Association of Azospirillum with Grass Roots

The association between grass roots and Azospirillum brasilense Sp 7 was investigated by the Fahraeus slide technique, using nitrogen-free medium. Young inoculated roots of pearl millet and guinea grass produced more mucilaginous sheath (mucigel), root hairs, and lateral roots than did uninoculated sterile controls. The bacteria were found within the mucigel that accumulated on the root cap and along the root axes. Adherent bacteria were associated with granular material on root hairs and fibrillar material on undifferentiated epidermal cells. Significantly fewer numbers of azospirilla attached to millet root hairs when the roots were grown in culture medium supplemented with 5 mM potassium nitrate. Under these growth conditions, bacterial attachment to undifferentiated epidermal cells was unaffected. Aseptically collected root exudate from pearl millet contained substances which bound to azospirilla and promoted their adsorption to the root hairs. This activity was associated with nondialyzable and proteasesensitive substances in root exudate. Millet root hairs adsorbed azospirilla in significantly higher numbers than cells of Rhizobium, Pseudomonas, Azotobacter, Klebsiella, or Escherichia. Pectolytic activities, including pectin transeliminase and endopolygalacturonase, were detected in pure cultures of A. brasilense when this species was grown in a medium containing pectin. These studies describe colonization of grass root surfaces by A. brasilense and provide a possible explanation for the limited colonization of intercellular spaces of the outer root cortex.     

Pectolytic enzymes in Rhizobium

A sensitive pectin agar plate assay was used to demonstrate low levels of pectolytic enzymes in infective and noninfective strains of Rhizobium. The possible relation of this characteristic to legume infection is discussed.     

Pectolytic enzymes in Rhizobium.

A sensitive pectin agar plate assay was used to demonstrate low levels of pectolytic enzymes in infective and noninfective strains of Rhizobium. The possible relation of this characteristic to legume infection is discussed.     

Effects of Rhizobium inoculation on growth and nodulation ofAlbizia falcataria (L.) Fosh. andAcacia mangium Willd. in the nursery

Three separate experiments were conducted in the nursery using grassland soil as a growing medium. The first experiment was conducted to assess the nodulation of the two legume trees grown in unamended soil, the second was done to determine the effects of N-fertilizer application on the interaction of the five Rhizobium isolates withA. falcataria and the third experiment was conducted to determine the effects of liming on theRhizobium × A. mangium interaction. Three local Rhizobium isolates, A₁₆, A₁₈, and A₁₄ were effective forA. falcataria with A₁₆ as the most promising strain under the conditions described. In general, application of combined nitrogen suppressed nodulation ofA. falcataria. Nodulation in the absence of combined N exceeded those fertilized with 30 kg N ha⁻¹ by 114.0%, 60 kg N ha⁻¹ by 209.6% and 100 kg N ha⁻¹ by 237.1%. Two local isolates, Am₁₀ and Am₂, and an introduced strain, NA 1533 from Australia were promising forA. mangium. Unlike inA. falcataria, the application of combined nitrogen at the rate of 100 kg N ha⁻¹ did not suppress nodulation inA. mangium. Liming the soil to pH 6.5 regardless of nitrogen fertilizer application improved the performance of the Rhizobium—A. mangium symbiosis.