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 electrondense material in aborted appressoria on roots of the nod-fix- genotype but not in appressoria on roots of the nod+fix+ genotype.