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Biochemical link between plant volatile organic compound (VOC) emissions and CO2 metabolism - from sub-molecular to ecosystem scales

Periodic Reporting for period 5 - VOCO (Biochemical link between plant volatile organic compound (VOC) emissions and CO2 metabolism - from sub-molecular to ecosystem scales)

Reporting period: 2021-10-01 to 2022-02-28

Plant metabolic processes exert a large influence on global climate and air quality through exchange of greenhouse gas CO2 and biogenic volatile organic compounds (VOCs). Despite their enormous importance, processes controlling plant carbon allocation into primary and secondary metabolism, such as respiratory CO2 emission and VOC synthesis, remain uncertain. Ongoing and future climate change can markedly alter these processes and thus enhance uncertainties. The global increase in severe drought is threatening ecosystem functioning thereby diminishing carbon sequestration and altering VOC emission, with detrimental climate feedbacks through ozone or secondary organic aerosol (SOA) formation. To meet the societal demand and preserve pristine ecosystem services, we need a deep understanding of ecosystem processes affected by climate change.

The overall goal of VOCO is to shed new light into regulatory responses of plants and ecosystems to global climate change, bridging scales from sub-molecular to ecosystem processes. VOCO aims to develop a novel technological and theoretical basis for carbon partitioning breaking new ground by innovative isotope labelling to establish mechanistic descriptions of plant primary and secondary metabolism, namely CO2 and VOC emissions. VOCO explores the role of climate change stresses on these processes in different plant functional groups to foster our understanding of vegetation response to climate extremes.
We developed a pioneering research facility (PTR-TOF-IRIS, Fig.1) coupling emergent technology from geosciences, such VOC mass spectrometry and 13CO2 isotope spectroscopy to resolve high temporal dynamics in VOCs and CO2 emissions. Combined with radically new approaches of 13C-position-specific feeding experiments of central metabolites, it enabled an unprecedented understanding of the metabolic linkage of plant CO2 and VOC emission. We gained novel insights into biochemical synthesis of VOC in response to environmental stress, such as the enhanced use of cytosolic pyruvate for plastidial isoprene biosynthesis under heat stress, and metabolic cross-talk between cytosolic and chloroplastic pathways for VOC biosynthesis. It enabled the quantification of the often-underestimated contribution of day-time CO2 emission during secondary metabolism and regulation of de novo and storage pool VOC emissions under stress.

This innovative setup enabled a series of climate change experiments, simulating e.g. heat waves and droughts on Mediterranean, temperate, boreal and tropical plant species. We further explored the impact of natural droughts during the severe 2018 heat wave, which significantly affected mid European forests. Recurrent hot drought shifted a pine forest to its tipping point, resulting in enhanced VOC emissions and significant tree mortality.

VOCO did set a new dimension with a world-wide first whole ecosystem labelling in a large-scale interdisciplinary drought experiment in the unique Biosphere 2 tropical rainforest (B2WALD campaign, Arizona, Fig. 2,3) with over 70 researchers covering molecular to ecosystem and atmospheric sciences. It opened new frontiers for assessing biogenic emissions of greenhouse gases and their isotopic compositions from different ecosystem components in an unparalleled 5-month drought experiment including a deep-water labelling during the recovery phase. B2WALD received substantial press, radio and TV coverage (> 40 reports; including Science Magazine News).

The overarching publication (Werner et al., 2021 Science, highlighted by an editorial) identifies significant flux dynamics as drought cascaded through different forest strata and the importance of diverse plant functional groups with different hydraulic strategies as key processes to increase ecosystem drought resistance. Drought-sensitive canopy trees strongly reduced water loss, but spared deep-water reserves until late drought thereby preventing rapid desiccation of the forest. Carbon allocation was down-regulated by increasing mean residence times, though fresh carbon investment in VOCs remained high, indicating their important functions. Atmospheric VOC dynamics traced increasing drought impact and dynamic soil-plant-atmosphere interactions. This has important implications for coupled climate-vegetation models, which mostly neglect diverse plant hydraulic responses and VOC dynamics, despite their impotent climate feedbacks. Notably, we achieved the first proof on the origin of independent regulation of different VOC enantiomers, i.e. the chiral versions of a VOC, emitted by plants (Byron et al. under revision in Nature). These deliver important information for global change related aspects, as these greenhouse gases can affect atmospheric chemistry and enhance global warming.

Thus, VOCO very successfully completed its objectives and went significantly beyond, delivering important new insights on key processes for ecosystem functioning from metabolic to ecosystem scale, which enhance our understanding of climate change impact and calls for advanced action to protect our pristine forest ecosystems revealing their important role for climate regulatory functions.
VOCO has set new ground with its pioneering research platform, which, combined with cutting-edge position specific labelling of central metabolites, has enabled an unprecedented real-time detection of (13C)-sub-molecular carbon partitioning into respiratory CO2 and VOC emission, bridging scales from molecular processes to whole-plant and ecosystems.

VOCO reached a new dimension with an unparalleled whole ecosystem labelling experiment (B2WALD), which has opened new, fascinating horizons for interdisciplinary research. Many insights of this unique large-scale experiment are yet to come, ranging from metabolic regulation of VOC emissions and soil microbial processes up to ecosystem and atmospheric exchange.

Our results are significantly advancing the research field, highlighting the importance of heterogeneity and plant functional diversity in complex forest system for sustainable functioning, revealing how drought and legacy effects impact ecosystem carbon sequestration. Furthermore, they emphasise the importance of VOC dynamics driven by soil-plant interaction. Particularly the observed early increase in monoterpene emissions under drought can increase atmospheric reactivity, potentially promoting cloud condensation nuclei, a feedback that could help relieve drought stress. Unexpected novel findings are the sustainable use of deep-water by drought-sensitive canopy trees, avoiding rapid depletion of ecosystem water reserves. Interestingly, unpredicted long water travel times in tree trunks and significant drought legacy was found. Thus, understanding diverse plant hydraulic strategies is key for predicting vegetation-driven changes in ecosystem fluxes and atmosphere feedbacks under climate change.

Another remarkable breakthrough was the first proof of independent biosynthetic regulation of different enantiomers of the same VOC. As VOCs strongly affect atmospheric chemistry, this is of major relevance for accurately predicting the impact of altered ecosystem-atmosphere exchange particularly in response to climate change.
Finally, VOCO has gained significant attention (> 55 conference presentations, > 40 media reports) and public outreach contributed to raise awareness on how climate change is endangering sustainable forest functioning.
Novel Measurement system of Workpackage 1