Co-application of tryptophan amino acid and bacillus simplex as a plant growth promoting bacteria on bread wheat yield in greenhouse condition

L-tryptophan is a precursor of auxin synthesis by PGPRs. In soil, this precursor derived from hydrolysis of soil organic matter protein. Lack of sufficient soil organic matter especially in rained condition could be limiting factor in this topic. For this purpose, an experiment was designed to evaluate the effect of tryptophan in the presence and absence of growth promoting bacteria on wheat growth. The results showed that in LB and root exudate mediums auxin was produced respectively 41.1 and 20.9 mg.L-1 by B. simplex 16 just when tryptophan added at 5 mM concentration. Bacterial inoculation of wheat seeds, addition of 100 mg.L-1 tryptophan and co-application of tryptophan and bacterial inoculation treatments respectively increased 9%, decreased 16% and decreased 20% the dry matter yield of wheat shoots compared to the control. Due to the use of tryptophan in both bacterial and non-bacterial inoculation conditions, root tissue density decreased 60% but root volume increased 70%. Microscopic studies and whole root images showed that both bacteria and tryptophan treatments caused morphological changes, especially in the hair roots section. According to the results of this study, the use of tryptophan at a concentration of 100 mg.L-1 is not recommended to increase wheat growth due to the potential for auxin stress and developmental disorders.