Plant growth responses to bacterization with selected Pseudomonas spp. strains and rhizosphere microbial development in hydroponic cultures

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Abstract
Increases in root and shoot fresh weight were observed when the nutrient solutions used for growth of tomato, cucumber, lettuce, and potato plants were bacterized with plant growth promoting Pseudomonas strains. It is hypothesized that plant growth promotion is due to the suppression of a deleterious rhizosphere microflora, especially deleterious endorhizosphere bacteria. The composition of the microbial community in artificial substrates, such as nutrient films and rockwool, is thought to be less complex than in soil. Therefore, development of a microflora deleterious for plant growth has to be anticipated. Microbial development in hydrocultures was simulated by preinoculating plant nutrient solutions with rhizosphere suspensions from plants that had been grown for different lengths of time in hydroponic cultures. After 16 days the plants in preinoculated nutrient solutions showed a growth reduction that correlated with the extent of root colonization by fluorescent pseudomonads in the endorhizosphere. Bacterization with plant growth promoting Pseudomonas sp. strain WCS417r counteracted this plant growth reduction. This increase in plant growth was accompanied by a dense colonization of the endorhizosphere by WCS417r and by a consequent displacement of indigenous endorhizosphere-inhabiting pseudomonads.Key words: Pseudomonas, endorhizosphere, hydroponics, plant growth, colonization.

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