Rhizobacteria and lettuce: Root colonization, plant growth promotion and biological control

Plant growth-promoting rhizobacteria (PGPR) can increase the productivity of several crops, including lettuce. The main objective of this study was to compare the root and/or root collar colonizing ability on the growth promotion ability. A method of evaluation of root colonization was tested with the main objective of speeding up screening trials for growth enhancement of lettuce. Additionally, the antagonism in vitro between rhizobacteria and the pathogen Fusarium sp. was tested to verify if growth promotion ability was determined by biological control. Sixty-four bacterial isolates of fluorescent pseudomonads from the rhizosphere of different lettuce varieties were tested in vitro to verify their capability of colonizing lettuce roots. The presence of a turbid, milky and narrow fog around the root zone indicated bacteria colonization. Only eight isolates colonized the root system, and thirty-eight did in the root collar region, as could be evaluated by visual means. A greenhouse trial was then carried out to verify the growth promotion capacity of the isolates in lettuce. The substrate was a mixture of soil and chicken manure, similar to the one used by producers. Twelve isolates promoted growth of plants. Among those, four increased root dry weight, nine enhanced the number of leaves and eleven colonized the root collar. The antagonism assay was performed in PDA medium and King's B medium. Although twelve isolates showed antagonistic activity against Fusarium sp., only three of them enhanced growth in lettuce and colonized the root collar.