Photosynthesis inPanicum milioides, a Species with Reduced Photorespiration

摘要

The capacity for C4photosynthesis inPanicum milioides, a species having reduced levels of photorespiration, was investigated by examining the activity of certain key enzymes of the C4pathway and by pulse-chase experiments with14CO2. The ATP$P1dependent activity of pyruvate,P1dikinase in the species was extremely low (0.14–0.18μmol mg chlorophyll−1min−1). Low activity of the enzyme was also found inPanicum decipiensandPanicum hians(related species with reduced photorespiration) and inPanicum laxum(a C3species). The antibody to pyruvate,P1dikinase caused about 70inhibition of the ATP$P1dependent activity of the enzyme inP.milioides. The activity of NAD-malic enzyme and NADP-malic enzyme inP.milioideswas equally low (approximately 0.1–0.2μmol mg chlorophyll−1min−1) and similar to the activity inP.decipiens,P.hiansandP.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 with14CO2only about 1of the labelled products appeared in malate and 2–3in aspartate. During a chase in atmospheric levels of CO2for up to 6 min there was a slight increase in labelling in the C4acids. The amount of label in carbon 4 of aspartate did not change during the chase, indicating little or no turnover of the C4acid via decarboxylation. The results indicate that under atmospheric conditionsP.milioidesassimilates carbon directly through the C3pathway. Photorespiration as indicated by the CO2compensation point may be repressed in the species by a more efficient recycling of ph