Effects of elevated CO₂ on the protein concentration of food crops: a meta‐analysis

Abstract Meta‐analysis techniques were used to examine the effect of elevated atmospheric carbon dioxide [CO 2 ] on the protein concentrations of major food crops, incorporating 228 experimental observations on barley, rice, wheat, soybean and potato. Each crop had lower protein concentrations when grown at elevated (540–958 μmol mol −1 ) compared with ambient (315–400 μmol mol −1 ) CO 2 . For wheat, barley and rice, the reduction in grain protein concentration was ∼10–15% of the value at ambient CO 2 . For potato, the reduction in tuber protein concentration was 14%. For soybean, there was a much smaller, although statistically significant reduction of protein concentration of 1.4%. The magnitude of the CO 2 effect on wheat grains was smaller under high soil N conditions than under low soil N. Protein concentrations in potato tubers were reduced more for plants grown at high than at low concentrations of ozone. For soybean, the ozone effect was the reverse, as elevated CO 2 increased the protein concentration of soybean grown at high ozone concentrations. The magnitude of the CO 2 effect also varied depending on experimental methodology. For both wheat and soybean, studies performed in open‐top chambers produced a larger CO 2 effect than those performed using other types of experimental facilities. There was also indication of a possible pot artifact as, for both wheat and soybean, studies performed in open‐top chambers showed a significantly greater CO 2 effect when plants were rooted in pots rather than in the ground. Studies on wheat also showed a greater CO 2 effect when protein concentration was measured in whole grains rather than flour. While the magnitude of the effect of elevated CO 2 varied depending on the experimental procedures, a reduction in protein concentration was consistently found for most crops. These findings suggest that the increasing CO 2 concentrations of the 21st century are likely to decrease the protein concentration of many human plant foods.