Drought is a major cause of soybean yield loss. Western North Dakota is characterized by low water or drought-like conditions, with soybean yields suffering as a result. While there may be multiple mechanisms to enhance drought tolerance in soybean, many have a negative effect on agronomic performance and yield during well-watered conditions compared to drought susceptible varieties. Interestingly, it has been shown that a greater ability to sustain symbiotic nitrogen fixation (SNF) under drought conditions improves soybean yield by 85% (Sinclair et al. 2007 and 2010). The ability to sustain SNF in soybeans has been shown to have a
genetic basis, and this trait has been observed in U.S. germplasm collections and is used in some breeding programs. Our long-term aim is to incorporate sustained SNF into NDSU varieties to improve yield in drought conditions. To take the first steps towards this, we will first investigate the ability to sustain SNF under drought in existing advanced NDSU germplasm. In addition, we will begin incorporating well characterized germplasm that has been shown to have an elite ability to sustain SNF under drought into the NDSU breeding program.

• Objective 1: Screen for the ability to sustain SNF under drought in NDSU advanced breeding lines.
• Objective 2: Initiate breeding populations using known germplasm with high drought SNF.

• Developed screening protocol for determining SNF in NDSU and other germplasm. This will allow for a new selection tool for determining which NDSU varieties are most suitable for Western North Dakota based both on yield and the ability to overcome drought conditions.
• Quantitative data for SNF phenotypes under drought and irrigated conditions for 43 NDSU advanced breeding lines.
• By the end of the 2024 funding year, we will have F4 populations with elite sustained SNF ability ready for testing in drought conditions.

Updated January 5, 2024:

Increasing soybean yield under drought through enhanced symbiotic nitrogen fixation (2023)-Miranda/Geddes

Objectives:
• Objective 1: Screen for the ability to sustain symbiotic nitrogen fixation under drought in NDSU advanced breeding lines.
• Objective 2: Initiate breeding populations using known germplasm with high drought sustained symbiotic nitrogen fixation.

Deliverables:
• Develop screening protocol for determining symbiotic nitrogen fixation in NDSU and other germplasm. This will allow for a new selection tool for determining which NDSU varieties are most suitable for Western North Dakota based both on yield and the ability to overcome drought conditions.
• Quantitative data for symbiotic nitrogen fixation phenotypes under drought and irrigated conditions for 43 NDSU advanced breeding lines.
• By the end of the 2024 funding year, we will have F4 populations with elite sustained symbiotic nitrogen fixation ability ready for testing in drought conditions.

Milestones: Collection of in-field samples from Williston in July/August 2023. Removal and counting of all nodules from field samples. Drying and weighing of all nodules from field trials. Drying and grinding of biomass from all field samples for N15 analysis. Analysis of field data. Development of greenhouse drought protocol. Repetition of field analysis done in greenhouse setting. Population initialization with sustained symbiotic nitrogen fixation materials from University of Arkansas. Advancement of early generation materials from these populations in Costa Rica.

Key indicators: Key indicators of success will be development of successful protocols that provide consistent results. Another will be determining the key phenotypes necessary to observe differences in symbiotic nitrogen fixation. These developed protocols will be necessary to phenotype the developed populations segregating for sustained symbiotic nitrogen fixation to determine if the crosses were successful.

Work completed:
In May 2023, three replications of 43 advanced experimental lines were planted in Nesson Valley near Williston, including two checks. This trial was planted in both dryland and irrigated conditions. At the ~R4 stage in early August, 10 entire plants were dug up from each plot. Root was separated from shoot, and transported back to Fargo in a refrigerated truck. The shoot biomass was placed in a dryer to remove moisture and nodules from roots were removed, counted, and weighed (wet weight) from each plant. This was completed in October. Nodules were then dried and weighed (dry weight). Dried shoot biomass is currently bring ground for N15 analysis. Preparation for greenhouse experiment is occurring now and should be started in January.
Germplasm from Arkansas contained the sustained nitrogen fixation trait bred into elite germplasm was acquired in early 2023. The first crosses were completed using North Dakota parents in summer 2023 in a growth chamber. F1 seed was grown in a growth chamber on the NDSU campus. F2 seed was harvested from the F1 plants and sent to Costa Rica for expediated generation advancement. Two generations of advancement are expected in the 23/24 American winter and F4 plants will be grown in North Dakota in 2024.

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Currently, yields in the Western part of ND lag significantly behind those in the Eastern half of the state due to low water conditions. Several farmers in Western ND already prefer NDSU varieties due to the glyphosate tolerance trait and the ability to save seed. This project will allow for screening for sustained SNF during drought in materials already in the advanced end of the breeding pipeline. These lines could then be selected primarily for their ability to sustain yield in drought conditions. Furthermore, we are ensuring an elite version of this trait will be incorporated into the NDSU breeding program to help to maximize yield under drought
conditions.