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Thomas D. Sharkey, Amy E. Wiberley, Autumn R. Donohue, Isoprene Emission from Plants: Why and How, Annals of Botany, Volume 101, Issue 1, January 2008, Pages 5–18, https://doi.org/10.1093/aob/mcm240
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Abstract
Some, but not all, plants emit isoprene. Emission of the related monoterpenes is more universal among plants, but the amount of isoprene emitted from plants dominates the biosphere–atmosphere hydrocarbon exchange.
The emission of isoprene from plants affects atmospheric chemistry. Isoprene reacts very rapidly with hydroxyl radicals in the atmosphere making hydroperoxides that can enhance ozone formation. Aerosol formation in the atmosphere may also be influenced by biogenic isoprene. Plants that emit isoprene are better able to tolerate sunlight-induced rapid heating of leaves (heat flecks). They also tolerate ozone and other reactive oxygen species better than non-emitting plants. Expression of the isoprene synthase gene can account for control of isoprene emission capacity as leaves expand. The emission capacity of fully expanded leaves varies through the season but the biochemical control of capacity of mature leaves appears to be at several different points in isoprene metabolism.
The capacity for isoprene emission evolved many times in plants, probably as a mechanism for coping with heat flecks. It also confers tolerance of reactive oxygen species. It is an example of isoprenoids enhancing membrane function, although the mechanism is likely to be different from that of sterols. Understanding the regulation of isoprene emission is advancing rapidly now that the pathway that provides the substrate is known.
Comments
We welcome Young’s clarification of the atmospheric chemistry of isoprene. Although we spent some time trying to put together a scheme that was both correct and accessible to the readers of Ann. Bot., it may well be that we got some of the atmospheric chemistry wrong. One area of agreement is that isoprene emission from plants has a significant effect on atmospheric chemistry and it will be very useful to know the biology of this phenomenon as one part of a comprehensive understanding of isoprene.
Thomas D. Sharkey, Amy E. Wiberley, and Autumn R. Donohue, Dept. Biochem. Mol. Biol., Michigan State University, East Lansing, MI 48824 USA ([email protected]) (note new contact information)
Conflict of Interest:
None declared
Sir,
As an atmospheric chemist working on the impact of biogenic emissions, I very much welcomed this invited review on isoprene emission from plants, particularly the opportunity to read a summary of the advances in the understanding of the biological aspects of isorpene emission (phylogeny, biosynthesis etc.) that are often absent from literature in my field. However, I feel it is necessary to clarify the description of tropospheric chemistry and isoprene oxidation presented by the authors.
In their "Why Isoprene Emissions Matter" section, I am not clear whether the authors are stating that the source of OH (the most important tropospheric oxidizing agent) is from conversion of HO2 via reaction with NO. In fact, the major source of OH is from a photolysis channel of ozone that produces high energy oxygen atoms (O1D), followed by reaction with water vapour, such:
1. O3 + hv(lamda < 320nm) -> O2 + O1D
2. O1D + H2O -> OH + OH
Globally, isoprene tends to reduce OH concentrations, compared to no emissions (e.g. Spivakovsky et al., 2000), as it provides a direct sink. Thus, as the OH concentration largely determines the methane lifetime, isoprene can be assigned a global warming potential (Collins et al., 2002); i.e. its emission is relevant to climate as well as air quality.
Furthermore, the authors state that isoprene oxidation proceeds via H -atom abstraction by an OH radical (forming H2O). As isoprene is a (di- )alkene (2 sets of C=C bonds), oxidation by OH proceeds predominantly via an addition mechanism (see Wayne, 2000, pp333-334). Illustrating with the most simple alkene, ethene:
1. CH2=CH2 + OH (+M) -> .CH2-CH2OH (+M)
2. .CH2=CH2OH + O2 -> CH2(OO)-CH2OH
where M is N2 or O2 and CH2(OO)-CH2OH is equivalent to the RO2 peroxy radical described by Sharkey et al.
It is the presence of 2 C=C bonds that make isoprene so reactive in the atmosphere, enabling reactions with ozone and NO3 (produced at nighttime from NO2 + O3) as well as OH. The asymmetry of isoprene results in a great array of oxidation products, some of which retain a C=C bond and are thus still very reactive (e.g. methacrolein, methyl vinyl ketone); see Atkinson & Arey (2003) (and refs. therein) for review of the gas- phase chemistry.
Those further interested in the atmospheric chemistry of isoprene and its impacts should also see the following modelling articles: von Kuhlmann et al. (2004), Fiore et al. (2005), Horowitz et al. (2007), Zeng et al., (2008).
Whilst these details may not be of interest to all this Journal's readership, I feel it is important to correct the mistake in an article that addresses an interdisciplinary area.
Yours faithfully,
Paul Young
Centre for Atmospheric Science, University of Cambridge, UK
References:
Atkinson, R. & J. Arey (2003), Gas-phase tropospheric chemistry of biogenic volatile organic compounds: a review, Atmos. Environ., 37, S197-S219.
Collins, W. J. et al. (2002), The oxidation of organic compounds in the troposphere and their global warming potentials, Clim. Change, 52, 453 -479.
Fiore, A. M. et al. (2005), Evaluating the contribution of changes in isoprene emissions to surface ozone trends over the eastern United States, J. Geophys. Res., 110, D12303, doi:10.1029/2004JD005485.
Horowitz, L. W. et al. (2007), Observational constraints on the chemistry of isoprene nitrates over the eastern United States, J. Geophys. Res., 112, D12S08, doi:10.1029/2006JD007747.
Spivakovsky, C. et al. (2000), Three-dimensional climatological distribution of tropospheric OH: Update and evaluation, J. Geophys. Res., 105, 8931--8980.
von Kuhlmann, R. et al. (2004), Sensitivities in the global scale modelling of isoprene, Atmos. Chem. Phys., 4, 1-17.
Wayne, R. P. (2000), Chemistry of atmospheres, 3rd ed., OUP, Oxford, UK.
Zeng, G. et al. (2008), Impact of climate change on tropospheric ozone and its global budgets, Atmos. Chem. Phys., 8, 369-387.
Conflict of Interest:
None declared