CSIRO, P.O. Box 1600, Canberra City A.C.T. 2601, Australia

The capacity for C4 photosynthesis in Panicum milioides, a species having reduced levels of photorespiration, was investigated by examining the activity of certain key enzymes of the C4 pathway and by pulse-chase experiments with 14CO2. The ATP$P1 dependent activity of pyruvate,P1 dikinase in the species was extremely low (0.14–0.18 µmol mg chlorophyll–1 min–1). Low activity of the enzyme was also found in Panicum decipiens and Panicum hians (related species with reduced photorespiration) and in Panicum laxum (a C3 species). The antibody to pyruvate,P1 dikinase caused about 70% inhibition of the ATP$P1 dependent activity of the enzyme in P. milioides. The activity of NAD-malic enzyme and NADP-malic enzyme in P. milioides was equally low (approximately 0.1–0.2 µmol mg chlorophyll–1 min–1) and similar to the activity in P. decipiens, P. hians and P. laxum. Photosynthetic pulse-chase experiments under atmospheric conditions showed a typical C3-like pattern of carbon assimilation including the labelling of glycine and serine as expected during photorespiration. During the pulse with 14CO2 only about 1% of the labelled products appeared in malate and 2–3% in aspartate. During a chase in atmospheric levels of CO2 for up to 6 min there was a slight increase in labelling in the C4 acids. The amount of label in carbon 4 of aspartate did not change during the chase, indicating little or no turnover of the C4 acid via decarboxylation. The results indicate that under atmospheric conditions P. milioides assimilates carbon directly through the C3 pathway. Photorespiration as indicated by the CO2 compensation point may be repressed in the species by a more efficient recycling of photorespired CO2.

(Received June 8, 1982; Accepted July 22, 1982)
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