Further evidence for a correlation between nodulation genotypes in alfalfa (Medicago sativa L.) and mycorrhiza formation.

The vesicular-arbuscular (VA) mycorrhizal status of three alfalfa (Medicago sativa L.) nodulation genotypes (nod fix , nod fix and nod fix ) was investigated using pot cultures of three fungal species from two genera (Glomus monosporum Gerd. & Trappe, Glomus fasciculatum (Thaxter) Gerd. & Trappe emend. Walker & Koske and Gigaspora margarita Becker & Hall). Roots of the nod fix genotype developed complete VA mycorrhizas with each fungus. Roots of the nod fix and nod fix genotypes did not develop normal VA mycorrhizal symbioses. Roots of the nod fix genotype had significantly more appressoria than roots of the other genotypes. Aborted appressoria were unable to penetrate the cortical cells of roots of nod fix and nod fix genotypes. Measurement of appressorium length, width and approximate contact area revealed significant differences in the size of appressoria produced by each VA mycorrhizal fungus on each alfalfa nodulation genotype. Histological studies of nod fix and nod fix genotypes colonized by Glomus versiforme (Karsten) Berch showed the presence of electron-dense material in aborted appressoria on roots of the nod fix genotype but not in appressoria on roots of the nod fix genotype.