Evaluating the potential of combined inoculation of Trichoderma harzianum and Brevibacterium halotolerans for increased growth and oil yield in Mentha arvensis under greenhouse and field conditions

The present study was conducted to evaluate the potential of co-inoculation of Trichoderma harzianum (Th) with the plant growth promoting rhizobacteria (PGPRs) on the growth and essential oil yield of Mentha arvensis under pot as well as field conditions. For in vitro screening of compatible strains, cross streak assays were undertaken and further assessed to confirm their ability to form biofilm help in root colonization. in vitro interaction studies showed that Trichoderma harzianum (Th) did not antagonize with Stenotrophomonas spp (Az-30), Bacillus fiexus (Sd-30) and Brevibacterium halotolerans (Sd-6) strains individually, indicating the possibility of using these combinations for plant growth promotion studies. These compatible combinations were further screened in vivo under greenhouse conditions on M. arvensis seedlings. The results revealed that co-inoculation of Th and Sd-6 increased plant growth, oil content, leaf-stem ratio, photosynthetic pigments, and nutrient uptake resulting in higher oil yields over other microbial treatments. The density (colony forming unit) of Th, as well as rhizobacteria, was significantly higher when compared to the single inoculations and control in the rhizospheric soil. Scanning electron microscopy revealed significant colonization of microbial biofilms on the root surface particularly for the best combination of Th and Sd-6. These results suggest the synergistic interactions between Brevibacterium halotolerans and Trichoderma harzianum for survival and improving plant growth and yield. The association of Sd-6 and Th, when applied under field conditions showed increased plant growth which provides an evidence that Th and Sd-6 together enhance the growth promoting abilities of each other significantly improving growth and yield of the crop. A higher number of propagules of both Th and Sd-6 may exert similar crop growth benefits in the subsequent crops while higher population of Th may protect the subsequent crops from many phytopathogens.