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Hidden Partners: Mycorrhizal Fungi and Plants

Nitrogen Uptake

Research in the area of nitrogen uptake is still ongoing, but it is believed that mycorrhizas strongly absorb ammonium and organic nitrogen, with little absorption of nitrate. Hyphae have a higher capacity to take-up ammonium than non-mycorrhizal plant roots and some mycorrhizas emit enzymes that mobilize organic nitrogen, making it easier to obtain. Early research suggests that amino acids may aid in the transport of nitrogen from the hyphae to the plant. A pathway involving ammonium may also be used. However, much is still unknown.

Ectomycorrhizas have been shown to be especially important to plants in terms of nitrogen accumulation when growing in conditions where pH and other soil factors limit the availability of nitrogen.

The ability of mycorrhizal fungi to absorb organic and other sources of nitrogen that can be used directly by the host plant allows the plant to essentially 'shortcut' the nitrogen cycle. Thus, plants in mycorrhizal associations have an adaptive edge over their non-colonized competitors and can therefore survive in harsher conditions.

Phosphorus Uptake

Phosphorus is a relatively immobile element in soil and is one of the main elements that mycorrhizal fungi absorb and provide for their host plant. Mycorrhizas are able to absorb large amounts of phosphorus by decomposing its organic sources. The fungal hyphae also maximize the potential surface area available to any accessible phosphorous. Studies show mycorrhizal fungi increase the phosphorus concentration in their host plant by up to 40% when compared to non-colonized plants. Phosphorus is transferred from the fungi to the plant in its inorganic form. Research involving ectomycorrhizae found that the exchange rate of phosphorus depended on the concentration of phosphorus in the cytoplasm of the Hartig net cells, suggesting that osmotic pressure is what drives the exchange of phosphorus between plant and fungus.

Page 5: Ectomycorrhizal Fungi and the Wider Environmental Community
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