This study aimed to investigate if pretreatment seed with H2O2 affects promoting effect of Azospirillum on plant. The experiment was a factorial combination of two main factors, Azospirillum and hydrogen peroxide (0, 25, 50, and 80%), carried out under field conditions. Soaking seed in various concentrations of H2O2 solution resulted in enhanced stomata density and increased length and histological components of leaf. These changes in histological components were found to be positive since plants of H2O2-soaked seeds (H2O2 pretreatment) had higher fresh and dry weights, seed number per spike, and higher grain yield. Azospiriilum enhanced number of grain spikes-1 and total grain yield by 3% and 7%, respectively. Soaking seeds in 25% H2O2 solution enhanced the promoting effect of Azospirillum on increased vessels area and diameter. Soaking seeds in 50% H2O2 solution resulted in enhanced stimulating effect of Azospirillum on enhanced areas of vascular bundle and upper epidermis. The promoting effect of Azospirillum on the area of lower epidermis was largest when seed was pretreated with 80% H2O2. All three concentrations of H2O2 used had the same effect on Azospirillum–inoculated plant in terms of leaf area and chlorophyll a. Soaking seed in 80% H2O2 solution and then inoculated with Azospirillum resulted in the highest grain yields.
