Mycorrhizal phenotypes and the Law of the Minimum

Background/Question/Methods

Although mycorrhizas are generally mutually beneficial for both plants and fungi, many factors influence the symbiotic outcome of mycorrhizal associations; consequently mycorrhizal function forms a continuum from mutualism to parasitism. The location of a particular mycorrhizal symbiosis on this continuum can be considered its phenotype: an emergent property of interactions among plant and fungal genotypes and the environment. Soil fertility is a key environmental controller of mycorrhizal phenotypes. Mycorrhizas can be studied using an economic perspective, photosynthate and minerals are the commodities exchanged and the trade value of these resources is set by their availability in the environment. The ‘Law of the Minimum’ states that plant production may be controlled by a single essential resource that is in limiting supply. Determining 1) which resources are in limiting supply in the environment, and 2) whether mycorrhizal symbioses can enhance access to these resources, provides an ecologically and evolutionarily sound approach to predicting mycorrhizal function in ecosystems and their responses to environmental disturbances. We explore the usefulness of this approach in a series of experiments that manipulate availability of photosynthate, nitrogen, and phosphorus to test hypothesized consequences of the Law of the Minimum to allocation patterns and functioning of arbuscular mycorrhizal associations. 

Results/Conclusions

Manipulation of carbon, N and P availability using shade, CO₂ enrichment, and fertilization experiments generated the full spectrum of mycorrhizal phenotypes, from mutualism to commensalism to parasitism. Our findings support the hypothesis that shade decreases and CO₂ enrichment increases the production of photosynthate, and that this carbon currency ultimately drives mycorrhizal trading partnerships. Also, our results show that P-limitation is a strong predictor of plant benefit from AM symbioses and that P-enrichment diminishes this benefit. The influence of N-availability on mycorrhizal phenotypes is mediated by P-availability. In systems with ample photosynthate and dual N- and P-limitation, N-enrichment increases fungal biomass and may or may not generate biomass gain for host plants. This N-induced enhancement of AM fungal biomass disappears if P is not in limited supply. Responses of AM fungi to N and P fertilization in field experiments throughout the world follow these patterns, and thus indicate that the Law of the Minimum is a useful predictor of mycorrhizal phenotype.