Fluctuations in the fluorescent pseudomonad and actinomycete populations of rhizosphere and rhizoplane during the growth of spring wheat

Abstract

The total bacterial populations, fluorescent pseudomonads, and actinomycetes in the rhizosphere, rhizoplane, and endorhizosphere of two wheat lines (C-R₅B and C-R₅D) were investigated weekly during plant growth. The total numbers of bacteria (colony-forming units) were counted on a low-nutrient agar medium, while the numbers of fluorescent pseudomonads and actinomycetes were determined by means of selective media. Fluorescent pseudomonads generally constituted less than 0.5% of the total bacterial numbers and the percentage significantly decreased with time in the rhizosphere, rhizoplane, and root-free soil. The percentages of fluorescent pseudomonads were lower in the rhizoplane than in the rhizosphere, and in the endorhizosphere they were barely detectable. The rapid decline of fluorescent pseudomonads frequently observed in survival studies may be explained by the inability of the plant to support this group of bacteria during later development of its rhizosphere. Actinomycetes generally represented no more than 7% of the total bacterial numbers. Actinomycete percentages were highest in the rhizosphere and root-free soil and lowest in the endorhizosphere. Only minor differences were found in total bacterial numbers and in the numbers of fluorescent pseudomonads between the two wheat lines. However, numbers of actinomycetes were significantly different over the whole experiment between wheat line C-R₅B and C-R₅D in the rhizosphere, rhizoplane, and endorhizosphere. Key words: actinomycetes, bacteria, fluorescent pseudomonads, rhizosphere, sampling time.