Worldwide, plants release about 100 million tonnes of monoterpenes into the atmosphere. These volatile organic molecules include many fragrances such as the molecule pinene, known for its fresh pine scent. Since these molecules are very reactive and can form tiny aerosol particles that can grow into condensation nuclei for raindrops, natural emissions play an important role in our climate. For climate predictions, it is therefore important to know how monoterpene emissions will change as temperatures rise.
As with pinene, many monoterpenes occur in two mirror-image forms: (+) alpha-pinene and (-) alpha-pinene. Plants can release both forms of these volatile molecules directly after biosynthesis or from stores in the leaves. Because the two chiral and enantiomeric forms, respectively, have identical physical and chemical properties, they are often not considered separately in atmospheric models. However, in a new study just published in Nature, researchers led by the Max Planck Institute (MPI) for Chemistry were able to show that the two mirror-image molecules are released via different processes in the plant and that they react differently to stress, especially drought.
Three months of drought stress in an artificial rainforest
The results come from experiments conducted in a closed artificial tropical rainforest in Arizona: the Biosphere 2 complex. The complex was originally built to create a self-sustaining ecosystem. It allowed a team of scientists from the University of Freiburg, the University of Arizona and the Max Planck Institute for Chemistry to precisely control the chemical and climatic conditions of the forest and measure its responses. For three months, the research team put the forest under moderate and then severe drought stress. A labelling experiment led by Prof. Dr. Christiane Werner from the University of Freiburg and colleagues (#B2WALD) made it possible to follow the reactions of the plants and the synthesis of volatile substances.
Using gas chromatographs, Joseph Byron, a doctoral student at the MPI for Chemistry, determined the emissions of alpha-pinene, camphene, limonene, terpinene and isoprene every hour. Since he and his colleagues wanted to find out when plants emit which chiral form, they used photosynthesis: they let "heavy" carbon dioxide (13CO2) flow into the air of the biosphere at certain times. One carbon atom of the carbon dioxide contained an additional neutron, i.e. it was isotopically marked and thus provided information about the plant metabolism. At the Freiburg Chair of Ecosystem Physiology headed by Werner and Dr. Jürgen Kreuzwieser, the team then used a mass spectrometer coupled to the chromatograph to track which monoterpenes contained heavy carbon atoms and which did not.
Heavy carbon dioxide gives insight into plant metabolism
"To our surprise, many mirror molecules behaved differently under drought stress," comments first author Joseph Byron. "For example, (-) alpha-pinene was labelled, whereas (+) alpha-pinene, which we measured at the same time, was not." This means that the tropical rainforest ecosystem releases (-) alpha-pinene directly after synthesis, while the mirror molecule comes from stores in the plant.
Furthermore, the researchers found that as the drought progressed, not only were more monoterpenes released, but the maximum emissions shifted further into the afternoon and the plants released more monoterpenes from storage pools. And there could be a reason for this, suspects project leader and atmospheric researcher Jonathan Williams: "Presumably, the later release of the monoterpenes increases the probability that clouds will form over the forest. The warmer it gets during the day, the more the vertical mixing of the air increases. As a result, the reactive volatiles reach higher layers of air and have a greater chance of becoming aerosol particles and eventually cloud condensation nuclei."
Important findings for the Amazon rainforest
Max Planck researcher Williams sums up from the Biosphere 2 studies: "In order to be able to make accurate predictions about the reactions of an ecosystem to stress, we should measure and model emissions of chiral molecules separately in the future. This is especially important for the Amazon rainforest, for which climate models predict more droughts." The group leader from the MPI for Chemistry in Mainz adds: "I am fascinated by the fact that we can decipher internal, enzyme-driven physiological processes of the forest by measuring air composition. This will certainly help us to also elucidate effects that we have observed in the real rainforest." William's team has also been conducting research in the Brazilian rainforest at the Amazon Tall Tower Observatory ATTO.
Original publication: Byron, J. Kreuzwieser, J., Purser, G., van Haren, J., Ladd, S. N., Meredith, L. K., Werner, C., Williams, J. (2022): Chiral monoterpenes reveal forest emission mechanisms and drought responses. In: Nature. DOI: 10.1038/s41586-022-05020-5
The work was carried out in collaboration with scientists from the Max Planck Institute for Chemistry in Mainz, the University of Freiburg and the University of Arizona/USA. An interdisciplinary team of about 80 researchers was involved in the entire drought experiment.
The work of the Max Planck Institute for Chemistry was partly funded by the EU project ULTRACHIRAL, that of the team from the University of Freiburg by an ERC Consolidator Grant.
Further publication: Christiane Werner et al, Ecosystem fluxes during drought and recovery in an experimental forest, Science, December 17, 2021, DOI 10.1126/science.abj6789
Prof. Dr. Christiane Werner
Chair of Ecosystem Physiology
Institute of Earth and Environmental Sciences
Faculty of Environment and Natural Resources
Albert Ludwig University Freiburg
Phone: 0761/203- 8301
University and Science Communication
Albert Ludwig University Freiburg