Developing drought-tolerant plants continues to be a goal of plant breeders and crop specialists. Through research, much has been discovered about plant processes affected by drought and how that effect is manifested in crop growth and yield response.

Improving crop plant performance in the presence of drought has been a slow process. Current methods employed to reduce the adverse effects of drought generally are related to changes in plant architecture and manipulation of management practices.

In the past five years, several research efforts with soybeans have produced findings that may lead to improved plant performance during drought. Hopefully, these findings can be used by geneticists and breeders to develop/select varieties that consistently perform better under drought conditions.

Slower wilting

Tommy Carter, a USDA-ARS soybean breeder located at North Carolina State University in Raleigh, N.C., has spent a quarter century identifying and developing soybeans that stand up to dry weather. His work has centered on selecting soybean types that wilt slower and stay greener during drought.

Plants expressing these traits are being used in breeding efforts to develop soybean varieties that are better able to withstand the typical summer drought in the southern United States. Results from recent research indicate that slow wilting is possibly associated with deep rooting.

Water use efficiency

Improved drought tolerance may result from a higher water use efficiency, or the ability of a variety to produce a greater yield with less water than is required by other varieties. This quality is controlled by leaf traits that regulate transpiration or water loss during drought.

E. Vicki Hufstetler, soybean researcher in the Department of Crop and Soil Sciences at the University of Georgia in Athens, and colleagues (Crop Science, Vol. 47, 2007) recently determined that soybean genotypes differ in the amount of dry matter produced with a given amount of water during drought.

They also surmised that genotypes that exhibit a quicker recovery when drought stress is relieved could regain productive potential quicker than genotypes with slower recovery. This is an important quality for varieties grown in dryland environments that have intermittent drought periods.

Another interesting finding by Hufstetler and colleagues was that released varieties versus plant introductions and germplasm used in the study had the highest water use efficiencies.

This indicates two things. First, past soybean breeding efforts have either purposely or inadvertently resulted in soybean varieties that are efficient water users. Second, future breeding efforts to improve drought tolerance in soybeans should ensure that this level of water use efficiency is maintained in new varieties that may be developed and released.

Enhanced nitrogen fixation

Nitrogen fixation in soybeans is sensitive to even modest soil water deficits.

Tom Sinclair, professor in the Agronomy Department at the University of Florida in Gainesville, and colleagues (Field Crops Research, Vol. 101, 2007) collaborated on research that resulted in identifying two Maturity Group 4 soybean lines that are better able to fix nitrogen when yield-limiting soil water deficits occur. The two lines are adapted to southern growing conditions and are being released as germplasm that will be available to public and private soybean breeders.

They can be used in developing varieties that produce greater yields under dryland conditions as a result of decreased sensitivity of nitrogen fixation to soil water deficit. This trait presumably could also help to increase yield even in years with no obvious drought.

Selecting for drought tolerance

Theoretically, determining relative drought tolerance in soybeans can only be done in a controlled environment. The reason for this is simple: an accurate categorization of conditions during drought is required in order to assess the effect of drought on plant processes.

A growth chamber is used for absolute control of temperature, soil moisture, light, and daylength. This facility can be used for year-round evaluations and testing. However, it is better suited for testing plants during the vegetative period.

A fixed or portable rainout shelter is used for soil moisture control in the field, but all other climate and weather factors are essentially natural. Its use is limited to the normal growing season. It is better suited for assessing drought effects under natural conditions (except rainfall) during the entire soybean life cycle.

Use of irrigation versus no irrigation is often used to measure plant responses to drought. This is satisfactory if a consistent level and timing of drought are assured during every growing season, which is rarely the case. Thus, results are usually confounded by the different temperature and rainfall patterns across years even in drought-prone regions.

Irrigated versus nonirrigated comparisons are better suited for determining how yield is affected by adequate versus inadequate water during the growing season rather than for determining relative drought tolerance. This setup is especially suited for evaluating how new varieties with perceived drought tolerance perform under high-yield conditions where water is not limiting.

The ultimate goal, of course, is to have drought-tolerant varieties that also yield well in dryland systems when drought does not occur or is alleviated with irrigation. Soybean varieties resulting from incorporating the above drought-tolerant traits should in theory do just that.