ROOT COLONIZATION BY PSEUDOMONAS-FLUORESCENS INTRODUCED INTO SOIL AMENDED WITH BENTONITE

The influence of bentonite clay amendments to loamy sand on the colonization of wheat roots by introduced Pseudomonas fluorescens was studied. In one experiment, root colonization was determined during 6 weeks of plant growth. In a second experiment, colonization was studied after 1 week of plant growth, either directly after introducing the bacteria, or after a 6 week conditioning of the soil. A rhizosphere effect, i.e. the occurrence of higher cell concentrations in the rhizosphere soil than in the bulk soil, was observed in the presence or absence of bentonite clay. The size of the rhizosphere effect was usually smaller in the presence of bentonite. This suggested that bacteria were physically hindered, making it more difficult to invade the immediate root environment. However, due to the fact that bentonite enhanced survival considerably, the overall survival in the rhizosphere in bentonite-amended soil was still higher than in unamended loamy sand. No bentonite effects on the rhizoplane populations were observed. In an additional experiment P. fluorescens was found to be sensitive to moisture fluctuations. It is therefore possible that moisture fluctuations during plant growth seriously reduced survival of P. fluorescens.