YIELD OF WHEAT ACROSS A SUBAMBIENT CARBON-DIOXIDE GRADIENT

Yields and yield components of two cultivars of day-neutral spring whe at (Triticum aestivum L.) were assessed along a gradient of daytime ca rbon dioxide (CO2) concentrations from about 200 to near 350 mu mol CO 2 (mol air)(-1) in a 38 m-long controlled environment chamber. The ran ge in CO2 concentration studied approximates that of Earth's atmospher e since the last ice age. This 75% rise in CO2 concentration increased grain yields more than 200% under well-watered conditions and by 80-1 50% when wheat was grown without additions of water during the last ha lf of the 100-day growing season. The 27% increase in CO2 from the pre -industrial level of 150 years ago (275 mu mol mol(-1)) to near the cu rrent concentration (350 mu mol mol(-1)) increased grain yields of 'Ya qui 54' and 'Seri M82' spring wheats by 55% and 53%, respectively, und er 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, resultin g in improved water use efficiency as CO2 rose. Data suggest that risi ng CO2 concentration contributed to the substantial increase in averag e wheat yields in the U.S. during recent decades.