Plant cell walls form the bulk of forest biomass. These cellulose-rich structures undergo physical and chemical modifications as leaves grow and develop, die and decompose, or desiccate when drought strikes. A plant’s cell walls are coated in methyl and acetyl esters–which are released into the atmosphere as methanol and acetic acid by its leaves as the cell walls expand or shrink.
While these emissions are part of a plant’s normal functioning, climate change can accelerate their release as temperatures spike and plants undergo intense drought. However, new Berkeley Lab research suggests that the uptick in methanol and acetic acid production could actually benefit drought-stricken leaves, which utilize these carbon sources to revive photosynthesis—a carbon-dependent process that leaves forgo when their stomata close to save water—thereby limiting the gases’ entry into the atmosphere.
“It could be a mechanism by which plants cope with drought stress by being given the ability to maintain some energy production,” said Kolby Jardine, a research scientist at Berkeley Lab’s Climate and Ecosystem Sciences Division and co-author of the study recently published in the journal Trends in Plant Science.
His team suggests that measuring methanol and acetic acid emissions from individual plants or entire forests—using plant chambers, micrometeorological towers, drones, aircrafts, or satellites with gas sensors—could be a way to assess the physiological well-being of trees and how they deal with environmental stress.
Branching out from traditional approach
Typically, scientists measure a stem or leaf’s water status, or a leaf’s photosynthesis activity, or cell wall damage by tracing leakage in cellular content within plant tissues to gauge if a plant is drought-stressed.
“The way we’re starting to think about studying cell wall dynamics is moving away from traditional means where you have to destructively sample plant tissues,” Jardine says. “We’re thinking about atmospheric emissions from individual leaves to entire ecosystems as a non-invasive way to study changes in plant cell wall structure and function.”
Plant cell walls are decorated with methyl and acetyl esters. As plants grow and their cell walls modify, these carbon-based compounds of low molecular weight evaporate as methanol and acetic acid gas into the atmosphere.
These emissions spike under high temperatures and drought-like conditions. Methanol emissions, for instance, double with every 10-degree-Celsius rise in leaf temperature. The removal of these esters could modify the stiffness and flexibility of plant, according to Rebecca Dewhirst, a postdoctoral research fellow in Berkeley Lab’s Climate and Ecosystem Sciences Division and lead author of the study. “We think this [removal] may be important for water retention in the cell, especially during drought stress.”
To limit water loss, leaves close the stomata through which they breathe, hindering their ability to pull in carbon dioxide from the air and continue photosynthesis. This closure also limits the escape of carbon-based methanol and acetic acid that are produced in higher amounts during drought. Through processes of oxidation, these compounds get converted to carbon dioxide, and “all of a sudden you have an internal source of carbon dioxide that’s coming from your cell wall,” Jardine says. He believes this may help maintain photosynthesis even when access to atmospheric carbon dioxide gets cut off.
Putting down roots of change
While this mechanism for coping with drought is currently a working hypothesis proposed by the research team, they expect that aerial measurements of these gases could help scientists assess whether a forest is undergoing physiological stress. High levels of methanol or acetic acid in the air surrounding a patch of forest, for example, could indicate a plant community that is under drought stress. But these could also reflect the sign of an establishing forest with many young plants, or of a deciduous forest that is putting out new leaves at the start of a growing season or shedding wilting and dying old ones during the fall.
The team suspects that looking at a ratio of methanol and acetic acid in the air might help distinguish emissions caused by vegetation growth and decomposition from those linked with plant stress. The production of acetic acid, for example, may rapidly increase during drought-like conditions compared to that of methanol, which are more strongly linked to the growth of young leaves.
“We’re trying to look at a ratio that might go from really depleted in acetic acid in the growth phase but becoming much more enriched in acetic acid in the stress phase,” Jardine says. “We could use these signals as a new way to understand forest responses to changing climatic conditions.”
This work was supported by the U.S. Department of Energy, Office of Science Early Career Research Program (FY18 DOE National Laboratory Announcement Number: LAB 17-1761), Topic: Plant Systems for the Production of Biofuels and Bioproducts, Award number FP00007421.