Background
The author writes that "because free-air CO2 enrichment (FACE) systems avoid several modifications of the environment associated with various enclosure systems, many consider that FACE systems provide more realistic information on plant responses to rising atmospheric CO2 than do enclosure systems." However, he states that "many systems for prolonged exposure of plant communities to elevated CO2 concentrations, but perhaps especially FACE systems, produce large fluctuations in CO2 concentration in addition to increasing the mean concentration," the importance of which phenomenon to plant functions "remains uncertain."

What was done
In a study aimed at reducing this uncertainty, Bunce exposed single leaves of wheat (Triticum aestivum cv Choptank) and rice (Oryza sativa cv Akitakomachi) for 30 minutes "to pulses of elevated CO2 similar to the frequency, duration and magnitude of pulses observed in free-air CO2 enrichment systems," during and after which he measured both leaf stomatal conductances and photosynthetic rates.

What was learned
The U.S. Agricultural Research Service scientist determined that (1) "both species had 20-35% lower stomatal conductance for at least 30 minutes after the termination of the pulses," that (2) "in all cases photosynthesis was below the values expected for the observed sub-stomatal CO2 concentration," and that (3) "a single 2-second pulse of elevated CO2 concentration reaching a maximum of 1000 µmole mol^-1 decreased stomatal conductance in both species."

In a note of caution, however, Bunce states that "it is possible that plants may acclimate to more prolonged pulsing of high CO2 concentration, and eventually return to normal photosynthesis and stomatal conductance in FACE systems." But he reports that such did not occur "in cotton and wheat grown in the field with fluctuating CO2 concentrations in open top chambers," as he had determined in a prior experiment (Bunce, 2012).

What it means
"On the basis of these results," in the concluding words of Bunce, "it is probable that plants in many CO2 enrichment systems have lower photosynthesis and stomatal conductance than would plants exposed to the same mean CO2 concentration but without pulses of higher concentration." And this likelihood suggests that the positive impacts of the ongoing rise in the atmosphere's CO2 concentration may be considerably larger than what has long been believed to be the case.

Reference
Bunce, J.A. 2012. Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration. Photosynthetica 50: 395-400.