Experimental evidence for the isotope effect in photorespiration

Recent data on carbon isotope fractionation in photosynthesis are reviewed. Analysis of the carbon isotope composition in photosynthates and derivative products supports the hypothesis of the isotope effect in photorespiration, This hypothesis envisages the existence in a photosynthesizing cell of two carbon flows differing in isotope composition. One of these flows is enriched in C-12 and associated with the assimilation pathway of photosynthesis. The other flow enriched in C-13 circulates in the photorespiratory pathway. The relation between stimulated photorespiration and the carbon isotope composition of biomass supports this view. Our hypothesis of two interrelated isotope effects in photosynthesis leads to the conclusion that photosynthesis and photorespiration are coupled processes subject to periodical oscillations, where Rubisco acts as a main switch regulating these two pathways.