Exploring the ecological fates of isoprene oxidation products

Deciduous trees emit large quantities of isoprene every year. Atmospheric oxidation leads to the formation of phytotoxic compounds. We investigated the fates of two major isoprene oxidation products on plants.
Exploring the ecological fates of isoprene oxidation products

At the beginning of my PhD I assisted lab-based experiments focusing on the impact of ozone on different tobacco varieties. We could show that ozone tolerant tobacco species inhibit a sophisticated defense strategy against elevated ozone exposure. Glandular trichomes, which are small, hair-like structures, secret specialized diterpenoid metabolites to the leaf surface.  These intermediate volatile organic compounds (IVOC) cover the plan surface acting as ozone scrubber thus strongly reducing stomatal uptake of ozone (https://doi.org/10.5194/acp-16-277-2016).
This ingenious defense mechanism triggered a series of questions:

“What defense mechanisms do plants have against other toxic organic gases present in the atmosphere? Will compounds, which result from isoprene oxidation cause stress to plants? Are there any other clever defense mechanisms?” Deciduous trees emit large quantities of isoprene around the globe. The reason why plants emit isoprene still remains in the dark. Several hypotheses have risen up in the past decade. For example, some assumed that isoprene helps the plant to protect itself against oxidative stress.

Working on the setup of the lab experiments with
poplar plants

To answer our questions, we decided to expose poplar plants with the main isoprene oxidation products methyl vinyl ketone (MVK) and isoprene-hydroxy-hydroperoxide (ISOPOOH). The detection of ISOPOOH with mass spectrometers is not easy. The 1,2-ISOPOOH isomer converts to MVK at metal surfaces. Usually the reaction chamber of mass spectrometers consists of metal. Therefor we had to design a metal-free reaction chamber for our PTR-MS instrument first. Secondly, chemical ionization of ISOPOOH using the classical reagent ion H3O+ produces many fragment ions. So, we adapted our instrument for the use with another, even softer reagent ion NH4+. After extensive testing our instrument was ready for the detection of ISOPOOH. (https://doi.org/10.3389/fchem.2019.00191)

In several months of lab experiments we fumigated a total of 40 poplars with MVK and ISOPOOH. The first results were exciting: Under daylight conditions the poplars converted 1,2-ISOPOOH to MVK and methyl ethyl ketone (MEK). To decode how plants transform these substances, we decided to take samples from the leaves before and after treatment for off-line analysis at the Helmholtz Zentrum Munich.

Poplar plant in the teflon coated glass cuvette
The PTR3 measured eddy-covariant fluxes on top
of the measurement tower in Hyytiälä, Finland.

Spurred on by our lab results, we wanted to find out what the global impact of the discovered conversion of ISOPOOH in plants would be. Dylan B. Millet and Hariprasad D. Alwe from the University of Minnesota used our deposition velocities and conversion efficiencies of isoprene oxidation products as input into a global chemistry-transport model and simulate the global impact.

At the same time my colleagues headed to the forest measurement station in Hyyitälä, Finland. They installed our newest and most sensitive instrument, the PTR-3, on top of a 40 m high measurement tower. Over the course of a few weeks we measured eddy covariant fluxes of volatile organic compounds over the boreal forest. Back home we compared the simulation data with the field data from Hyytiälä and Michigan (US) and found astonishing similarities in the ratio of isoprene and MEK.

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