By Howell Medders

The elusive goal of drought-tolerance in a soybean variety appears much more attainable than it did just two years ago, thanks to research by University of Arkansas scientists funded by the Arkansas Soybean Promotion Board and the United Soybean Board.

Research by Dr. Larry Purcell, a crop physiologist, and Dr. Clay Sneller, a soybean breeder and geneticist, has documented that Jackson, a low-yielding 1950s variety, has an important drought-tolerance trait.

Preliminary results from tests this summer indicate that the gene or genes responsible for drought-tolerance in Jackson can be transferred to high-yielding varieties by conventional cross-breeding methods.

Purcell said the researchers have made what could be termed breakthrough discoveries about why Jackson soybean plants are more drought-tolerant than other varieties.

They discovered that nitrogen fixation is the weak link in the ability of soybean plants to produce seed when grown under low to moderate drought stress, he said. And they discovered why this link is stronger in Jackson than in other varieties.

"Jackson is able to use water more efficiently because it sends a lot more sugars and photosynthates to the nodules during drought," Purcell said.

"This allows the nodules to continue to fix nitrogen, and plants need a large amount of nitrogen to fill seed."

The nodules are colonies of Bradyrhizobia bacteria on the roots of soybean plants that give the plant the ability to fix nitrogen from the atmosphere.

An early accomplishment of the soybean drought-tolerance research team was to develop a screening method to identify the level of a plant's ability to fix nitrogen under drought stress. The method is based on a measurement of urides, a product of nitrogen fixation, in plant tissue.

Using this new procedure, some 2,200 soybean genotypes from the USDA germplasm collection are being screened in the search for a variety with greater drought-tolerance of nitrogen fixation than Jackson.

The new knowledge and tools developed so far are already being used by Sneller, a soybean breeder, in a breeding program with the goal of developing more drought-tolerant varieties adapted to Arkansas conditions.

Sneller made crosses of Jackson with several high-yielding, drought-sensitive varieties and is evaluating data on the first generations of progeny in search of a potential parent line with the right combination of genes that could reduce the risk of growing soybeans without irrigation.