What was done
The authors grew hybrid poplar (Populus x euroamericana) saplings for one full growing season in a FACE system located in Rapolano, Italy, where the air's CO2 concentration was increased by approximately 200 ppm, while they periodically measured rates of leaf development and isoprene emission.

What was learned
Centritto et al. say their study demonstrated that "isoprene emission is reduced in elevated CO2, in terms of both maximum values of isoprene emission rate and isoprene emission per unit of leaf area averaged across the total number of leaves per plant," the latter amounting to a reduction of approximately 34%.  When isoprene emission was summed over the entire plant profile, however, the reduction in the emission rate of isoprene was not nearly so great because of the greater number of leaves on the CO2-enriched saplings, which resulted in a whole-canopy isoprene emission reduction of only 6%.  "However," as they state, "Centritto et al. (1999), in a study with potted cherry seedlings grown in open-top chambers, and Gielen et al. (2001), in a study with poplar saplings exposed to FACE, showed that the stimulation of total leaf area in response to elevated CO2 was a transient effect, because it occurred only during the first year of growth."  Hence, they conclude that "it may be expected that with similar levels of leaf area, the integrated emission of isoprene would have been much lower in elevated CO2."

What it means
In the words of the authors, "our data-set as well as that reported by Scholefield et al. (2004), in a companion experiment on Phragmites growing in a nearby CO2 spring, mostly confirm that isoprene emission is inversely dependent on CO2 [concentration] when this is above ambient, and suggests that a lower fraction of C will be re-emitted in the atmosphere as isoprene by single leaves in the future."  Consequently, these two new studies join that of Rosenstiel et al. (2003) in suggesting a future CO2-induced reduction of isoprene-prompted increases in tropospheric ozone pollution and methane-induced global warming, as described in that earlier paper and our review of it.

References
Centritto, M., Lee, H. and Jarvis, P.  1999.  Interactive effects of elevated [CO2] and water stress on cherry (Prunus avium) seedlings.  I.  Growth, total plant water use efficiency and uptake.  New Phytologist 141: 129-140.

Gielen, B., Calfapietra, C., Sabatti, M. and Ceulemans, R.  2001.  Leaf area dynamics in a poplar plantation under free-air carbon dioxide enrichment.  Tree Physiology 21: 1245-1255.

Rosentiel, T.N., Potosnak, M.J., Griffin, K.L., Fall, R. and Monson, R.K.  2003.  Increased CO2 uncouples growth from isoprene emission in an agriforest ecosystem.  Nature 421: 256-259.

Scholefield, P.A., Doick, K.J., Herbert, B.M.J., Hewitt, C.N.S., Schnitzler, J.-P., Pinelli, P. and Loreto, F.  2004.  Impact of rising CO2 on emissions of volatile organic compounds: isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring.  Plant, Cell and Environment 27: 393-401.