On host regulation of the vesicular-arbuscular mycorrhizal symbiosis.

We investigated mycorrhizal infections caused by Glomus elunicalum Becker & Gerd. in sunflower (Helianthus annuus L.) plants given small concentrations of phosphate. In pure sand given no additional phosphate, infection was only poorly developed. As phosphate availability was increased, the extent of infection also initially increased. We hypothesized that the extent of infection should be regulated by the nutritional status of the shoot. However, by performing a split-root experiment, we found that the root rather than the shoot was responsible for this regulation.