Yield responses of wild C3 and C4 crop progenitors to subambient CO2: a test for the role of CO2 limitation in the origin of agriculture

Limitation of plant productivity by the low partial pressure of atmospheric CO2 (C-a) experienced during the last glacial period is hypothesized to have been an important constraint on the origins of agriculture. In support of this hypothesis, previous work has shown that glacial C-a limits vegetative growth in the wild progenitors of both C-3 and C-4 founder crops. Here, we present data showing that glacial C-a also reduces grain yield in both crop types. We grew four wild progenitors of C-3 (einkorn wheat and barley) and C-4 crops (foxtail and broomcorn millets) at glacial and postglacial C-a, measuring grain yield and the morphological and physiological components contributing to these yield changes. The C-3 species showed a significant increase in unthreshed grain yield of similar to 50% with the glacial to postglacial increase in C-a, which matched the stimulation of photosynthesis, suggesting that increases in photosynthesis are directly translated into yield at subambient levels of C-a. Increased yield was controlled by a higher rate of tillering, leading to a larger number of tillers bearing fertile spikes, and increases in seed number and size. The C-4 species showed smaller, but significant, increases in grain yield of 10-15%, arising from larger seed numbers and sizes. Photosynthesis was enhanced by C-a in only one C-4 species and the effect diminished during development, suggesting that an indirect mechanism mediated by plant water relations could also be playing a role in the yield increase. Interestingly, the C-4 species at glacial C-a showed some evidence that photosynthetic capacity was upregulated to enhance carbon capture. Development under glacial C-a also impacted negatively on the subsequent germination and viability of seeds. These results suggest that the grain production of both C-3 and C-4 crop progenitors was limited by the atmospheric conditions of the last glacial period, with important implications for the origins of agriculture.