URBANA, Ill. (U.S.A.) — In a decisive new study, crop scientists at the University of Illinois Urbana-Champaign and USDA’s Agricultural Research Service show that soybeans are doubly damaged by drought and ozone pollution, both of which are increasing across the globe due to climate change.
The study settles the question of whether drought stress could actually protect soybean plants from the damaging effects of ozone, which include leaf bronzing, leaf drop, and yield loss. Here’s the logic: Leaves are speckled with thousands of tiny openings called stomata that let carbon dioxide in and water vapor out. When they sense drought, plants close their stomata to conserve water. Some early studies suggested that by causing stomata to close, drought may also prevent ozone from entering leaves and damaging sensitive tissues.
“We found that really wasn't the case. There was a significant negative impact of ozone on yield, regardless of drought,” said study co-author Lisa Ainsworth, C.A. Ewing Chair of Crop Physiology and professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois.
To arrive at that conclusion, Ainsworth’s team grew soybeans in the field for three years under ozone concentrations of 100 parts per billion — similar to summer concentrations in agricultural regions of China and India today — while excluding about 40% of each season’s rainfall.
The team leveraged the world’s longest-running Free Air Concentration Enrichment facility, SoyFACE, established in 2001 through a collaboration between USDA-ARS and U. of I. The facility allows researchers to manipulate and hold constant the atmospheric and weather conditions in a production field, something that’s typically only possible to simulate on small scales using highly artificial equipment like growth chambers.
The researchers measured physiological characteristics related to photosynthesis and hormone signaling in the growing plants, as well as yield at the end of each season. Elevated ozone consistently reduced photosynthetic efficiency and reduced yield, with fewer and smaller seeds in each pod.
Hormone signaling was important to study because — as a counterpoint to the idea that drought could cause stomata to close and protect plants from ozone — scientists have hypothesized that ozone may disrupt the hormone abscisic acid, which conveys the drought signal from roots to stomata. That would mean stomata stay open, allowing ozone to pour in and damage photosynthetic machinery. But that’s not what Ainsworth’s team found.
“The abscisic acid concentration was linearly correlated with how much moisture was in the soil, regardless of whether plants were grown in ambient or elevated ozone,” she said. “So there was no evidence that ozone prevented plants from responding to drought.”
So, even though abscisic acid was working properly and stomata were responding to drought, ozone still got in and damaged tissue. Why?
Ainsworth says stomata don’t typically close all the way, even in the driest years. At 100 parts per billion, any amount of ozone that gets in is problematic.
“Once it’s inside a leaf, ozone can turn into other reactive oxygen species that can enter cells and produce signals that say, ‘Hey, we're under attack,’ and lead to changes in gene expression," she said. “Under very high ozone concentrations, a hypersensitive response can be triggered, causing cells to die.”
Although U.S. Corn Belt ozone concentrations are lower than the 100 parts per billion tested in the study, the threshold for damage is only around 40 parts per billion. Ainsworth says Corn Belt concentrations routinely exceed that level during the growing season, and are projected to increase through 2070. She also says farmers may not recognize ozone damage because the symptoms could be mistaken for diseases or other environmental damage. She hopes her latest study will raise awareness among farmers, breeders, and policymakers.
“Drought and ozone pollution are just part of our growing environment today. Understanding how these factors interact will help us design practices that could mitigate their effects. As we're thinking about targets for crop improvement, regardless of whether there's drought or not, improving ozone tolerance will be key.
“Ozone is a short-lived pollutant, which means we have the ability to do something about it in the short term. Our work shows that 100 parts per billion ozone has a very big impact on seed development in soybean. If we want to have an immediate way to increase yields, reducing ozone pollution is a good strategy,” Ainsworth said. “This is solvable.”
The study, “Drought does not mitigate reductions in soybean photosynthesis and yield caused by elevated ozone,” is published in Plant Physiology [DOI: 10.1093/plphys/kiaf350]. This study was supported by USDA-ARS funding to the Global Change and Photosynthesis Research Unit.
Ainsworth is also affiliated with the Department of Plant Biology in the College of Liberal Arts and Sciences, the Realizing Increased Photosynthetic Efficiency project, the Carl R. Woese Institute for Genomic Biology, the National Center for Supercomputing Applications, and the Center for Advanced Study at U. of I.
Research in the College of ACES is made possible in part by Hatch funding from USDA’s National Institute of Food and Agriculture.