The development of endomycorrhizal root systems VIII. Effects of soil phosphorus and fungal colonization on the concentration of soluble carbohydrates in roots

Concentrations of phosphorus in shoot and soluble carbohydrates (fructose, glucose, sucrose and fructans) in root were measured in non-mycorrhizal and vesicular-arbuscular (VA) mycorrhizal (Glomus mosseae) leek plants (Allium porrum) raised at six concentrations of soil phosphate. In conditions when an increased concentration of soil phosphate reduced VA mycorrhizal infection, the concentrations of soluble carbohydrates in the root were at a maximum. Therefore the hypothesis that greater concentrations of soluble carbohydrates in roots favour VA mycorrhizal infection is discounted. There was a specific effect of VA mycorrhizas, in that infected roots contained a larger concentration of sucrose than did uninfected roots, in plants with similar phosphorus concentrations in dry matter of shoots. We conclude, first, that increased phosphorus supply from either phosphate addition to soil or VA mycorrhizal infection increases concentration of soluble carbohydrates in leek roots and, secondly, that the VA mycorrhizal root behaves as a particularly strong physiological sink when there is an excess concentration of sucrose in the host

How useful is the mutualism-parasitism continuum of arbuscular mycorrhizal functioning?

BACKGROUND A recent review in this journal puts forward the premise that our recent studies have resulted in our questioning the validity of the so-called mutualism-parasitism continuum of functioning of arbuscular mycorrhizas. This premise is incorrect and appears largely to result from a misunderstanding of terminology. SCOPE AND CONCLUSIONS We clarify a comment in one of our publications that influenced the previous review, which contains several statements that do not accurately represent our views. Our research has overturned not the continuum concept itself, but some past ideas about the balance of resources traded between AM fungi and plants. Of course, we recognize that outcomes of AM symbiosis in relation to the non-mycorrhizal (NM) state are strongly influenced by many environmental factors. Nevertheless, underlying resource trade is always a key determinant of costs and benefits of the symbiosis for both partners. In this context, we address uncertainties and contradictory ideas about mechanisms, causes, effects and outcomes in AM symbioses that occur in the literature, and issues of relevance of research at different scales. We also discuss semantics that can cause confusion. Finally, we assess how useful the mutualism-parasitism continuum is for design of hypothesis-driven experiments to disentangle the complex interactions that determine growth of AM plants, i.e. the so-called emergent properties.F. Andrew Smith & Sally E. Smit