Effects of CO₂ on Cereal Grain and Protein Production

It is sometimes claimed that while cereal grain crops may be more productive in a high-CO₂ world of the future, the amount of protein they produce may be lower, making their grains less nutritious. Investigating this debate, Veisz et al. (2008) grew seven cereal grain crops -- winter barley (Hordeum vulgare, cv. Petra), winter wheat (Triticum aestivum, cvs. Libellula, Mv Regiment, Mv Mambo), winter durum wheat (Triticum durum, cv. Mv Makaroni), spring wheat (Triticum aestivum, cv Lona), and spring oats (Avena sativa, cv. Mv Pehely) -- in a phytotron at the Agricultural Research Institute of the Hungarian Academy of Sciences at ambient and enriched atmospheric CO₂ concentrations (380 and 750 ppm, respectively) under well-watered conditions or drought conditions, where water was withheld from the 10th day after heading, during which time soil volumetric water content dropped from approximately 25% to 6%, after which they measured a number of crop characteristics at harvest.

Veisz et al. report that the plants grown in the CO₂-enriched air "produced more organic matter, being taller, with more spikes and a higher grain number per plant than those grown at the present CO₂ level," and that "thanks to the more intensive incorporation of carbohydrate, there was an increase in the mean grain mass and in the grain yield per plant" in the CO₂-enriched air. However, there was a concomitant decrease in the protein concentration of the grains produced in the high CO₂ treatment. Nevertheless, the net effect was still positive, as noted in the following paragraph.

For the several cereal varieties averaged together, grain yield under the well-watered conditions rose by 12.37% (from 2.83 to 3.18 g/plant) in response to atmospheric CO₂ enrichment, while grain protein concentration dropped from 17.04% to only 16.23%, which resulted in a net increase of 7% in total grain protein production. Likewise, grain yield under the water-stressed conditions rose by 30.68% (from 1.76 to 2.30 g/plant) in response to atmospheric CO₂ enrichment, while grain protein concentration dropped from 21.63% to 19.70%, which led to a net increase of 19% in total grain protein production.

The increase in cereal-grain biomass production that accompanied the rise in the air's CO₂ content in this study was more than adequate to more than compensate for the much smaller decrease in grain protein concentration that occurred concomitantly in these crops, which resulted in a net increase in total protein harvest, which is generally typical of all such studies.