Growth promotion of greenhouse tomatoes with Pseudomonas sp. and Bacillus sp. biofilms and planktonic cells

The effects on plants of commercial applications of biofertilizer and biocontrol agents containing planktonic rhizobacteria cells are inconsistent mainly due to an unsuccessful competition with existing endogenous microbial communities. The present study investigated rhizobacteria inoculants in the form of biofilms, which are dense colonies of single or multi-species microbial cells, adherent to either biotic or abiotic surfaces, and encased in a self-produced matrix composed of extracellular polymeric substances. Bacillus sp., Pseudomonas sp., or a mixed inoculum of both bacteria at the planktonic or biofilm stage of growth were applied to greenhouse-grown tomato (Solanum lycopersicum L. cv Trust) plants fertilized with a half strength Hoagland solution or a modified solution lacking phosphate or iron. Based on multivariate analysis of variance, availability of nutrients, bacteria, and stages of bacterial growth significantly affected plant variables known to be contributing factors to greenhouse tomato yield. Results suggest that Bacillus sp. and Pseudomonas sp. biofilms are more effective inoculants than planktonic bacteria cells. Pseudomonas sp. applied as a biofilm increased the height and root dry weight of tomato plants, but it inhibited the positive effects of inoculation on tomato leaf number and tomato root length. Path analysis identified positive causal paths from (Pseudomonas) biofilm and planktonic cells, to the tomato plant growth variables. Presence of Bacillus sp. alone or in a mixture inhibited the positive effects of inoculation on tomato plant leaf area and root dry weight. Nevertheless, the effect of Bacillus sp. on root length was relatively positive. Results suggest that the use of innovative rhizobacteria biofilm technologies could prove advantageous as biofertilizer agents, and alleviate the dependence of greenhouse tomato producers on non-biological agro-chemicals.