Research Highlights

Soybeans that 'Sweat' Less Tolerate Drought More

By Laura Temple

Some people sweat, or perspire, more than others while scouting soybeans on a hot summer day, releasing moisture through skin pores. 

While soybeans don’t perspire like that, they do transpire. 

Transpiration is the process of gas exchange between the atmosphere and a leaf. A leaf takes in carbon dioxide and releases oxygen through stomata, or pore-like structures on plant leaves. The leaf also releases water as it evaporates through open stomata. A key part of photosynthesis, transpiration also helps plants regulate temperature.

“Stomata can open and partially close to regulate water loss. This activity impacts how a plant saves or uses water, which tells us how plants handle drought conditions,” says Avat Shekoofa, assistant professor with the Department of Plant Sciences, University of Tennessee Institute of Agriculture. “We are looking at plant physiology to identify soybean varieties that can tolerate drought.”

Shekoofa cites unstable weather patterns as a key reason to study how different soybean varieties use water and to discover how they could more effectively use available water in stressful conditions. 

“Unpredictable weather patterns have created common issues where rain is not coming at the right time to maximize soybean yield,” she explains. “Often drought conditions exist during pod fill, and lack of drought tolerance traits in plants causes yield loss. We are studying how the environment impacts soybean production.”

Her long-term research, funded by the Tennessee Soybean Promotion Board, measures water loss, wilting and leaf temperature in soybeans to identify lines that handle drought conditions well. 

“We are phenotyping varieties, or observing how genetics interact with the environment, to select for drought tolerance traits that soybean breeders can use as they develop new varieties,” she says. 

The study started by measuring water loss of 17 common Tennessee soybean varieties in the greenhouse. Nine of these varieties with wide variation in water loss were selected for field trials under rainfed, irrigated and simulated drought conditions created using rainout shelters over the plots. 

Shekoofa says soybean varieties demonstrate a wide range of water loss at different growth stages, based in part on stomata activities and transpiration rate. 

“If soybeans use lots of water as small plants, during vegetative growth, less water will be available in the soil to support reproductive growth and pod fill later in the season if drought stress occurs,” she explains. “Plants can adjust how much water they release, or transpire. By adjusting early in the season, plants conserve water for later, when drought stress is extreme.”

Wilting and Leaf Temperature Indicate Stomata Activity

In both the greenhouse and the field, Shekoofa and her team recorded wilting score (WS) for each soybean variety under water-deficit stress frequently, with 0 representing no wilting, and 5 representing extremely wilted. When water is limited, the degree of wilting correlates to how often stomata open to release water and take in carbon dioxide. 

The team observed that some soybean varieties wilt much more slowly than others under drought conditions. This indicates that stomata stay partially closed or open less frequently, conserving water. Under drought conditions, these slow-wilting soybean varieties yielded 22 to 29% more than fast-wilting varieties that lose water more quickly.

During the growing season, a thermal camera mounted on a drone measured canopy temperature of the soybeans, as well. Changes in leaf temperature correlate to water loss. The water released through stomata during transpiration reduces canopy temperature.

In the drought-simulation field trials, the team also measured soybean recovery from drought stress. They irrigated extremely wilted soybeans, scoring above 4 on the rating scale, at their R5 growth stage with 1.5 inches of water. They have observed significant differences among soybean varieties in stomatal activity and plant wilting recovery 24 and 48 hours after that water was applied. Wilting scores and canopy temperatures dropped at different rates, indicating how well the soybeans can bounce back from that stress. Yield results correlated with recovery rates.

“Soybean lines expressing slow wilting managed drought stress conditions later in the season well,” Shekoofa says. “They showed lower leaf canopy temperatures after drought recovery and produced higher yields. In unstable environments, soybean varieties that save water will be more sustainable to produce.

She adds, “The information gained over time from this research will help both soybean breeders and farmers improve soybean variety selection to manage unpredictable drought conditions during the season.”

In 2021, this research is expanding to include a wider range of maturity groups, as well as high oleic soybean varieties. Soybean lines that show slow-wilting traits are being incorporated into breeding efforts to develop and improve drought-tolerant soybean varieties. 

Published: Aug 16, 2021