Yield of wheat across a subambient carbon dioxide gradient

Yields and yield components of two cultivars of day-neutral spring wheat ( Triticum aestivum L.) were assessed along a gradient of daytime carbon dioxide (CO₂) concentrations from about 200 to near 350 μmol CO₂ (mol air)^–1 in a 38 m-long controlled environment chamber. The range in CO₂ concentration studied approximates that of Earth's atmosphere since the last ice age. This 75% rise in CO₂ concentration increased grain yields more than 200% under well-watered conditions and by 80–150% when wheat was grown without additions of water during the last half of the 100-day growing season. The 27% increase in CO₂ from the pre-industrial level of 150 years ago (275 μmol mol^–1) to near the current concentration (350 μmol mol^–1) increased grain yields of 'Yaqui 54' and 'Seri M82' spring wheats by 55% and 53%, respectively, under well-watered conditions. Yield increased because of greater numbers of grains per spike, rather than heavier grains or numbers of spikes per plant. Water use increased little with CO₂ concentration, resulting in improved water use efficiency as CO₂ rose. Data suggest that rising CO₂ concentration contributed to the substantial increase in average wheat yields in the U.S. during recent decades.