Rapid conversion of isoprene photooxidation products in terrestrial plants

Eva Canaval,Eva Maria Partoll,Jörg-Peter Schnitzler,Ina Zimmer,Markku Kulmala,Dylan B Millet,Elisabeth Georgii,Armin Hansel,Hariprasad D Alwe,Tetyana Nosenko,Thomas Karl,Lukas Fischer

doi:10.1038/s43247-020-00041-2

Abstract

Isoprene is emitted from the biosphere into the atmosphere, and may strengthen the defense mechanisms of plants against oxidative and thermal stress. Once in the atmosphere, isoprene is rapidly oxidized, either to isoprene-hydroxy-hydroperoxides (ISOPOOH) at low levels of nitrogen oxides, or to methyl vinyl ketone (MVK) and methacrolein at high levels. Here we combine uptake rates and deposition velocities that we obtained in laboratory experiments with observations in natural forests to show that 1,2-ISOPOOH deposits rapidly into poplar leaves. There, it is converted first to cytotoxic MVK and then most probably through alkenal/one oxidoreductase (AOR) to less toxic methyl ethyl ketone (MEK). This detoxification process is potentially significant globally because AOR enzymes are ubiquitous in terrestrial plants. Our simulations with a global chemistry-transport model suggest that around 6.5 Tg yr−1 of MEK are re-emitted to the atmosphere. This is the single largest MEK source presently known, and recycles 1.5% of the original isoprene flux. Eddy covariance flux measurements of isoprene and MEK over different forest ecosystems confirm that MEK emissions can reach 1–2% those of isoprene. We suggest that detoxification processes in plants are one of the most important sources of oxidized volatile organic compounds in the atmosphere.

Highlights

Isoprene is emitted from the biosphere into the atmosphere, and may strengthen the defense mechanisms of plants against oxidative and thermal stress
Under low nitrogen oxide (NOx) conditions the main product of isoprene peroxy radicals HO–C5H8O2 (ISOPOO) reacting with HO2 is isoprene hydroxy hydroperoxide (ISOPOOH), with the 1,2-ISOPOOH isomer formed in highest yield[5]
We investigated the fate of isoprene oxidation products when exposed to poplar leaves (Fig. 2, Supplementary Figs. 1–4)

Summary

Introduction

Isoprene is emitted from the biosphere into the atmosphere, and may strengthen the defense mechanisms of plants against oxidative and thermal stress. There, it is converted first to cytotoxic MVK and most probably through alkenal/ one oxidoreductase (AOR) to less toxic methyl ethyl ketone (MEK) This detoxification process is potentially significant globally because AOR enzymes are ubiquitous in terrestrial plants. Nguyen et al.[8] applied the eddy covariance (EC) technique to quantify the rapid dry deposition of ISOPOOH and other oxidized BVOC to a temperate forest. They determined a daytime mean deposition velocity (vd) for ISOPOOH + IEPOX of 2.5 cm s−1. We apply chemistrytransport modeling to assess atmospheric implications of our findings (see “Methods” section for details)

Methods

Results

Conclusion