Quality versus quantity: optimizing evaluation of ectomycorrhizae for plants under stress

Many single environmental conditions or stresses have been evaluated with respect to formation and survival of ectomycorrhizae (EM). Current interests in atmospheric change include plant responses to pollutants such as ozone, sulfur dioxide, and acidic deposition in the presence of additional stress such as water deficit or plant disease. Stresses that result in formation of fewer EM or death of fine roots are evaluated routinely by quantifying EM, and results are often correlated with parameters that describe host physiology and growth. However, effects of stresses may be subtle, or identification of early plant responses may be desired. In such cases, quantification of EM may not reveal changes in EM formation by individual species of fungi if roots are colonized by other species with no net change in total numbers. Differences among frequencies of morphotypes suggest changes in species diversity of EM-forming fungi. Although the relative benefits of individual species to the host are largely unknown, information provided by qualitative assessment of EM may offer insight to define plant responses to stress and suggest additional research related to benefits of individual EM-forming fungi.