Research

Background

Mycorrhizal fungi have a symbiotic relationship with plants. When mycorrhizal fungi colonize a plant’s root tissues, they can provide additional nutrients, such as phosphorus, to the plant. There is wide variation in the impact that this relationship has on plant growth and stress tolerances, suggesting that there may be an opportunity to select for mycorrhizal colonization as part of the breeding process, or to create tailored inoculants for certain forage species and environments.

Objectives

The objectives of this study are to:

• Determine whether mycorrhizal fungi colonization can be used as a trait that can be quickly assessed to improve breeding of alfalfa and sainfoin for forage production and stress tolerance
• Quantify the relationship between mycorrhizal fungi colonization, forage production, and forage quality
• Determine whether mycorrhizal fungi colonization is a heritable trait in alfalfa and sainfoin
• Compare how individuals from each legume species that vary in mycorrhizal fungi colonization rates respond to nutrient limitation and drought

What they Did

100 individual plants from 30 (alfalfa) and 10 (sainfoin) populations displaying a range of growth were measured for plant size, forage quality, and mycorrhizal colonization, including hyphae, arbuscules (structures which allow nutrient exchanges with plants), and vesicles (structures which store carbohydrates from plants).

These plants were then transplanted into greenhouse conditions where 60 plants per species were divided equally into a control treatment, a moisture limited treatment, and a low-nutrient treatment. After four months, plant growth was measured along with forage quality, root traits, and mycorrhizal colonization. DNA was also extracted to determine which types of mycorrhizae were present and whether these populations changed under plant stress.

From the remaining 40 plants, 10 with the highest and 10 with the lowest degree of mycorrhizal colonization were crossed to generate seeds from high and low colonization lines. The resulting seeds were inoculated with mycorrhizae from native rangeland, started in the greenhouse, and then transplanted into a former forage field at the University of Saskatchewan. These were harvested after roughly two months of growth and plant biomass, root traits, and mycorrhizal colonization were measured.

The final part of this project used 16 paired saline and non-saline grassland sites in Alberta and Saskatchewan in the brown, dark brown, and black soil zones. At each site, soil samples were taken, analyzed for soil properties and used to develop mycorrhizal inoculants. The inoculants were subsequently used on Rugged alfalfa (salt tolerant), CDC Maverick barley, and tall wheatgrass and compared to no inoculant, a commercial inoculant, and an inoculant cultured from the tame breeding nursery. Plants were grown under saline and normal soil conditions in the greenhouse.

What they Learned

Mycorrhizal colonization varied between alfalfa and sainfoin and by fungal structure. Higher levels of hyphae and vesicles were generally linked to larger plants, especially in alfalfa, but colonization had little effect on forage quality.

Stress affected the two forage species differently. Drought reduced overall colonization in alfalfa, while greater vesicle colonization was linked to more shoot growth under low nutrients and less shoot death under drought. In sainfoin, greater colonization and denser roots were associated with higher crude protein when nutrients were limiting. Denser roots were also associated with reduced dieback whereas greater fungal colonization was linked to lower fibre.  Sainfoin hosted more types of fungal species, particularly under low nutrients, but this did not increase yield.

Colonization showed some potential to be inherited. High-colonization lines had greater colonization and larger plants in both alfalfa and sainfoin, although results differed between field and greenhouse trials.

Some native grassland inoculants improved barley growth under saline conditions, but the best-performing inoculant depended on where and when the soil was collected. Results for alfalfa and tall wheatgrass were more variable and not linked to saline conditions.

What it Means

The benefits of mycorrhizal fungi depend on the forage species, fungal structure, and growing conditions. Measuring individual fungal structures provided more useful information than measuring total colonization alone.

In alfalfa, greater vesicle colonization may support growth under low nutrients and reduce tissue dieback during drought.

In sainfoin, mycorrhizal fungi appear to support nutrient acquisition, while root tissue density may be a useful trait for identifying drought-tolerant plants.

These findings suggest that breeding for beneficial plant–fungus relationships may help improve forage growth and stress tolerance, although more testing is needed.

Native grassland inoculants also show promise for saline soils, but their performance varies by forage species and by the location and timing of soil collection.

While these results are not ready for immediate use on farms, they provide a foundation for future research. Over time, this work could support the development of more resilient forage varieties and targeted inoculants that improve forage productivity under drought, low-nutrient, or saline conditions.

This project was also supported by Saskatchewan Cattle Association and NSERC.