Phytostimulation Properties of Indigenous Plant Growth-promoting Bacteria from Licorice (Glycyrrhiza glabra L.): Benefits for Seed Germination and Seedling Growth

To evaluate the effects of four licorice plant growth-promoting bacteria (PGPB), i.e. Pantoea agglomerans (S72), Serratia rubidaea (S28) Pseudomonas azotoformans (E101), Pseudomonas frederiksbergensis (E56) on licorice seed germination and seedling growth, two experiments were conducted at the Research Laboratory and Greenhouse, University of Tehran in 2019. Treatments were bacterial strains in eight levels (sole or in combination) and growing media at four levels of M1 (sand + perlite (1: 1)), M2 (soil + cow manure), M3 (soil + mycorrhiza fungi) and M4 (soil + mycorrhiza fungi+ cow manure). Germination quality criteria and some morphological traits of two-month-old seedlings were evaluated. The results of seed germination showed that in most of the evaluated traits, bacterial treatments performed better than the control, although in some cases, there were no significant differences with the control. Also, in some cases, endophytic bacteria had an effective role in improving seed germination index compared to rhizospheric bacteria. According to the experiment results of seedling growth, M4 in combination with two endophytic bacteria (E101 and E56), of the Pseudomonas genus in licorice plants, had a superior performance in improving the initial growth and establishment qualities of the licorice plants. Regarding most of the traits, the co-application of mycorrhiza with S28 (M3B3) showed the lowest values. The results of this study indicated the potential use of licorice endophytic bacteria as a source of biofertilizer for the improvement of licorice seed germination and growth and, if possible, for similar applications in other plant species. Abbreviations M1 (sand + perlite (1: 1)), M2 (soil + cow manure), M3 (soil + mycorrhiza fungi) and M4 (soil + mycorrhiza fungi + cow manure), Plant growth-promoting bacteria (PGPB), Indole-3-acetic acid (IAA), B1 (Pantoea agglomeransz (S72)), B2 (Serratia rubidaea (S28)), B3 (Pseudomonas azotoformans (E101)), B4 (Pseudomonas frederiksbergensis (E56)