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 (Ca ) 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 Ca limits vegetative growth in the wild progenitors of both C3 and C4 founder crops. Here, we present data showing that glacial Ca also reduces grain yield in both crop types. We grew four wild progenitors of C3 (einkorn wheat and barley) and C4 crops (foxtail and broomcorn millets) at glacial and postglacial Ca , measuring grain yield and the morphological and physiological components contributing to these yield changes. The C3 species showed a significant increase in unthreshed grain yield of ~50% with the glacial to postglacial increase in Ca , which matched the stimulation of photosynthesis, suggesting that increases in photosynthesis are directly translated into yield at subambient levels of Ca . 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 C4 species showed smaller, but significant, increases in grain yield of 10-15%, arising from larger seed numbers and sizes. Photosynthesis was enhanced by Ca in only one C4 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 C4 species at glacial Ca showed some evidence that photosynthetic capacity was upregulated to enhance carbon capture. Development under glacial Ca also impacted negatively on the subsequent germination and viability of seeds. These results suggest that the grain production of both C3 and C4 crop progenitors was limited by the atmospheric conditions of the last glacial period, with important implications for the origins of agriculture.