Nanoparticles and plant growth promoting rhizobacteria (PGPR) modulate the physiology of onion plant under salt stress

The effects of rhizobacteria and Ag-nanoparticles were studied on the growth of onion seedlings under induced salt stress. The onion seedlings were treated with 50mM NaCl for 7days. The PGPR strains Bacillus pumilus and Pseudomonas moraviensis were used to inoculate one month old onion seedlings by adding broth culture in rhizosphere soil. One month after transplantation. Ag-nanoparticles (5ppm) were applied to rhizosphere soil for 5 days. Number and weight of leaves, length of shoots, weight of roots, leaves and bulb were measured. 38 days are growth period after transplantation, the treated plants were analyzed for chlorophyll, carotenoids, sugar, protein, proline, flavonoids and phenolics content of onion leaves and bulb. Bacillus pumilus in association with Ag-nanoparticle performed better for growth stimulation of the onion plants. The soil moisture was higher in salt stressed plants but the PGPR inoculated plants and silver nanoparticles alone and also in combinations with PGPR exhibited decrease in the salt induced retention in soil moisture. Silver-nanoparticle combination with PGPR and alone increased the total chlorophyll and carotenoids contents under salt stress. Both Ag-nanoparticle and PGPR exhibited maximum increase in protein content of bulb, decreased the leaf flavonoids but had significant increase in the bulb flavonoid contents. The PGPR being more effective. The Ag-nanoparticle significantly increased the sugar and proline contents. Bacillus pumilus proved to be more effective under unstressed conditions to all growth parameters but Pseudomonas moraviensis effectively coped under salinated conditions. PGPR strains overcame the salt induced inhibition in growth parameters of plans. New proteins appear to be synthesized both by PGPR as well as Ag-nanoparticles to combat adverse effects of salt on plant growth. (author)