August 13, 2013 | David F. Coppedge

Invisible Influence of Plants Coming to Light

The air is filled with substances we barely notice – molecules produced by plants – but they have profound effects on climate and ecology.

They’re called volatile organic compounds, or VOCs – molecules with a low boiling point that evaporate out of plants and waft through the air.  Scientists know about a few of them, but not all.  We can easily smell the aroma of an orange orchard or pine forest or the perfume of honeysuckle, pleasing smells that lift our spirits, a few that make us stay away.  Plants produce hundreds of other VOCs, though, that until recently were not even detected.  What is the invisible influence of plants in their ecological niches, or for the planet as a whole?

First, we would have to know what VOCs exist.  A new study published in Science shows that we have have just scratched the surface detecting them.  A team from Europe and America found hundreds of new ones, mostly complex oxygenated hydrocarbons with some pure hydrocarbons, the effects of which are unknown and unaccounted for in climate models:

Numerous volatile organic compounds (VOCs) exist in Earth’s atmosphere, most of which originate from biogenic emissions. Despite VOCs’ critical role in tropospheric chemistry, studies for evaluating their atmosphere-ecosystem exchange (emission and deposition) have been limited to a few dominant compounds owing to a lack of appropriate measurement techniques. Using a high–mass resolution proton transfer reaction–time of flight–mass spectrometer and an absolute value eddy-covariance method, we directly measured 186 organic ions with net deposition, and 494 that have bidirectional flux. This observation of active atmosphere-ecosystem exchange of the vast majority of detected VOCs poses a challenge to current emission, air quality, and global climate models, which do not account for this extremely large range of compounds. This observation also provides new insight for understanding the atmospheric VOC budget.

And that’s just from a study of the air above an orange grove in California.  PhysOrg noted some of the implications of this discovery:

Going in, the team expected to find a few new compounds in the air—they were surprised, however, to find they had detected hundreds of VOCs. Their effort marks the first time a research team has been able to identify the entire exchange of VOCs in a portion of the atmosphere.

The team notes that their research effort was limited to one site and that monitoring other sites could result in vastly different findings. More importantly, it appears their technique could be used to monitor sites all around the world, giving environmental scientists much more information about what is going on in the atmosphere, including of course, global warming.

The scientists said that 90% of VOCs in the atmosphere come from “biogenic sources” (called BVOCs).  Because of the difficulty of detecting and measuring them, PhysOrg said that “researchers have primarily focused almost exclusively on a chosen few, such as isoprene, methanol, and various terpenes.”

The team realizes this new knowledge must be factored into climate models and ecological considerations.

Our results show that the mass balance of VOCs in the orange orchard ecosystem is highly incomplete when considering only emission and deposition of commonly measured dominant BVOCs. The contribution from hundreds of individually minor compounds is important. Future research is required to determine if this is also true for ecosystems that emit larger quantities of isoprene and/or monoterpenes.

Moreover, those hundreds of BVOCs are on the move, fluctuating in concentration and direction of movement throughout the day.  Plants both emit and receive them – an important part of their signaling other plants in their neighborhood, attracting pollinators, and deterring pests.  The global effects of trillions of plants filling the atmosphere with these compounds is poorly known.

For more on plant-emitted VOCs, see the entries from 2/21/06, 4/26/07, 8/31/08, 9/25/08 and 3/31/11.

Remember that the BVOCs are not just incidental smells from plants, like the smell of paint or asphalt, but are produced specifically by plants for purposes like communication.  This means that most (if not all) are encoded by genes, using information on how to manufacture them.  Plants know more about organic chemistry than professors!  Concocting some of these same molecules in a lab would be a tricky, multi-step process, and probably could not be done at atmospheric temperature.

One important lesson of this story is that there is still a lot out there we don’t understand or even know about.  How many of you knew that hundreds of complex molecules composed of carbon, hydrogen and oxygen were passing your nose each time you went outdoors?  It’s like being oblivious to radio signals without a receiver, only unlike radio, these signals might be affecting you without your awareness.  Are these molecules involved in regulating climate?  Who knows?  All we know is that there’s another potentially large factor that was not included in current climate models.  Best not jump off the deep end without sufficient information.



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