COLUMBIA, Mo. -- The key to improving drought tolerance in soybeans may be found in the crop's seldom-seen root system, said a University of Missouri agronomist.

"If we are to be competitive in the world soybean market, we need cultivars that are more drought-tolerant and bred specifically for Missouri soil types and climatic conditions," said Robert Sharp, MU professor of agronomy.

Sharp and fellow MU agronomists Henry Nguyen, Grover Shannon and David Sleper are combining their expertise in plant physiology, molecular genetics and plant breeding to make such cultivars a reality. The team received a three-year grant from the Missouri Soybean Merchandising Council to screen and characterize soybean germplasm for drought tolerance.

"There is evidence that soybeans with certain rooting characteristics are less susceptible to yield loss when faced with water deficit," Sharp said. "Our goal is to identify germplasm lines having drought-tolerance characteristics in the rooting system, tag the genes that determine the drought-tolerance traits, and then use molecular marker technology to incorporate those traits into elite soybean lines for cultivar development."

Screening soybeans for drought tolerance required the researchers to develop a research system that both mimics natural conditions and allows them to observe root growth.

"We're growing the plants in 4-foot-tall, 9-inch diameter Plexiglas cylinders," said Tracy Scanlan, a master's student in agronomy working with Sharp on the project. "The transparent tubes allow us to visually monitor root growth and analyze root response as the plants adapt to a rate of dry-down that simulates drought conditions in the field."

Although the soybean has a taproot, it's not an inherently deep-rooted plant, Sharp said. "In order to avoid drought stress, the taproot must keep ahead of soil drying," he said. "The final depth of the tap root is one criteria we're evaluating, as is the root system's plasticity, which is its ability to dynamically respond as the soil profile dries."

Each group of screened soybeans is grown for about six weeks in the Plexiglas cylinders, Scanlan said. All plants are watered until they reach the trifoliate stage, at which time half receive no more water throughout the trial.

"When the growing period ends, we measure each plant's depth of rooting," she said. "We also measure the plant's total leaf area to assess the impact of the soil-drying treatment on shoot development."

During the next year, Sharp said, they plan to screen 10 to 15 soybean lines. Additional screening in growth chambers will help to narrow the lines that will undergo genetic testing.

"The genetic testing will help us locate the regions of the soybean genome where the genes that determine drought tolerance are found," he said. "We can then tag those genes and incorporate them into our elite soybean cultivars."

Sharp anticipated that with steady progress, the group could, in six to eight years, develop a plant-breeding program for soybeans with improved drought tolerance lines.

Jason L. Jenkins is a senior information specialist with the University of Missouri Extension Service (573-882-2980 or JenkinsJL@missouri.edu).