Use of plant growth promoting bacteria as an efficient biotechnological tool to enhance the biomass and secondary metabolites production of the industrial crop Pelargonium graveolens L'Her. under semi-controlled conditions

Pelargonium graveolens L'Her. well known as rose-scented geranium is used worldwide in perfume, cosmetic, pharmaceutical and food industries. The essential oil and phenolic concentrations for this species were reported to be very low which requires their improvement. The effect of three plant growth promoting bacteria namely Pseudomonas rhizophila 5211, Halomonas desertis G11 and Oceanobacillus iheyensis E9 on P. graveolens transplants were evaluated with single, dual and consortium modalities under semi controlled conditions. Significant positive effects on the growth related parameters, leaf and root biomass, photosynthetic pigments and secondary metabolites production were observed with inoculum-specific response. The highest root biomass was recorded with the dual inoculation [H. desertis G11 + O. iheyensis E9] while the inoculum [P. rhizophila S211 + O. iheyensis E9] showed the highest ability to enhance fresh leaf biomass (12.27 +/- 0.61 g) leading to an improvement rate of 43.34 % over the control. A significant increase in the amounts of total phenolics and flavonoids contents in the inoculated plants over the control was revealed for all the treatments. The consortium was the most efficient to enhance the contents of total phenolics (105.62 +/- 2.95 mg GAE/g DW) and flavonoids (48.55 +/- 1.25 mg RE/g DW) resulting in an increase over the control of 67.35 % and 53.59 %, respectively. The essential oil synthesis was enhanced significantly for all the inoculated transplants compared with the control conditions. The dual inoculations resulted in the highest levels of essential oil contents in P. graveolens leaves, followed by the consortium than individual inoculants. The dual inoculation [H. desertis G11 + O. iheyensis E9] gave the highest essential oil amount (0.49 +/- 0.03 %) with an increase of 68.96 % over the control (0.29 +/- 0.01 %). The GC-MS analyses of the essential oils resulted in the identification of 23 compounds. In all treatments, citronellol (25.84-40.02 %), geraniol (9.67-11.96 %), citronellyl formate (5.35-10.14 %), 2-phenylethyl tiglate (4.58-7.13 %), epi-gamma-eudesmol (4.38-5.87 %) and linalool (2.56-4.07 %) are the main compounds with qualitative and quantitative variation. Interestingly, the three dual inoculations gave the highest amounts of citronellol, citronellyl formate and citronellyl tiglate compounds. The highest concentration of the high-value citronellol (40.02 %) which defines the commercial value of P. graveolens was recorded with the inoculum [H. desertis G11 + O. iheyensis E9] resulting in an increase rate of 54.87 % over the control (25.84 %). This study suggests the efficiency of the studied inoculums as promising biofertilizers with significant potentials to enhance P. graveolens biomass and secondary metabolites production which present an alternative biotechnological tool to limit the excessive use of chemical fertilizers.