Nutrient use efficiency is something that should be at the forefront of our minds, especially with implementation of schemes such as the ReefSafe initiative, but also production costs and soil health. The more efficiently the fertilizer is utilized by the plant, the less applied, reducing the risk of environmental damage and decreasing costs of use. Mycorrhiza, a fungi that has been well-researched over the decades provides a solution to improving this nutrient use efficiency.
Roots represent the other half not usually seen in crops, but out of sight should not mean out of mind, especially in this instance. The root system provides or supports an array of vital functions; anchoring the crops, supplying moisture and nutrients, and facilitating the exchange of many growth promoting substances to the plants shoot tissue. In fact, most interactions between the plant and the surrounding environment occur between the roots and soil.
Mycorrhiza is termed as the mutualistic interaction (symbiosis) between these roots and soil-borne fungi. The most widely distributed association in plants is that of endomycorrhizas, described as inter- and intracellular (occurring within or inbetween cells) fungal growth developing into the root cortex, creating specific fungal structures commonly referred to as vesicles or arbuscles. These structures lead to endomycorrhiza’s more universally recognized name, Vesicular Arbuscular Mycorrhiza (VAM) or Arbuscular Mycorrhiza Fungi (AMF), which is used interchangeably.
In exchange for plant produced photosynthetic products, the VAM or AMF provides their host plant with mineral nutrients and water it has obtained with its fungal hyphae called mycelium. The mycelium emerging from the plants root system can acquire nutrients from soil that is otherwise inaccessible to the roots, with fungal hyphae being of a much thinner profile and able to penetrate narrower soil pores. Alternatively, some mycorrhiza hyphae produce exudates (organic acids) that solubilise nutrients like Phosphorous that have been bound in the soil profile in an inorganic, insoluble, and otherwise unobtainable form for plant uptake.
The mineral nutrients these fungal hyphae acquire are translocated back into the arbuscles, the functional site of nutrient exchange, where the fungi also receive photosynthetic products like carbohydrates in exchange.
Nutrient uptake and tissue levels have been recorded to be enhanced post-inoculation in a variety of host plants grown in wide variety of soils. Those most enhanced are Phosphorous, Nitrogen, Zinc and Copper, with Potassium, Calcium and Magnesium showing the greatest response in acidic soils. In studies focusing on shoot uptake of Phosphorous (P), uptake and growth increased in the studied host plants when compared to uninoculated plants and the soil solution from the hyphae regions showed increased solubilized P compared to samples taken from uninoculated plants. Increased shoot growth, P concentration and uptake of P was recorded in all soils of various P concentrations to begin with, with the increase obviously being the most pronounced in low P soil to begin with .When applying phosphorous fertilization, inoculated plants showed increased growth, raised P concentration in soil and P uptake into plant tissue compared to uninoculated plants exposed to the same level of fertilization. This showed that the presence of VAM increased the efficiency of applied Phosphorous fertilizer for the plants. Nitrogen and Potassium soil levels and uptake also increased in VAM inoculated plants in comparison to those that were uninoculated.
Plants benefit from inoculation with VAM because of the increased ability to grow and produce in relatively deficient environments. VAM modifies the transpiration rates and composition of the microflora in the rhizosphere, as well as directly enhancing the host plants growth by positively affecting nutrient acquisition and therefore plant growth.
There is an increasing desire to minimize the adverse effects of excess Phosphorous on the surrounding environments, including waterways, as well as still providing the crops with adequate levels of P for development and growth. This will require new methods to reduce inputs of P while maximizing the efficiency of the nutrients supplied via the uptake mechanisms used by the plants.
Recent studies and advancements in microbiology offer these very solutions, with incorporation of VAM into cropping systems providing an alternative option for sustaining crop yields while reducing fertilizer inputs, like Phosphorous.