Beneficial microbes for improving circularity and yield in hydroponic crop cultivation

Microbial communities living in the proximity of crop roots in the soil shape the biochemical composition of the rhizosphere root zone and thereby affect the susceptibility of the crop to pathogenic organisms and the availability of mineral and organic nutrients. In hydroponic culturing conditions the context in which the roots grow is very different, which likely affects the composition of the rhizosphere microbiome and the response of the plant root system. In a previous experiment, we found that rhizosphere microbes isolated from hydroponically-grown lettuce promoted biomass production. Here we asked whether the lettuce roots inoculated initially with beneficial bacteria from hydroponically-grown plants would propagate the beneficial bacteria and serve as an inoculum for future lettuce growth stimulation. Analyses of the fresh and dry weights of lettuce shoot and lettuce roots showed that plants inoculated with rhizosphere microbes were not different from one another. Root electrolyte leakage measurements indicated that plants were presumably stressed yet not significantly different across the treatments. The inoculation with microbial rhizosphere extract did not alter the total phenolic content in root exudate, and hence no evidence was obtained for a differential response from the lettuce plants. The results show that growth-promoting activity is not transferred from one cultivation to the next and that re-inoculation with the original microbial source is likely a prerequisite for reproducing growth stimulation.