Mainz (Germany): Similar to pinene, several monoterpenes may be found in (+) alpha-pinene and (-) alpha-pinene, which are mirror images of one another. Both of these volatile compounds can be released by plants either immediately upon biosynthesis or from storage pools in leaves. In atmospheric modelling, the two chiral or enantiomeric forms are frequently not taken into account individually because they have identical physical and chemical characteristics.
However, a recent study from the Max Planck Institute has demonstrated that the two mirror-image molecules are released in the plant by several processes and that they react differently to stress, particularly drought.
Three months of drought stress in an artificial rainforest
The results come from experiments conducted at an enclosed artificial tropical rainforest within the Biosphere 2 complex, in Arizona which was originally built to create a self-sustaining ecosystems. This facility allowed a team of scientists from the Max Planck Institute for Chemistry, University of Freiburg and the University of Arizona to precisely control the chemical and climatic conditions of the forest and measure its responses. For three months, the science team put the forest under moderate and then severe drought stress.
Using gas chromatographs, Joseph Byron a PhD student on the project from the Max Planck Graduate School, determined hourly emissions of alpha-pinene, camphene, limonene, terpinene and isoprene. To determine when the plants emitted which chiral form, the researchers used isotopically labelled CO2 to track photosynthentic carbon and introduced "heavy" carbon dioxide (13CO2) to the air of the biosphere at certain times. Using a mass spectrometer coupled to the chromatograph, the team could then track which monoterpenes contained heavy carbon atoms and which did not. This revealed which labelled compounds were being made and released instantaneously by the ecosystem and which unlabeled species were coming from storage pools.
Heavy carbon dioxide provides insight into plant metabolism
"To our surprise, many mirror molecules behaved differently under drought stress," comments the paper's first author Joseph Byron "Thus, (-) alpha-pinene was labelled, whereas (+) alpha-pinene, which we measured simultaneously, was not." This means that the tropical rainforest ecosystem released (-) alpha-pinene directly after synthesis, while the mirror molecule comes from storage pools in the plant.
More drought leads to a diurnal shift in monoterpene emissions
In addition, the researchers found that as the drought progressed, not only were more monoterpenes released, but also the maximum in emissions shifted to later in the afternoon, and the plants released more monoterpenes from storage pools. And there may be a reason for this, suspects project leader and atmospheric scientist Jonathan Williams: "We suspect that the later release of monoterpenes increases the likelihood that clouds will form over the forest. The warmer it gets during the day, the more the vertical mixing of the air increases, allowing the reactive volatiles to reach higher layers of air where they have a greater chance to become aerosol particles and eventually cloud condensation nuclei."
Max Planck researcher Williams concludes from the Biosphere 2 studies, "To predict ecosystem responses to stress accurately, we should measure and model emissions from chiral molecules separately in the future. This is especially important for the Amazon rainforest, for which climate models predict more droughts in the future." The group leader from the Max Planck Institute 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 elucidate effects we've observed in the real rainforest as well." Williams' team has also been conducting research in the Brazilian rainforest at the Amazon Tall Tower Observatory ATTO. —ANI