Kites, balloon collect aerial data for soybean drought tolerance

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A kite's no toy to Larry Purcell.

Following in the footsteps of Benjamin Franklin, Purcell flies kites in the name of science.

Purcell, holder of the Altheimer Chair for Soybean Research for the University of Arkansas System Division of Agriculture, is trying to understand the mechanisms that allow some plants to tolerate drought better than others. The Arkansas Soybean Promotion Board supports his research project.

One of the keys was to measure the temperatures of plants under specific conditions of moisture, weather and other factors. "The plant canopy is cooler when water is available," Purcell said. "The plants get warmer when water becomes scarce."

The cooling is the result of transpiration, Purcell said, the mechanism by which plants draw water from the soil, transport it through the plant and evaporate through stomates, tiny holes in the leaves and stems that enlarge or shrink to control movement of moisture and gases. When water is scarce, evaporation is restricted and the plant's temperature rises.

Research has shown some soybean plants remain cooler than others under drought conditions. "The cooler plants have some mechanism that allows them to use water more efficiently," Purcell said.

"Some soybean genotypes do not wilt as quickly because they conserve water in the soil, providing a reserve during a drought," Purcell said. "Other plants use deeper rooting to draw water from deeper strata in the soil. Both traits allow a plant to continue active growth, which results in water evaporating from the leaves."

Purcell has identified molecular markers for these genetic traits and has discovered that those same molecular markers are associated with higher yields.

Early work on drought tolerance required measuring temperature and rating the color of the plants from the ground. It was a painstaking process of walking through each field during mid-day when weather conditions might be most consistent.

The problem, Purcell said, was that atmospheric conditions often changed before he could record the data from a field. The sun changed position and the temperature would rise as the day progressed. In addition, wind speed could change or clouds might roll in and cause inconsistent temperature readings.

The result was that measurements he took at 11 a.m. were not comparable to those he recorded at 1 p.m. or 3 p.m. Purcell needed a way to rapidly record data from thousands of soybean plants under the same ambient conditions.

The answer was aerial photography. "Aerial sensing allows us to measure an entire field at once," Purcell said.

From the air, an infrared camera can detect temperature changes as tiny as a tenth of a degree Fahrenheit and cover a field in minutes instead of hours. A visible light camera can record changes in greenness that indicate early maturity when drought stressed during seed fill.

Purcell first tried using a remote-controlled drone that could mount the cameras and fly predetermined routes over his research fields. But he soon ran up against complex and restrictive FAA regulations.

So Purcell turned to technology that originated more than 2,500 years ago in China. As it turns out, kite aerial photography, or KAP, has a considerable following, and he found a lot of help from members of KAP organizations.

Purcell uses three kites, each one adapted to flying at different wind speeds. When there's little or no wind, he uses a six-foot diameter balloon with a three-point tether. The aim for each vehicle is to provide the most stable platform possible for photography.

Once launched, Purcell and his research assistants can walk the kite or balloon across a field while the camera shoots a sequence of infrared or visible light photos. From launch to recovery, he said, this system can record data from a field in 30 minutes.

The photos are later downloaded to a computer where the infrared pixels are correlated with temperatures and average values are assigned to segments of a field. Visible colors of the plants indicate water use. The color photos are being used to identify genotypes that do not have a shortened seed-fill period in response to drought.

The results are further correlated with genetic markers that are known to be associated with plant traits, such as how fast they wilt in a drought or how availability of water affects yield.

Purcell said the results are promising. For example, infrared measurements of plant temperatures reveal drought stress before the soybeans show any physiological sign of it.

"Our analysis of these traits is part of the pathway toward building a useful tool for plant breeders who are trying to develop drought tolerant varieties," Purcell said.

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