Background
"Because providing EC [elevated concentrations of carbon dioxide] more hours per day is expensive," according to Bunce, "many free air CO2 enrichment systems provide EC only during the daytime."  Consequently, since he had previously found that "several studies have indicated that plant growth may respond to [CO2] during the dark (reviewed in Bunce, 2003)," he decided "to test whether the seed yield of soybean would be the same whether EC was provided continuously (ECdn) or only in the daytime (ECd)."

What was done
Well fertilized plants exposed to normal precipitation while being grown from seed to maturity out-of-doors in open-top chambers were continuously fumigated with either ambient air (AC) or with air enriched with an extra 350 ppm of CO2 either 24 hours per day (ECdn) or 14 hours per day centered on solar noon (ECd) for a total of four growing seasons.

What was learned
Bunce says that "ECdn increased seed yield by an average of 62% over the four years compared with the ambient CO2 treatment, while ECd increased seed yield by 34%," indicative of the fact that the CO2-induced yield enhancement in the ECdn treatment was fully 82% greater than that in the ECd treatment (62% / 34% = 1.82).

What it means
Bunce's final conclusion about the matter, which seems a grand understatement to us, is that "these results suggest that caution should be used in interpreting soybean yield data obtained using systems in which EC is provided only in the daytime."  Indeed, they should raise a really red flag about all plants studied in this way, for results obtained by daylight-only CO2 enrichment may greatly underestimate what will actually occur in a future CO2-enriched world, where elevated CO2 concentrations will obviously be present both day and night.

Reference
Bunce, J.A.  2003.  Responses of seedling growth to daytime or continuous elevation of carbon dioxide.  International Journal of Plant Science 164: 377-382.