Arbuscular Mycorrhizal Fungi (AMF) are fungi that form a beneficial relationship with the roots of most plants. This partnership, or symbiosis, is a natural process in soils worldwide. The fungus extends its network of microscopic threads, called hyphae, into the soil, effectively increasing the plant’s root system and allowing it to access more water and nutrients. In this exchange, the plant provides the fungus with sugars from photosynthesis, and in return, the fungus supplies the plant with mineral nutrients like phosphorus, enhancing its ability to grow and resist stress.
The Concept of AMF Strains
Just as there are many breeds of dogs, there are different “strains” within a single AMF species. These strains possess unique genetic makeups that dictate their specific characteristics and abilities as a result of evolution in different environments. A well-known species like Rhizophagus irregularis, for example, comprises numerous distinct strains, each with a slightly different genetic code.
This genetic variation leads to differences in how strains function, meaning two strains of the same species might have different levels of effectiveness. This diversity explains why a generic, one-size-fits-all approach to AMF inoculants may not yield the best results. Therefore, matching a strain to a particular situation is an important consideration.
Functional Roles of Different Strains
The genetic diversity among strains translates into functional diversity, meaning different strains are specialized for different tasks. The capabilities of a strain determine the benefits it confers upon its host plant. These specialized functions include:
- Acquiring and transferring phosphorus by producing specific enzymes that unlock the nutrient from soil particles, making it available for root uptake.
- Enhancing a plant’s tolerance to drought by developing extensive hyphal networks that absorb and hold water in the soil.
- Contributing to a plant’s defense system by protecting roots from soil-borne pathogens or by strengthening the plant’s own immune responses.
- Improving soil structure by producing a sticky protein called glomalin, which binds soil particles together into stable aggregates.
- Acquiring other micronutrients like nitrogen and zinc that are essential for plant growth.
This functional specificity underscores that a strain’s performance is tied to its unique genetic adaptations.
Selecting the Appropriate Strain
Choosing the right AMF strain requires considering the plant’s needs and its environment. A primary factor is the host plant itself, as different plants have co-evolved with certain fungi. The ideal AMF for a vegetable crop may differ from what is best for a hardwood tree.
Soil conditions are another important consideration, as factors like pH, nutrient levels, and texture influence which strains will thrive. A strain that performs well in acidic, sandy soil might struggle in alkaline, clay-based soil. Analyzing the soil can help in selecting a pre-adapted strain.
The grower’s primary objective also plays a role. If the goal is to increase crop yield, a strain known for phosphorus uptake might be best. If water scarcity is the challenge, a strain recognized for enhancing drought resistance would be more appropriate.
When purchasing commercial inoculants, you can choose between single-strain or multi-strain products. Single-strain products are for targeted applications where a specific benefit is desired. Multi-strain inoculants offer a broader range of functions, increasing the likelihood that at least one strain will establish a successful symbiosis.
Application and Environmental Factors
Proper application is necessary for the fungi to establish a relationship with plant roots. Commercial inoculants come in powders, granules, and liquids. An effective method is to apply the inoculant directly to the roots during transplanting to ensure immediate contact. Other methods include coating seeds or mixing the inoculant into the soil.
Several environmental factors influence the success of AMF inoculation. High levels of synthetic phosphorus fertilizer can inhibit the symbiosis, as plants have less incentive to partner with fungi when the nutrient is readily available.
Physical soil disturbance is also detrimental to the fungal network. Tilling breaks up the delicate hyphae, disrupting the connection between the fungi and plant roots. Practices that minimize soil disturbance, like no-till farming, help preserve the mycorrhizal network. The use of certain fungicides can also harm or kill beneficial AMF.