Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

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The capacity for C_4 photosynthesis in Panicum milioides, a species having reduced levels of photorespiration, was investigated by examining the activity of certain key enzymes of the C_4 pathway and by pulse-chase experiments with ^<14>CO_2. The ATP+p_i dependent activity of pyruvate,P_i 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 C_3 species). The antibody to pyruvate,Pi dikinase caused about 70% inhibition of the ATP+p_i 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 0.1-0.2μmol 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 C_3-like pattern of carbon assimilation including the labelling of glycine and serine as expected during photorespiration. During the pulse with ^<14>CO_2 only about 1% of the labelled products appeared in malate and 2-3% in aspartate. During a chase in atmospheric levels of CO_2 for up to 6 min there was a slight increase in labelling in the C_4 acids. The amount of label in carbon 4 of aspartate did not change during the chase, indicating little or no turnover of the C_4 acid via decarboxylation. The results indicate that under atmospheric conditions P. milioides assimilates carbon directly through the C_3 pathway. Photorespiration as indicated by the CO_2 compensation point may be repressed in the species by a more efficient recycling of photorespired CO_2.
日本植物生理学会の論文

Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

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