What was done
Well-watered and fertilized white cabbage (Brassica oleracea subsp. capitata cv. Lennox) plants were grown from seed for 25 days in 1-liter pots filled with Sphagnum peat and sand (3:1 v/v) that were housed in growth chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm.  One group of plants in each CO2 treatment experienced no larval insect feeding, while one group experienced 48 hours of feeding by larvae of the crucifer specialist Plutella xylostella L. (Lepidoptera: Yponomeutidae), and another group experienced 48 hours of feeding by larvae of the generalist Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae), after which a number of volatile organic compounds (VOCs) that were released from each group of plants were collected from the air surrounding them and analyzed.

What was learned
With respect to biomass production, the doubled CO2 concentration increased plant dry weight by 47% in the no-larval-feeding treatment, while it increased it by 13% in the P. xylostella-feeding treatment and by 33% in the S. littoralis-feeding treatment.  With respect to larval-induced damage to the plants, there was no difference between the ambient-air and CO2-enriched treatments.  In addition, the authors report that "total monoterpene emission per shoot dry weight was approximately 27% reduced from plants grown at elevated CO2 and this proportion was not affected by insect damage."

What it means
It is difficult to say for sure what these observations ultimately imply about a CO2-enriched world of the future.  As noted in our Editorial of 27 Aug 2003, there are a host of potential phenomena that could be impacted by changes in biological VOC emissions; but the net result(s) is(are) by no means clear.  In addition, other studies have recorded both CO2-induced increases (Staudt et al., 2001) and decreases (Loreto et al., 2001) in monoterpene emissions from Quercus ilex seedlings, while Constable et al. (1999) found no effect of elevated CO2 on monoterpene emissions from Ponderosa pine and Douglas fir trees.  Much more research will be needed before we will be able to conclude anything substantive about this subject with more than marginal confidence.

References
Constable, J.V.H., Litvak, M.E., Greenberg, J.P. and Monson, R.K.  1999.  Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming.  Global Change Biology 5: 255-267.

Loreto, F., Fischbach, R.J., Schnitzler, J.P., Ciccioli, P., Brancaleoni, E., Calfapietra, C. and Seufert, G.  2001.  Monoterpene emission and monoterpene synthase activities in the Mediterranean evergreen oak Quercus ilex L. grown at elevated CO2 concentrations.  Global Change Biology 7: 709-717.

Staudt, M., Joffre, R., Rambal, S. and Kesselmeier, J.  2001.  Effect of elevated CO2 on monoterpene emission of young Quercus ilex trees and its relation to structural and ecophysiological parameters.  Tree Physiology 21: 437-445.