Ectomycorrhiza, Friend or Foe?
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Ectomycorrhiza, Friend or Foe? ˚ gren,* Riitta Hyvo¨nen, and Preetisri Baskaran Go¨ran I. A Department of Ecology, Swedish University of Agricultural Science, P.O. Box 7044, 750 07 Uppsala, Sweden
ABSTRACT Many ecology textbooks present the interaction between mycorrhizal fungi and their host plants as the archetype of symbiosis or mutualism. However, mycorrhiza drains carbon directly from the plant and also competes with the plant for soil inorganic nitrogen. We developed hypotheses based on a simple model to qualitatively investigate when, in a nitrogen-limited system, the fungal partner returns sufficient extra nitrogen to compensate for the amount of carbon allocated to it by the plant. We showed when the mycorrhizal association can be beneficial to the plant, but also when mycorrhizal immobilization of soil inorganic nitrogen can be a limitation. The amount of carbon and nitrogen that the mycorrhizal fungus can obtain from soil organic matter, by producing extracellular enzymes, is also important. Saprotrophic capability decreases the value of the fungus to the plant, as fungal uptake of soil carbon augments the use of the plant-supplied
carbon and increases the fungal requirement for N. The stoichiometric mismatch between low-N soil organic matter and high-N fungal biochemistry turned out to be a bottleneck in making the fungus a net provider of additional N to the plant. The most important properties determining the usefulness to a plant of a mycorrhizal symbiont are plant nitrogen use efficiency and the amount of inorganic N taken up per unit extra fungal growth. The fraction of carbon the fungus allocates to its own growth, relative to its investment in exocellular enzymes, is also a critical property. Our results show that plants could benefit from the association with the fungus, which could explain the ubiquitous nature of this association between fungi and plants.
HIGHLIGHTS
INTRODUCTION
Mycorrhiza can, depending on environment, be a benefit or a cost to the host plant Saprotrophic capabilities of mycorrhiza is an important aspect Supply of inorganic nitrogen by the fungi to the host plant may be the major benefit.
Received 8 October 2018; accepted 24 January 2019 ˚ designed the study and performed the Author Contributions GIA ˚ , RH and PB wrote the paper. calculations. GIA *Corresponding author; e-mail: [email protected]
Key words: allocation; mycorrhiza; parasitism; plant growth; symbiosis.
nitrogen;
Many ecology textbooks present the interaction between mycorrhizal fungi and their host plants as the archetype of symbiosis or mutualism (for example, Odum 1971; Ricklef 1993). However, some studies show that mycorrhizal plants grow less well than non-mycorrhizal plants (for example, Correˆa and others 2006; Hoeksema and others 2010) and that mycorrhizal fungi can immobilize large quantities of nutrients, causing nutrient deficiency in the host plant (Na¨sholm and others 2013). These processes might not be mutually exclusive, as there is likely a gradient between mycorrhiza being benefi
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