Super-optimal CO2 reduces seed yield but not vegetative growth in wheat

Although terrestrial atmospheric CO2 levels will not reach 1000 mu mol mol(-1) (0.1%) for decades, CO2 levels in growth chambers and greenhouses routinely exceed that concentration. CO2 levels in life support systems 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 (approximate 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. Yield increases were primarily the result of an increased number of heads per square meter. Further elevation of CO2 to 2500 mu mol mol(-1) reduced seed yield by 22% (P < 0.001) in cv. Veery-10 and by 15% (P < 0.001) 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 per seed by 11%. The toxic effect of CO2 was similar over a range of light levels from half to full sunlight. Subsequent trials revealed that super-optimal CO2 during the interval between 2 wk before and after anthesis mimicked the effect of constant super-optimal CO2. Furthermore, near-optimal CO2 during the same interval mimicked the effect of constant near-optimal CO2. Nutrient concentration of leaves and heads was not affected by CO2. These results suggest that super optimal CO2 inhibits some process that occurs near the time of seed set resulting in decreased seed set, seed mass, and yield.