Diazotroph establishment and maintenance in the Sorghum – Glomus – Azospirillum association

Abstract

Sorghum (Sorghum bicolor (L.) Moench cv. Bok 8) plants were grown in either a high peat or a low organic matter substrate and inoculated with a pigmented strain of Azospirillum brasilense (Cd). Half of the plants were also inoculated with the vesicular–arbuscular mycorrhizal (VAM) fungus Glomus etunicatum, while the remaining plants were fertilized with levels of phosphorus that had been found to be comparable to P input resulting from mycorrhizal colonization. Plants were harvested 1, 3, 5, and 7 weeks postemergence. Total plant dry weight was statistically similar (p > 0.05) between the two treatments for each substrate at all harvests. For each substrate, Azospirillum counts per gram of root were significantly (p < 0.05) higher in mycorrhizal roots at all harvests than in P-fertilized roots, and also higher in the high peat substrate compared with the low organic matter substrate. The ratio of the total number of inoculated A. brasilense cells in the Glomus-colonized compared to the P-fertilized roots was positively and significantly correlated (r = 0.81; p < 0.05) with % VAM colonization. Colonization of sorghum roots by G. etunicatum thus enhanced the establishment and persistence of A. brasilense in the endorhizosphere of sorghum.Key words: Azospirillum brasilense, endomycorrhizae, Glomus etunicatum, rhizosphere, Sorghum bicolor.