Although terrestrial atmospheric CO2 levels will not reach 1000 mu mol
mol(-1) (0.1%) for decades, CO2 levels in growth chambers and greenho
uses routinely exceed that concentration. CO2 levels in life support s
ystems in space can exceed 10 000 mu mol mol(-1)(1%). Numerous studies
have examined CO2 effects up to 1000 mu mol mol(-1), but biochemical
measurements indicate that the beneficial effects of CO2 can continue
beyond this concentration. We studied the effects of near-optimal (app
roximate to 1200 mu mol mol(-1)) and super-optimal CO2 levels (2400 mu
mol mol(-1)) on yield of two cultivars of hydroponically grown wheat
(Triticum aestivum L.) in 12 trials in growth chambers. Increasing CO2
from sub-optimal to near-optimal (350-1200 mu mol mol(-1)) increased
vegetative growth by 25% and seed yield by 15% in both cultivars. Yiel
d increases were primarily the result of an increased number of heads
per square meter. Further elevation of CO2 to 2500 mu mol mol(-1) redu
ced seed yield by 22% (P < 0.001) in cv. Veery-10 and by 15% (P < 0.00
1) in cv. USU-Apogee. Super-optimal CO2 did not decrease the number of
heads per square meter, but reduced seeds per head by 10% and mass pe
r seed by 11%. The toxic effect of CO2 was similar over a range of lig
ht levels from half to full sunlight. Subsequent trials revealed that
super-optimal CO2 during the interval between 2 wk before and after an
thesis mimicked the effect of constant super-optimal CO2. Furthermore,
near-optimal CO2 during the same interval mimicked the effect of cons
tant near-optimal CO2. Nutrient concentration of leaves and heads was
not affected by CO2. These results suggest that super optimal CO2 inhi
bits some process that occurs near the time of seed set resulting in d
ecreased seed set, seed mass, and yield.