Effects of light and temperature on photosynthetic carbon isotope discrimination in Pinus sylvestris

Background/Question/Methods: Stable isotopes have become a fundamental tool for understanding carbon fluxes at the scale of the leaf, plant, and ecosystem. However, a limited understanding of how environmental factors generate variation in stable isotopes of carbon at the leaf scale currently impedes our ability to interpret patterns at the plant and ecosystem scales. We studied 1) the effects of changes in light and temperature on the magnitude of leaf carbon isotope fractionation and 2) the time necessary to reach steady state given such environmental changes. To do so, we generated light (100 – 1200 umol m^-2 sec^-1) and temperature (10 – 33 °C) response curves in potted individuals of Pinus sylvestris L. (Scots Pine) using simultaneous online measurements of leaf level gas exchange and carbon isotope discrimination.  

Results/Conclusions: Carbon isotope discrimination varied between 9.6 and 22.4 ‰ (VPBD). Discrimination decreased non-linearly as a function of light, approaching an asymptote above ~ 500 umol m^-2 sec^-1. In contrast, we observed no consistent response to temperature. Carbon isotope discrimination reached steady state rapidly (70 ± 7 s) in response to a step change in light, whereas the photosynthetic gas exchange response was significantly slower (1205 ± 288 s). The consistent differences in the temporal response of photosynthesis versus carbon isotope fractionation raise compelling questions about the underlying biochemical and physical processes driving the processes, as well as how to account for such differences in other isotope applications. Constraining the magnitude and time of the isotopic response to changing environmental conditions can contribute to our understanding of underlying physiological processes at the scale of the leaf and plant, as well as how to model patterns at the scale of the ecosystem.