Photorespiration: In the balance

Proc. Natl Acad. Sci. USA http://doi.org/9z4 (2015)

The balance between photosynthetic carbon gain and photorespiratory carbon loss is a key determinant of the productivity of C₃ plants. A study of plant stable isotope signatures suggests that the ratio of photorespiration to photosynthesis in C₃ plants has fallen by around 25% since 1900, in line with the 100 ppm rise in atmospheric carbon dioxide concentrations seen over the same period.

Ina Ehlers, of Umea University, Sweden, and colleagues carried out a series of laboratory experiments with sunflower to determine the intramolecular distribution of the stable isotope deuterium in photosynthetically derived glucose in plants exposed to different concentrations of carbon dioxide. They find that the intramolecular deuterium distribution shifts with CO₂ concentration, and can thereby serve as a proxy for the ratio of photorespiration to photosynthesis. Applying this logic, they examine the deuterium signature of archived (field grown) sugar beet sugars collected between 1890 and 2012. They find that the ratio of photorespiration to photosynthesis declined significantly over the measurement period, such that net photosynthetic carbon gain rose by 35%. Comparison of modern and herbarium samples of several other C₃ species revealed a similar trend.

The researchers suggest that their findings provide much-awaited experimental evidence for the ‘CO₂ fertilization’ effect — the hypothesis that the post-industrial rise in atmospheric CO₂ concentrations has stimulated plant carbon gain.