What was done
Over two consecutive years, the authors measured isoprenoid emissions from two species of mature oak trees - the deciduous downy oak (Quercus pubescens Willd.) and the evergreen holm oak (Quercus ilex L.) - growing close to a natural CO2 spring in central Italy, where atmospheric CO2 concentrations averaged about 1000 ppm, and at a nearby control site where the air's CO2 content was unaffected by the spring.

What was learned
Rapparini et al. report that long-term exposure to high levels of atmospheric CO2 did not significantly affect actual isoprenoid emissions from the trees, i.e., emissions experienced under prevailing environmental conditions at the time of measurement.  However, they say that "when leaves of plants grown in the control site were exposed for a short period to an elevated CO2 level by rapidly switching the CO2 concentration in the gas-exchange cuvette, both isoprene and monoterpene basal emissions were clearly inhibited," where basal emissions are defined as those that occur at standard measuring conditions of 30°C air temperature and 1000 µmol m^-2 s^-1 light intensity.

What it means
In the words of the authors, "these results generally confirm the inhibitory effect of elevated CO2 on isoprenoid emission."  In addition, they note that the absence of a CO2 effect on actual emissions might indicate "an interaction with multiple stresses," such as the "recurrent droughts" that are typical of the Mediterranean climate in which the experiment was conducted, which stresses are known to enhance isoprenoid emissions.  Hence, evidence continues to mount for the beneficial phenomenon of CO2-induced decreases in isoprene emissions described in our reviews of the papers of Rosentiel et al. (2003) , Scholefield et al. (2004) and Centritto et al. (2004).

References
Centritto, M., Nascetti, P., Petrilli, L., Raschi, A. and Loreto, F.  2004.  Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free-air CO2 enrichment.  Plant, Cell and Environment 27: 403-412.

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.