Biochemical components of the photosynthetic CO2 compensation point of higher plants.

Barley, Panicum milioides and Panicum maximum were exposed to ¹⁴CO₂ near their photosynthetic CO₂ compensation points and their respective ¹⁴C-products were determined. In short exposure times Panicum maximum had 100% of its ¹⁴C in malate and aspartate whereas Panicum milioides and barley had 16 and 3% of their respective ¹⁴C in C₄ organic acids. Near the respective CO₂ compensation points a linear relationship occurs in plotting the ratio of glycine, serine, and glycerate to C₄ organic acids. The ratio of ribulose 1,5-bisphosphate oxygenase to phosphoenolpyruvate carboxylase is linear with their CO₂ compensation points. The photosynthetic CO₂ compensation point apparently is controlled by the activity of enzymes producing photorespiration metabolites and the activity of phospheonolpyruvate carboxylase.