Growth, Yield and Water Use of Wheat under Elevated Carbon Dioxide

Crops grown under high CO₂ gave, on average, about a 50% increase in yield. This increase occurred irrespective of the sowing time or year. The May to November rainfalls were a dry 148 mm in 2008 and a more normal 264 mm for 2009. The harvest index of these crops—the proportion of growth that goes to grain—was not reduced with high CO₂ so the plants were actually operating more efficiently with the extra carbon available to them in the atmosphere.
The yield response suggests that CO₂ will help reduce the impact of higher temperatures and lower rainfalls, even in the low rainfall regions of Australia. However, higher yields come with lower grain protein content which is part of a physiological adaptation to having more CO₂. The plant invests less N in proteins associated with photosynthesis so that when grain filling starts, there is less N to move to the grain. Our sites were well-fertilized with N, but the grain protein contents still slipped from 15.3% (2008) and 15.5% (2009) under normal conditions to 13.4% (2008) and 13.5% (2009) under elevated CO₂. Reductions in grain mineral content and changes in other aspects of grain quality were also noticed.