Recent research has found ways to produce soybean seeds that have improved yields under drought conditions. A group of researchers published their research in the November-December issue of Agronomy Journal. The researchers were M. Jyostna Devi and Thomas Sinclair, North Carolina State University; Pengyin Chen, University of Arkansas; and Thomas Carter, Jr. USDA-ARS, Raleigh, NC.

Soybeans are the third largest crop in the United States, after corn and wheat, and drought is one of the greatest threats to crop profitability. A limitation in developing soybean varieties with higher yields under drought conditions is that the genetic base of elite U.S. soybean breeding programs is narrow. The new study focused on three specific characteristics that could make crops more drought tolerant and result in yield increases:

1. The transpiration rate of the plants under dry-air conditions (i.e., how much water the leaves lose);
2. How quickly the transpiration rate changed as soil conditions became more dry; and,
3. How well the plants fixed nitrogen in drying soil.

Scientists first observed soybean lines with delayed wilting under drought conditions in the 1980s. This wilting delay indicated a resistance to drought. Yet, there was no clear understanding of what allowed these lines to be drought-tolerant. Thus, breeding drought-resistant soybean was difficult.

Most soybean types have been found to lose more water through their leaves under dry-air conditions. Previous research by Sinclair suggests that some varieties limit water use early in the season under dry-air conditions. This saves water for later in the growing season to “complete crop growth and potentially increase crop yield.”  In this new study, the researchers identified new soybean varieties developed in breeding programs that express this water-savings characteristic.

Soybeans are a type of plant called legumes. Legumes process nitrogen from the air and put it into the soil, which improves soil health over time. This is called nitrogen fixation, and is a major advantage of soybean as a crop since it reduces the need for nitrogen fertilizers. However, nitrogen fixation in soybean is especially sensitive to soil drying. In fact, in most years and locations in the United States, this sensitivity to drought results in the loss of at least some soybean yield. 

Sinclair’s previous research showed that nitrogen fixation is the most important drought tolerant trait. Increased “drought tolerant nitrogen fixation” was predicted to result in yield increases in 85% or more of the years in most areas of the United States.

 Again, breeding has now produced new soybean varieties that have drought-tolerant nitrogen fixation and higher yields under drought conditions. Overall, this research not only explained the basis for yield increase of recently developed drought-tolerant varieties, but offered insight in genetic resources for specific traits to further increase soybean yields.