Carbon isotopic fractionation does not occur during dark respiration in C-3 and C-4

The magnitude of possible carbon isotopic fractionation during dark respiration was investigated with isolated mesophyll cells from mature leaves of common bean (Phaseolus vulgaris L.), a C-3 plant, and corn (Zea mays L.), a C-4 plant. Mesophyll protoplasts were extracted from greenhouse-grown leaves and incubated in culture solutions containing different carbohydrate substrates (fructose, glucose, and sucrose) with known delta(13)C values. The CO2 produced by protoplasts after incubation in the dark was collected, purified, and analyzed for its carbon isotope ratio. From observations of the isotope ratios of the substrate and respired CO2, we calculated the carbon isotope discrimination associated with metabolism of each of these substrates. In eight of the 10 treatment combinations, the carbon isotope ratio discrimination was not significantly different from 0. In the remaining two treatment combinations, the carbon isotope ratio discrimination was 1 parts per thousand. From these results, we conclude that there is no significant carbon isotopic discrimination during mitochondrial dark respiration when fructase, glucose, or sucrose are used as respiratory substrates.