Notes on Types of Bacteria Associated with Plant Roots

The occurrence of greater numbers of micro6rganisms in the vicinity of plant roots than in soil free from root penetration has become generally established (1-5). The magnitude of the increase in micropopulations on root surfaces or in the rhizosphere has been shown to be influenced by age of plant or stage of plant development, type of plant, disease resistance or region of the rhizosphere (3, 6-7), but to remain relatively uninfluenced by soil fertilization procedures (7-10). With the exception of detailed information concerning the association of symbiotic nitrogen-fixing bacteria with the leguminous plants, knowledge of types of bacteria associa.ted with root development is still limited, although there has been a number of valuable contributions to this subject. From cultural studies Starkey has concluded that all groups of bacteria do not respond equa.lly to the presence of plant roots, and from direct microscopic examinations he concluded that coccoid cells predominate, whereas spore-forming rods were seldom found (3, 11, 12). Lohnis (13) and Smith (14) have noted increased numbers of B. radiobacter in soil in the presence of plant roots. Lochhead (15) has reported Corynebacteria and Mycobacteria, common in soil generally, to be depressed in the rhizosphere, whereas Mycoplana is increased. Krassilnikov (16, 17) also noted that different groups of micro6rganisms responded differently to plant development, with cellulose-decomposing types active, and the predominating types nonspore forms. Azotobacter was found poorly distributed in the rhizosphere of corn, and absent in that of wheat; B. mnycoides a.nd B. megatherium were found in neither. But Poschenreider (18) found Azotobacter distributed in the rhizosphercs of a great variety of plants, and Truffaut and Vladykov (19) found both B. mycoides and B. megatherium and other Bacillus spp., and also Azotobacter and other special nutritional types generally prevalent in the rhizosphere of wheat. Verona and Luchetti (20), Isakora (21) and others have recognized that qualitative differences exist between soil microfloras and root microfloras, and Thom (22), in a broader discussion of the localization of microorganisms, noted that most of the organisms in contact with root surfaces belong to species active in the decomposition of fresh organic matter, and not with species associated with the breakdown of humus residues in the soil. These citations indicated a field which will require extensive study before the broader agreements between different root microfloras and the valid specific differences or peculiarities of each rhizosphere can become established. Our investigations conducted during the course of the last two years upon the