By Carol Brown
Plants need water to grow and how they use water is a key factor in their development. It’s particularly important in cash crops, as yield can be affected by how efficiently crops use water, especially under drought conditions.
Felix Fritschi, University of Missouri crop physiology and genetics professor, has been studying how to improve soybean water use efficiency for some time. Through research supported by the Missouri Soybean Merchandising Council, Fritschi is searching within soybean lines for genetic traits that are associated with strong water use efficiency, or WUE, which helps them tolerate drought conditions better.
“We are looking for the efficiency with which soybeans convert water into biomass and ultimately yield,” Fritschi says. “They don’t use water for that directly, rather plants need water to absorb nutrients, cool their leaves and for the photosynthesis process. We are trying to measure the amount of biomass produced per unit of water they lose through transpiration.”
Improving soybean drought tolerance is complicated and it starts with finding out how efficient the plant is at using water. Scientists have found a way to measure WUE through molecular indicators from the photosynthesis process.
“As plants photosynthesize, they use carbon dioxide. As a result of this process, different masses of carbon exist in the plant,” Fritschi explains. “One carbon atom is not necessarily the same weight as another. The plant structure is made up of molecules that include carbon 12 and carbon 13. We compute the ratio of these two weights and use it as an indicator of water use efficiency.”
Seeking Good WUE Traits
Fritschi began his search for soybeans with traits that confer good water use efficiency several years ago by casting a wide net. He is searching through the diverse, natural genetic variation within the USDA soybean germplasm collection of approximately 20,000 accessions.
“Many accessions in the collection are not agronomically sound — they may not yield well, or the seeds shatter or they have other undesirable traits,” he says. “But some may have high water use efficiency in their gene composition. We’re looking for that physiological trait that allows the plant to produce a lot of biomass per unit of water.”
After finding promising traits, genetic studies come next to map the quantitative trait loci, or QTL, to mark the area where the trait would appear, which helps hasten the breeding process. This identified germplasm can then be crossed with soybean cultivars that do have desirable agronomic traits — eventually arriving with a soybean that can produce good yields with high WUE.
University of Missouri Assistant Professor Andrew Scaboo specializes in soybean breeding. He is working with Fritschi on this project by providing germplasm material from his program and helping with the breeding process.
“Some of the populations that Dr. Fritschi is using for his studies were developed through our breeding program,” says Scaboo. “He’s found some unique plant introductions from the germplasm collection with interesting traits for water use efficiency, and we’re crossing some of our breeding lines with that material.”
Taking It to the Field
The team is growing the soybean crosses in several field studies to test parent plants in more detail. This can confirm, or reject, WUE genetics under regulated field conditions. Fritschi has created two types of water-limiting scenarios in the field to conduct these studies: rain-out shelters and limiting rooting depth. He also maintains sites under rain-fed conditions to use as the control comparison.
He describes the rain-out shelter as a mobile greenhouse on a track in the field. They can move the shelter over certain research plots during a rain event, excluding rain from particular areas, then move the shelter away after the rain is done.
“We also have a field set up to limit the plants’ root growth to different soil depths,” Fritschi says. “We excavated channels in the field, put down a layer of plastic and covered it with soil. The roots grow until they reach the plastic layer. With this, we’re influencing how much soil moisture is available for the plant over the course of the season.”
The field gets the same amount of rain, but the roots don’t have as much water available in the soil profile as they would in a field that wasn’t modified. It’s not as controlled as the rain-out shelter, he says, but it provides a chance to increase stress in the field.
Fritschi and his team are making strides in finding soybean traits that use water most efficiently. He continues the work to develop soybean cultivars that will be drought tolerant along with demonstrating good yield potential and other desirable traits. It’s not an easy process to find the proverbial needle in the haystack, but he has narrowed down the search.
“The effects of drought on soybeans and their ability to recover is a challenge and the mechanisms that confer drought tolerance are quite complex,” he remarks. “We certainly appreciate the long-standing support from the Missouri Soybean Merchandising Council, which allows us to continue this important work.”
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