text: Two years later, Bunce (2005), grew soybean plants from seed to maturity out-of-doors in open-top chambers exposed to normal precipitation while continuously fumigating them with either ambient air (AC) or with air enriched with an extra 350 ppm of CO 2 either 24 hours per day (ECdn) or 14 hours per day centered on solar noon (ECd) for a total of four entire growing seasons . And this sustained effort, in Bunce's words, revealed that "ECdn increased seed yield by an average of 62% over the four years compared with the ambient CO 2 treatment, while ECd increased seed yield by 34%," indicative of the fact that the CO 2 -induced yield enhancement in the 24-hour CO 2 enrichment treatment was almost twice as great as that of the daylight-only CO 2 enrichment treatment.