Stomatal uptake and stomatal deposition of ozone in isoprene and monoterpene emitting plants

Volatile isoprenoids were reported to protect plants against ozone. Tounderstand whether this could be the result of a direct scavenging of ozoneby these molecules, the stomatal and non-stomatal uptake of ozone was esti-mated in plants emitting isoprene or monoterpenes. Ozone uptake by holmoak (Quercus ilex, a monoterpene emitter) and black poplar (Populus nigra,an isoprene emitter) was studied in whole plant enclosures (continuouslystirred tank reactors, CSTR). The ozone uptake by plants was estimatedmeasuring ozone concentration at the inlet and outlet of the reactors, aftercorrecting for the uptake of the enclosure materials. Destruction of ozone atthe cuticle or at the plant stems was found to be negligible compared to theozone uptake through the stomata. For both plant species, a relationshipbetween stomatal conductance and ozone uptake was found. For the poplar,the measured ozone losses were explained by the uptake of ozone throughthe stomata only, and ozone destruction by gas phase reactions with iso-prene was negligible. For the oak, gas phase reactions of ozone with themonoterpenes emitted by the plants contributed significantly to ozonedestruction. This was confirmed by two different experiments showing a)that in cases of high stomatal conductance but under low CO 2 concentra-tion, a reduction of monoterpene emission was still associated with reducedO 3 uptake; and b) that ozone losses due to the gas phase reactions only canbe measured when using the exhaust from a plant chamber to determinethe gas phase reactivity in an empty reaction chamber. Monoterpenes cantherefore relevantly scavenge ozone at leaf level contributing to protectionagainst ozone