S06 - Session O9 - Transplanting bacterial community inocula on hydroponic lettuce.

Authors: Danny Geelen, Thijs Van Gerrewey, Nico Boon, Brechtje de Haas *

The different components of the root zone (plant, microbiome, and rooting medium) can be engineered to enhance crop production sustainably. The application of plant growth-promoting rhizobacteria (PGPR) can contribute to plant growth enhancement. However, multispecies microbial inocula may be more effective in improving plant performance than single species PGPRs. To this end, in soilless systems, the proper selection of the plant growing medium raw materials plays a crucial role in the effectiveness of bacterial community amendment. Therefore, we investigated the effect of transplanting bacterial community inocula on hydroponic lettuce ( Lactuca sativa L.) root-associated bacterial community functioning. The lettuce plants were grown in ten plant growing media with varying raw materials. The results showed that the plant growing medium composition determined plant performance and that successful bacterial amendment was a key driver for improved plant performance. The effectiveness of bacterial amendment depended on the bacterial source, but it also depended on the interaction with the plant growing medium. We found that the raw materials had distinct resident bacterial community structures that were batch-dependent. However, amending a bacterial community to the plant growing media allowed more control over the final root-associated bacterial community structure than a single species PGPR inoculum. Both plant growing medium composition and inoculation affected diversity. These changes in diversity were correlated to plant performance, indicating that a high diversity promoted plant growth. The results support the concept of creating bacterially enhanced plant growing media to maximize root-associated microbiome functioning to secure high plant performance in a hydroponic environment.