Plant and Soil

, Volume 363, Issue 1–2, pp 411–419 | Cite as

The continuum concept remains a useful framework for studying mycorrhizal functioning

  • Nancy Collins Johnson
  • James H. Graham
Review Article



Recent studies have questioned the validity of the mutualism-parasitism continuum of mycorrhizal function. This paper re-evaluates the continuum model and analyzes these concerns.

Scope and Conclusions

Three insights arise from this analysis. First, the continuum model defines mycorrhizal function as an emergent property of complex interactions. The model identifies resource trade and symbiotic control as key determinants of the costs and benefits of the symbiosis for plants and fungi, and the interaction of these factors with the environment ultimately controls mycorrhizal function. Second, analysis of carbon costs and phosphorus benefits is too narrow a focus to accurately predict mycorrhizal function. Analysis of plant and fungal fitness responses in ecologically and evolutionarily relevant systems are required to elucidate the full range of nutritional and non-nutritional factors embodied within mycorrhizal functioning. Finally, the definition of the term ‘parasitism’ has evolved. Some fields of science maintain the original definition of a nutritional relationship between host and parasite while other fields define it as a +/- fitness relationship. This has generated debate about whether the continuum of mycorrhizal functioning should properly be called a positive–negative response continuum or a mutualism-parasitism continuum. This controversy about semantics should be resolved, but it does not overturn the continuum concept.


Benefits Costs Emergent properties Mutualism Mycorrhizal growth response (MGR) Parasitism Resource trade Symbiotic control 



Mycorrhizal growth response



We thank Theo Ruissen for his encouragement and helpful insights throughout all phases of this project. We are grateful to Tomas Herben and the students and faculty at the Department of Botany at Charles University in Prague, Czech Republic for their thought-provoking discussion. We thank Gail Wilson and her students in the graduate soil ecology seminar at Oklahoma State University. Thanks to Daniel Engelmoer, Jan Jansa, Rick Johnson, E. Toby Kiers, R. Michael Miller, Pål Axel Olsson, Marcel van der Heijden, Radka Sudová, Erik Verbruggen, and Gijsbert Werner for their helpful comments on an earlier version of this manuscript. Financial support was provided by the Fulbright Commission of the Czech Republic and the National Science Foundation of the USA (DEB-0842327).


Emergent property

When individual components interact to create distinct collective properties and functions that are not manifested unless the interacting system is observed in its entirety.


Reproductive success measured as the relative genetic contribution of individuals to future generations.

Mycorrhizal function

The net effect of a mycorrhizal symbiosis for the fitness of both plants and their fungal partners.


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Copyright information

© Springer Science+Business Media B.V. 2012

Authors and Affiliations

  1. 1.School of Earth Sciences and Environmental SustainabilityNorthern Arizona UniversityFlagstaffUSA
  2. 2.Citrus Research and Education CenterUniversity of FloridaLake AlfredUSA

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