Respiration that occurs in plants in the light. It differs from dark respiration in that it does not occur in the mitochondria and is not coupled to oxidative phosphorylation. The rate of C0
2 release by photorespiration in C
3 plants can be three to five times greater than that released by dark respiration. Since the process does not generate ATP it appears to be extremely wasteful. It has been estimated that photosynthetic efficiency could be improved by 50% if photorespiration were inhibited. In C
4 plants photorespiration is hardly detectable, possibly because synthesis of *
glycolic acid, the substrate for photorespiration, is much lower in C
4 plants (about 10% of that of C
3 plants). This could be because the concentration of C02 in the bundle sheath cells is so high that oxidation (instead of carboxylation) of ribulose bisphosphate is prevented.
Some C
3 plants, e.g. the grass Panicum miliodes, have a leaf structure comparable in some respects to the Kranz structure of C
4 plants. Such plants also tend to have reduced rates of photorespiration. These observations have stimulated research into the possibility of breeding C
3 plants with slower photorespiration rates.
