This paper summarizes a multinational collaborative project to search for natural, intimate associations between rhizobia and rice (Oryza sativa L.), assess their impact on plant growth, and exploit those combinations that can enhance grain yield with less dependence on inputs of nitrogen (N) fertilizer. Diverse, indigenous populations of Rhizobium leguminosarum bv. trifolii (the clover root-nodule endosymbiont) intimately colonize rice roots in the Egyptian Nile delta where this cereal has been rotated successfully with berseem clover (Trifolium alexandrinum L.) since antiquity. Laboratory and greenhouse studies have shown with certain rhizobial strain-rice variety combinations that the association promotes root and shoot growth thereby significantly improving seedling vigour that carries over to significant increases in grain yield at maturity. Three field inoculation trials in the Nile delta indicated that a few strain-variety combinations significantly increased rice grain yield, agronomic fertilizer N-use efficiency and harvest index. The benefits of this association leading to greater production of vegetative and reproductive biomass more likely involve rhizobial modulation of the plant's root architecture for more efficient acquisition of certain soil nutrients [e.g. N, phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), zinc (Zn), sodium (Na) and molybdenum (Mo)] rather than biological N 2 fixation. Inoculation increased total protein quantity per hectare in field-grown grain, thereby increasing its nutritional value without altering the ratios of nutritionally important proteins. Studies using a selected rhizobial strain (E11)

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.

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.

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 with A. falcataria and the third experiment was conducted to determine the effects of liming on the Rhizobium x A. mangium interaction.Three local Rhizobium isolates, Al6 , A~8, and Al4 were effective for A. falcataria with Ai6 as the most promising strain under the conditions described. In general, application of combined nitrogen suppressed nodulation of A. falcataria. Nodulation in the absence of combined N exceeded those fertilized with 30kgNha ~by 114.0%,60kgNha Jby209.6% and 100kgNha ~by237.1% .Two local isolates, Amt0 and Am2, and an introduced strain, NA 1533 from Australia were promising for A. mangium. Unlike in A. falcataria, the application of combined nitrogen at the rate of 100 kg N ha-~ did not suppress nodulation in A. mangium. Liming the soil to pH 6.5 regardless of nitrogen fertilizer application improved the performance of the Rhizobium-A. mangium symbiosis.

The objectives of this study were to document by light and electron microscopy the events leading to the successful establishment of an associative symbiosis of Azospirillum brasilense Sp 7 with the roots of a grass host under axenic conditions and to define physiological and biochemical interactions of the association suggested by the ul trastructural studies. The association between grass roots and Azospirillum brasilense Sp 7 was investigated by the Fatiraeus glass slide technique for axenic plant culture. Young inoculated roots of pearl millet and guinea grass produced more mucigel, more root hairs and more lateral roots than did the uninoculated controls. Within 12 h after inoculation cells of A. brasilense Sp 7 were embedded in the mucigel at the root cap and along the root axis. The bacteria were also firmly adsorbed to root hairs and epidermal cells but supplementing the medium with combined nitrogen x