Evaluation of the effect of growth-promoting rhizobacteria isolated from soils on the ionic contents of drought-stressed tomato (Lycopersicon esculentum Mill.)

Applying biological agents for increasing plant growth and their resistance to different tensions is of particular importance. In the present study, an attempt was made to isolate and identify plant growth-promoting bacteria from soil samples and to evaluate their ability to help the tomato plant cope with drought stress. Soil samples were collected from wheat and barley farms around Karaj and after isolation and initial purification of phosphate solubilizing, nitrogen fixing, and siderophore producing strains, they were isolated using PVK media culture, nifH gene PCR, and CAS-agar media culture, respectively. The identified isolates were then analyzed in terms of their drought tolerance in an environment containing polyethylene glycol. Finally, tomato seedlings were inoculated with bacterial isolates (belonging to the Bacillus, Brevibacterium, and Actinomycetes genera) before they were grown under drought stress at three osmotic pressure levels of 0, -0.4, and -0.8 MPa. Then, sodium and potassium ion contents of leaves, roots, and stems were measured using a flame photometer while calcium contents of the same organs were assayed through atomic absorption measurement. Results showed that all five isolates capable of dissolving phosphate, nitrogen fixation, and siderophore production were able to withstand drought up to -0.8 MPa osmotic pressure. All these isolates significantly affected drought stress and the intensity of ions reduction in leaves, roots, and stems compared with the control plants. The results of the present study showed that inoculation of the bacterial isolates considering their ability in siderophore production and nitrogen fixation may help the plant to resist drought stress.