Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

Abstract

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.