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^†

Abstract

Isoprene basal emission (the emission of isoprene from leaves exposed to a light intensity of 1000 m m m m mol m ----2 s ----1 and maintained at a temperature of 30 ∞ ∞ ∞ ∞ C) was measured in Phragmites australis plants growing under elevated CO 2 in the Bossoleto CO 2 spring at Rapolano Terme, Italy, and under ambient CO 2 at a nearby control site. Gas exchange and biochemical measurements were concurrently taken. Isoprene emission was lower in the plants growing at elevated CO 2 than in those growing at ambient CO 2 . Isoprene emission and isoprene synthase activity (IsoS) were very low in plants growing at the bottom of the spring under very rich CO 2 and increased at increasing distance from the spring (and decreasing CO 2 concentration). Distance from the spring did not significantly affect photosynthesis making it therefore unlikely that there is carbon limitation to isoprene formation. The isoprene emission rate was very quickly reduced after rapid switches from elevated to ambient CO 2 in the gas-exchange cuvette, whereas it increased when switching from ambient to elevated CO 2 . The rapidity of the response may be consistent with post-translational modifications of enzymes in the biosynthetic pathway of isoprene formation. Reduction of IsoS activity is interpreted as a long-term response. Basal emission of isoprene was not constant over the day but showed a diurnal course opposite to photosynthesis, with a peak during the hottest hours of the day, independent of stomatal conductance and probably dependent on external air temperature or temporary reduction of CO 2 concentration. The present experiments show that basal emission rate of isoprene is likely to be reduced under future elevated CO 2 levels and allow improvement in the modelling of future isoprene emission rates.

The use of branch enclosures to assess direct and indirect effects of elevated CO2 on photosynthesis, respiration and isoprene emission of Populus alba leaves

“…Isoprene is formed predominantly from photosynthetic carbon fixation (Sharkey & Yeh 2001) and it was expected that the two processes be simultaneously stimulated by increasing availability of CO2. However, exposure to or growth at elevated CO2 often reduce isoprenoid emission by vegetation (Loreto & Sharkey 1990, Loreto et al 2001a, Scholefield et al 2004, Rosen stiel et al 2003, with few exceptions (e.g., Sharkey et al 1991, Rapparini et al 2001. This uncoupling between the two processes may be due to an inhibition of isoprene syn thase activity under elevated CO2 (Schole field et al 2004) or to a reduction of the availability of isoprene synthase substrate (predominantly dimethylallyl diphosphate (DMADP), Rosenstiel et al 2003).…”

“…A portion of the leaf was clamped in the cuvette of a portable Li-Cor 6400 (LiCor, Lincoln, NE, USA) gas exchange sys tem as described by Scholefield et al (2004). This system allows very fast changes of CO2 concentration (380 ppm or 900 ppm) and source ( 12 CO2 or 13 CO2) or light intensity while controlling all other environmental pa rameters.…”

The Relationship between the Methyl-Erythritol Phosphate Pathway Leading to Emission of Volatile Isoprenoids and Abscisic Acid Content in Leaves

“…However, no evidence of changes in total carotenoids and in xanthophylls deepoxidation status Scholefield et al, 2004) were previously found upon fosmidomycin feeding for short periods (up to 1 h). Similar results were found in these experiments, being the deepoxidation status of xanthophylls independent of fosmidomycin, while the total amount of xanthophylls was slightly but not significantly reduced following fosmidomycin feeding (data not shown).…”

Isoprene emission from terrestrial ecosystems in response to global change: minding the gap between models and observations

“…The fundamental logic of such models is that changes in NPP will produce more or less biomass capable of emitting isoprene, and changes in climate (principally temperature) will stimulate or inhibit emissions per unit of biomass. These models tend to ignore the discovery that there are direct effects of changes in the atmospheric CO 2 concentration on isoprene emission that tend to work in the opposite direction to that of stimulated NPP (Sanadze 1964;Monson & Fall 1989;Rosenstiel et al 2003;Centritto et al 2004;Rapparini et al 2004;Scholefield et al 2004;Possell et al 2005; figure 1). Progress has been made in the past few years on the biochemical mechanisms underlying this direct response (Rosenstiel et al 2003(Rosenstiel et al , 2004, and it is now possible to propose strategies for incorporating these effects into surface emission models.…”