A century of research on carbon isotope discrimination during photosynthesis: from early breakthroughs to future goals.

Starting with its discovery in 1929, the stable nuclide 13C has been a critical tool for developing our understanding of photosynthesis. Early surveys of carbon isotopes in plants led to questions about whether the isotopic ratio of a given plant is ultimately controlled by an active response to its environment or by the relatively static biochemistry of photosynthesis. This led to a model of the carbon isotope composition of plant tissue meant to describe the integrated effects of physical diffusion, the biochemical action of enzymes, and the physiological response of stomata on net carbon fixation. In the decades since, recognition of the CO2 rise has highlighted the role of photorespiration, inviting a more thorough analysis to isolate the effect of CO2 concentration on 13C uptake and fixation. Plant growth chamber experiments performed across multiple levels of CO2 quantified the dependency of net carbon isotope discrimination on CO2, and showed the effect to be both significant and independent of the effect of water limitation. These advances pave the way for the determination of ancient CO2 levels from the stable isotope composition of fossilized plant remains, particularly for understudied, yet fossil-rich, periods of geologic time.