Influence of light intensity regime on the symbiotic functioning of arbuscular mycorrhizae in high phosphorus soils.

Arbuscular mycorrhizal fungi (AMF) form a symbiotic relationship with plants roots called mycorrhizae, through which the fungus acquires photosynthate from the host plant in return for hyphal assisted uptake of nutrients. Although arbuscular mycorrhizae is generally mutualistic, benefitting both the plant and the fungus, the availability of light and nutrients can shift this relationship to one that is more parasitic. In particular, mycorrhizae may become parasitic on plants when the cost of supporting the symbionts exceeds the benefit, such as in high phosphorus (P) conditions where hyphal extension of the root system does not increase nutrient uptake. Light availability impacts plant growth as well as the development of root symbionts and may affect where mycorrhizae falls on the parasitism/mutualism spectrum. Despite the importance of light levels in the mycorrhizal symbiosis, little is known about how varying light levels in greenhouse studies influence mycorrhizal function in high P soils. Greenhouse studies exploring the growth response of plants to AMF often fail to report light intensity levels, which could impact the quality of their data. To investigate how plants would respond to mycorrhizae in high P soils under different light regimes, we conducted a greenhouse study where we grew plants with and without AMF in high and low light conditions, totaling ten plants in each of four treatments. We found that AMF decreased plant biomass in the unshaded treatment, as expected by a parasitic symbiosis in high P soil, but did not have an effect on biomass in the shaded treatment. Examination of mycorrhizal colonization in root samples revealed that colonization levels correlated with plant biomass. These results suggest that in high P conditions, root colonization and the parasitic action of AMF on plants is greater in high light than in low light. We conclude that nutrient availability and light are both important controls on plant growth response to AMF and light levels should be reported in future greenhouse studies exploring the function of AMF