What was done
In a mini-review of the authors' previous research - involving 16 open-top chamber experiments conducted on spring wheat in Denmark, Finland, Sweden and Switzerland between 1986 and 1996 - the individual and combined effects of elevated CO2 and ozone, plus the similar effect of water stress, on the relationship between grain yield and protein content are discussed.

What was learned
The grain protein content of spring wheat was negatively and linearly associated with grain yield.  Thus, elevated CO2, which generally stimulated grain yield, tended to reduce grain protein content.  In contrast, elevated ozone, which typically reduced grain yield, tended to increase grain protein content.  In addition, one and the same relationship described both effects, as well as the effect of water stress, which when severe enough to depress grain yield, increased grain protein content.

What it means
The knowledge gained from this review clearly indicates there is nothing unusual about the oft-observed tendency for atmospheric CO2 enrichment to slightly depress grain protein concentrations in wheat; this phenomenon is merely a universal response to increased grain production that can be brought about equally as easily by decreases in tropospheric ozone concentration or the severity of soil moisture stress.  Hence, in situations where significant yield increases are anticipated, such as would be expected when CO2 increases, O3 decreases or soil moisture is plentiful, extra nitrogen fertilization may be required to keep grain protein concentrations in the range desired.