Global food security increasingly hinges on crops’ ability to withstand drought. But are scientists and producers focusing on the right metric when measuring crop-relevant drought? Not exactly, according to new research from University of Illinois scientists, who urge the scientific community to redefine the term.

“Plants have to balance water supply and demand. Both are extremely critical, but people overlook the demand side of the equation, especially in the U.S. Corn Belt,” said Kaiyu Guan, Blue Waters professor in the Department of Natural Resources and Environmental Sciences and the National Center for Supercomputing Applications at the U of I.

The demand Guan refers to is atmospheric dryness, often expressed as vapor pressure deficit (VPD). The drier the air, the more moisture is sucked out of pores, or stomata, in plant leaves. Plants have to open stomata to take in carbon dioxide as their food, but if they sense the atmosphere is too dry, they’ll close pores to avoid drying out. Keeping stomata closed too long leads to reductions in photosynthesis, plant growth and grain yield.

The kicker? Plants shut down stomata due to atmospheric dryness even when there’s an adequate supply of moisture in the soil.

“If you only consider rainfall and soil moisture, which is how most people think about drought, that’s mostly describing the supply side. Of course, if you have low soil moisture, plants will be stressed by how much water they get. But the supply is often pretty sufficient, especially here in the Corn Belt,” Guan said. “However, the demand side from the atmosphere can also severely stress plants. We need to pay more attention to that drought signal.”

Redefining drought in the Corn Belt

Kaiyu Guan

Guan’s first study, published in Agricultural and Forest Meteorology, used data from seven sites across the Corn Belt to conclude VPD accounts for nearly 90% of the changes in crop stomatal conductance, a proxy for drought stress, and approximately 85% of changes in gross primary productivity, a measure of productivity.

His group is working on follow-up studies evaluating the role of irrigation in increasing supply and decreasing demand, but for now, Guan said breeding for improved water use efficiency could be an important part of the solution.