Photorespiration during Leaf Ontogeny

Abstract

Three principal oxygen uptake processes in leaves are presently recognized. These are the oxygenase reaction of ribulose- 1, 5-bisphosphate carboxylase/oxygenase (RuBPCO) and the associated metabolism of phosphoglycollate, the Mehler reaction, and the oxygen uptake associated with “dark” respiration which may continue in the light (cf. Sections 7.3 and 10.8; Canvin et ah 1980). The oxidation of carbon compounds to CO₂ in the light via glycollate metabolism, induced by the activity of the photosynthetie system, is known as photorespiration. It could include the direct oxidation of organic acids by the two photosystems, the production of higher concentrations of oxidizable substrates, the increase in the level of substrates available for mitochondrial oxidation, and the oxygen dependent oxidation of ribulose-l,5-bisphosphate (RuBP) by RuBPCO (Schaedle 1975). In brief (Heber and Krause 1980), glycollate is formed in the chloroplast at the expense of sugar-phosphate intermediates of the Calvin cycle: oxygen competes with CO₂ at the active sites of RuBPCO. Instead of being carboxylated, RuBP is oxygenated to form phosphoglycerate and phosphoglycollate. The latter is hydrolysed to glycollate. After export from the chloroplast, the glycollate undergoes a series of reactions taking place in the peroxisome, mitochondrion, and, finally, again in the chloroplast. There, phosphoglycerate is produced, linking glycollate metabolism to the Calvin cycle. A peculiar feature of photorespiration is that, in contrast to mitochondrial, dark respiration, it does not evolve but consumes energy.