Effects of phosphorus on formation of the vesicular-arbuscular mycorrhizal symbiosis

Abstract

The effect of phosphorus (P) on formation of the vesicular-arbuscular mycorrhizal (VAM) symbiosis was examined. Using a sand-hydroponic culture chamber, increasing P levels were supplied lo soybean (Glycine max L. Merrill) seedlings. It was found that indirect supply of P up to 600 mM did not affect the germination of Glomus mosseae chlamydospores in the rhizosphere. Similar results were found for germination of either G. mosseae or Gigaspora margarita spores in the rhizosphere of soybean growing in soil amended with increasing P levels. These data suggest that P controls VAM formation at some stage after spore germination in the rhizosphere has occurred. Increasing P supply to the soil enhanced carbon exchange rate, sugar content of the roots and decreased sugars in the root exudates. Bioassays using root extracts from plants supplied with different P levels showed differential effects on germination of G. mosseae and G. margarita spores in vitro. Extracts from plants well supplied with P had high levels of sugars and inhibited spore germination. It was proposed that P controls root colonization through its effect on host carbon metabolism. In addition, the relationship between P availability and plant growth response to mycorrhizal formation is discussed.