Isoprenoid emission in trees of Quercus pubescens and Quercus ilex with lifetime exposure to naturally high CO2 environment
The long-term effect of elevated atmospheric CO 2 on iso- prenoid emissions from adult trees of two Mediterranean oak species (the monoterpene-emitting Quercus ilex L. and the isoprene-emitting Quercus pubescens Willd.) native to a high-CO 2 environment was investigated. During two con- secutive years, isoprenoid emission was monitored both at branch level, measuring the actual emissions under natural conditions, and at leaf level, measuring the basal emissions under the standard conditions of 30 ∞ C and at light intensity of 1000 m mol m - 2 s - - - 1 . Long-term exposure to high atmo- spheric levels of CO 2 did not significantly affect the actual isoprenoid emissions. However, when leaves of plants grown in the control site were exposed for a short period to an elevated CO 2 level by rapidly switching the CO 2 con- centration in the gas-exchange cuvette, both isoprene and monoterpene basal emissions were clearly inhibited. These results generally confirm the inhibitory effect of elevated CO 2 on isoprenoid emission. The absence of a CO 2 effect on actual emissions might indicate higher leaf temperature at elevated CO 2 , or an interaction with multiple stresses some of which (e.g. recurrent droughts) may compensate for the CO 2 effect in Mediterranean ecosystems. Under elevated CO 2 , isoprene emission by Q. pubescens was also uncoupled from the previous day's air temperature. In addi- tion, pronounced daily and seasonal variations of basal emission were observed under elevated CO 2 underlining that correction factors may be necessary to improve the realistic estimation of isoprene emissions with empirical algorithms in the future. A positive linear correlation of isoprenoid emission with the photosynthetic electron trans- port and in particular with its calculated fraction used for isoprenoid synthesis was found. The slope of this relation- ship was different for isoprene and monoterpenes, but did not change when plants were grown in either ambient or elevated CO 2 . This suggests that physiological algorithms may usefully predict isoprenoid emission also under rising CO 2 levels.