Two soybean breeding lines developed by the University of Arkansas Division of Agriculture offer improved yields under drought conditions.

The high-yield, maturity-group-V breeding lines R01-416F and R01-581F were released recently as germplasm. The genetic material is available to public and private soybean-breeding programs for use in developing improved varieties.

“Quite a few public and private breeders have already requested seed for crossing in their own breeding programs,” says Pengyin Chen, leader of the agriculture division's soybean-breeding program. “By sharing germplasm with other breeders, you get it back into the farmers' hands quicker and in many different forms suitable for a variety of growing conditions,” says Larry Purcell, University of Arkansas Altheimer chair for soybean research.

Drought tolerance in the new lines is based on nitrogen (N) fixation, a physiological function that allows soybeans to convert atmospheric N into a form that can be used by the plants. Nitrogen is very important for yields because protein development depends on it, and soybean seed is about 40% protein.

BUT FIXATION IS very sensitive to drought, Purcell says. “In most soybean cultivars, N fixation decreases very early under drought conditions — before other functions like photosynthesis and leaf expansion.”

Thomas Sinclair of the University of Florida, who collaborated on the research to develop the new germplasm lines, first identified the link between drought stress and cessation of N fixation. After that initial discovery, a research team of members from several states was assembled by the United Soybean Board to improve drought tolerance, Purcell says.

In 1993, Arkansas researchers crossed “Jackson,” a variety with drought-tolerant N fixation, and KS4895, a high-yielding, maturity group-IV variety. The cross resulted in lines in which N fixation is prolonged about as long as photosynthesis and other plant activities during drought.

Since the cross was made, the resulting lines were screened for yield at locations in Arkansas, North Carolina, Florida and Georgia. The best of these were evaluated for N fixation in drought conditions. Purcell says R01-416F and R01-581F have yields comparable to commercial varieties in the same maturity group that were used as check cultivars.

THE BEST GROUP IV varieties have yields as high as 60-65 bu./acre, Purcell says. Chen says R01-416F and R01-581F produce yields that are 90% of those top commercial varieties. “During moderate drought, these genotypes have a distinct advantage for yields, even under irrigated conditions,” Purcell says.

“The drought tolerance and N fixation ability are unique,” Chen says. “In addition, these two germplasm lines are well adapted to southern growing conditions and cultivation practices. Breeders can make a very quick incorporation of the drought-tolerant N-fixation trait into their elite lines.”