St. John's wort root-associated beneficial bacteria with vermicompost augment specialized metabolites without penalizing biomass yield by improving photosynthesis and soil microbial properties

Reintroducing beneficial rhizosphere microbes associated with wild medicinal plants back into the rhizosphere during cultivation is expected to increase the efficacy of herbal medicine. However, the effectiveness of this approach for St. John’s wort (Hypericum perforatum L.) has not been explored, nor has it been evaluated alongside other plant biostimulants, such as seaweed extract and non-host-specific plant-beneficial rhizobacteria, that may also impact plant growth and specialized metabolites. To fill these knowledge gaps, in present study a two-year factorial randomized block design experiment was conducted using vermicompost amendment and plant biostimulants, derived from microbial origin (consortia of host plant-associated and non-host-specific plant-beneficial rhizobacteria) and macroalgae-based (seaweed extract). Dry biomass yield of main medicinal material (Hyperici herba) significantly (P = 0.05) influenced by the interaction effect; while contents of specialized metabolites by individual treatments. Seaweed extract significantly maximized the contents of specialized metabolites, particularly pseudohypericin and hypericin; however, it substantially reduced the biomass yield. In both year, biomass yield was not significantly (P > 0.05) influenced by the individual treatment of non-host-specific plant-beneficial rhizobacteria; however, in combination with vermicompost amendment at 5 Mg ha⁻¹ the biomass yield numerically increased compared to the seaweed extract treatment. Nevertheless, specialized metabolites were minimally increased by non-host-specific plant-beneficial rhizobacteria. Unlike seaweed extract and non-host-specific plant-beneficial rhizobacteria, host plant-associated rhizobacteria simultaneously enhanced specialized metabolites, especially hyperforin, and the biomass yield either alone or in combination. Across both years, the maximum biomass yield was achieved under the combined application of host plant-associated plant-beneficial rhizobacteria and vermicompost amendment at 5 Mg ha⁻¹. This treatment also improved net photosynthetic rate, internal water use efficiency, soil microbial biomass carbon, and respiration compared to control and rest of the treatments. Thus, the integrating host plant-associated plant-beneficial rhizobacteria with vermicompost amendment offers a sustainable approach to increase the efficacy of St. John’s wort herbal medicine without compromising the biomass yield.