Effects of interactions between different culture fractions of 'phosphobacteria' and Rhizobium on mycorrhizal infection, growth, and nodulation of Medicago sativa.

Interactions between cell-free culture supernatants, cells, and the whole cultures of Rhizobium and phosphobacteria with endomycorrhizal fungi and their effects on growth and nutrition of Medicago sativa grown in a low-phosphate soil were studied. A satisfactory nodulation was greatly dependent on the mycorrhizal symbiosis. Cell-free supernatants of Rhizobium and phosphobacteria improved plant growth, nodulation and mycorrhiza formation. The activity of phosphobacterial culture seemed to be due mainly to the supernatant and the possibility of plant hormones contained in this culture fraction being involved in such interactions is discussed. An increase of the overall pool of soluble P in soil by the inoculated phosphobacteria cells was not found in the conditions of this experiment. It was noteworthy that the best positive effect was achieved by the treatment which consisted of the whole cultures of Rhizobium, phosphobacteria, and the mycorrhizal fungi applied all together.