Publisher Summary The chapter presents a discussion on both rhizosphere and rhizoplane. Rhizosphere is defined as the soil adjacent to roots with a different physical, chemical, and biological environment from the bulk soil. The rhizoplane is defined as the actual root surface or soil interface. The chapter describes plant responses, the nature and dynamics of rhizosphere populations and their determinants, the management of the plant, and management of microorganisms. The chapter also explores the impact of soil conditions and agronomic practices on the rhizoshere microflora and suggests areas that require more study in order to achieve effective management. The chapter also illustrates different methods that are used by agronomists, plant pathologists, and microbiologists in order to modify the rhizosphere. In most cases, when microbiologists and plant pathologists needs to change the rhizosphere in order to improve plant growth they often think in terms of introducing microorganisms that possess the desired characteristics, be they antibiotic or siderophore production, or the ability to stimulate plant growth.

The rhizosphere and its management to improve plant growth

Antibiotic-resistant strains of Pseudomonas fluorescens and Bacillus subtilis, produced by transposon Tn5 mutagenesis and transformation with plasmid pFT30, respectively, were characterized. Both strains grew at a rate comparable to that of the wild-type strains, and the antibiotic resistance remained stable for over 50 generations without selective pressure. During the growing season, the survival of these strains was studied in two soils of different texture cropped with wheat. The B. subtilis populations declined rapidly in both soils and then stabilized at the levels of added spores. P. fluorescens showed a slow, steady decline in both soils; survival was better in the finer-textured soil, a silt loam, than in the coarser loamy sand. For both bacteria, some translocation to deeper soil layers was observed. No significant rhizosphere effects were detected in either of the two soils.

Survival of Pseudomonas fluorescens and Bacillus subtilis introduced into two soils of different texture in field microplots

Chapter 8 – Role of Root-Colonizing Bacteria in Plant Growth

Publisher Summary This chapter describes nitrogen fixation in C-3 and C-4 grasses. It mentions that nitrogen-fixing bacteria, which have been found in association with grasses are all capable of fixing nitrogen in soil or culture medium without the plant. Therefore, they are generally included in the group of free-living N 2 -fixing bacteria. Emphasis is given on bacteria for which specific associations with tropical grasses have been shown. Nitrogen fixation in digitaria decumbens and paspalum notatum cores has been confirmed by the incorporation of 15 N 2 . Acetylene reduction assays over a 24-hour period in cores indicating fixation of more than 100 g N/ha per day have been obtained with several tropical forage grasses, rice, and occasionally with wheat and rye grown under tropical conditions. N 2 fixation estimated by the excised root method in maize and sorghum has been shown to be several-fold higher than the amount of N 2 fixation obtained from core assays. The chapter discusses various factors affecting nitrogen fixation in grasses—such as, (1) seasonal and diurnal fluctuations, (2) plant genotype, (3) temperature, (4) oxygen, (5) combined nitrogen.

Nitrogen Fixation In Grasses

The recognition of plant growth‐promoting rhizobacteria (PGPR), a group of beneficial plant bacteria, as potentially useful for stimulating plant growth and increasing crop yields has evolved over the past several years to where today researchers are able to repeatedly use them successfully in field experiments. Increased growth and yields of potato, sugar beet, and radish have been reported. The most effective strains of PGPR have been fluorescent Pseudomonas spp. Growth increases appear to result from the suppression of soil microorganisms that are deleterious to plant growth. A possible mode of action is through the production of the siderophore pseudobactin, that chelates iron in the plant rhizosphere making it unavailable to the harmful microflora. Commercial applications of PGPR are being tested and are frequently successful; however, a better understanding of the microbial interactions that result in plant growth increases will greatly increase the success rate of field applications.

Beneficial plant bacteria

Seed of maize, tomato, and wheat was inoculated with cultures of Azotobacter, Clostridium, and a nitrogen-fixing facultative Bacillus and grown in a nutrient-deficient sand and a highly fertile silt loam. In sand, wheat showed a significant positive response to inoculation with Azotobacter and Clostridium but maize and tomato were unaffected by inoculation. When inoculated seed was planted in Lima silt loam there were significant increases in the growth of maize, tomato, and wheat to treatment with Clostridium, inoculated maize and wheat responded to Azotobacter inoculation while only wheat responded to inoculation with the facultative Bacillus. In pure-culture studies of the ability of these cultures to establish upon plant roots it was shown that Azotobacter did not colonize the roots of lucerne, maize, tomato, or wheat to any great extent. Bacillus and Clostridium were moderate colonizers of plant roots reaching from 1 to 20 per cent the levels reached byPseudomonas fluorescens on the same plants.

A. D. Rovira

Papers

Microbial inoculation of plants