Paper Reviewed
Cunniff, J., Jones, G., Charles, M. and Osborne, C. 2017. 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. Global Change Biology 23: 380-393.

For those concerned about the modern rise in atmospheric CO2 and its potential impacts on climate, a solution most commonly put forward is to slow or even halt CO2 emissions in an effort to reduce the CO2 concentration of the atmosphere. Notwithstanding the enormous social and economic challenges such a reversal would present, is there any virtue from an agricultural perspective of reducing the CO2 content of the atmosphere?

A new study by Cunniff et al. (2017) sheds some important light on this question. Contrary to most plant growth studies involving CO2, which tend to examine its impact at higher concentrations, the team of four researchers investigated the effects of subambient CO2 levels. More specifically, they measured "the yield and physiological responses of the modern day representatives of two wild C3 and C4 crop progenitors to glacial and postglacial atmospheric CO2."

To accomplish their objective, they grew Triticum boeoticum (C3, einkorn wheat), Hordeum spontaneum (C3, wild barley), Panicum miliaceum (C4, broomcorn millet) and Setaria viridis (C4, foxtail millet) from seed to harvest in controlled environment chambers under a CO2 concentration of either 180 or 270 ppm, corresponding to levels associated with the last glacial maximum and during the preindustrial Holocene, respectively. The main focus of their efforts was on grain yield, given the importance of this parameter to human agriculture.

So what did the researchers learn?

As shown in the figure below, the grain yield of all crop species was significantly reduced at the lower CO2 concentration (180 ppm) compared to that experienced during the preindustrial Holocene (270 ppm), with declines of approximately 32, 34, 9 and 23 percent for einkorn wheat, wild barley, broomcorn millet and foxtail millet, respectively. What is more, all individual component factors affecting grain yield measured by Cunniff et al., including the number of tillers, percent of fertile tillers, seed number, percent viable seeds, seed size, total dry matter and harvest index, were also reduced in each plant at the lower CO2 value (although not all reductions were significant). The only exceptions were the number of tillers and percent fertile tillers for foxtail millet, the former of which were greater at lower CO2, while the latter remained identical at 100 percent. Other findings by the researchers included depressed values of light-saturated rates of photosynthesis, reduced seed germination rates and a decline in water use efficiencies at the lower, as opposed to the higher, CO2 concentration.

Commenting on their work, Cunniff et al. say that "the effects of CO2 concentration on yield in our experiment are likely to be conservative," due to the fact that they did not account for differences in air temperature experienced between the late glacial and Holocene CO2 epochs (the former of which periods was likely some 4 to 5 °C cooler than the latter), which temperature difference would have reduced yields even more. In light of these facts, their findings do not bode well for the future of agriculture if policies designed to limit, halt or even reverse the rise in atmospheric CO2 are enacted, especially given projections of future global food needs, which indicate that a near-doubling in global food production is necessary to adequately feed the rising global human population just a few short decades from now.

Figure 1. Total yield of the C3 and C4 species grown under glacial and preindustrial CO2 levels. Significance is indicated as * = <0.05 and *** = <0.001. Adapted from Cunniff et al. (2017).