Study the bio-fertilizer effects on the quantitative yield and hormonal changes of soybean (Glycine max Merrill) under different irrigation regimes

Introduction Oilseeds after grains are the second most important source of energy for human societies and the meal produced from the industrial process of oil production is also considered to be one of the important items in livestock feed, poultry and aquaculture in terms of protein richness. (Yassari et al., 2014). Soybean (Glycine max) is an annual dicotyledonous, from Poaceae family and one of the most important oily seeds that is widely used in agriculture and industry (Yassari et al., 2009). Abiotic stresses affect different aspects of plant growth, such as reduction and delay in germination, decrease in development rate, decrease in plant organs growth, and decrease in plant life duration and finally decrease in dry matter production. Among abiotic stresses, drought stress is considered to be the most influential type of stress in the production of oil seeds in the world and can greatly reduce production on many arable lands. One of the primary effects of drought is the reduction of water content of plant tissues (Ghanbari et al., 2016). Soil microorganisms are effective in fixing nitrogen in air and dissolving insoluble phosphates, as well as synthesizing growth stimulating hormones such as indole acetic acid, gibberellins, cytokinins, as well as the synthesis of vitamins and amino acids, and increases yields (Viscardi et al., 2016). Considering that most of the country's lands are affected by drought stress and soybeans are susceptible to drought stress, as well as due to the use of biofertilizer as a kind of drought stress resistance strategy and its effect on soybean growth and development, this study was carried out to investigate the effect of Azotobacter chroococcum in combination with Pseudomonas putida on yield and yield components of soybean and its hormonal changes. Material and methods The present experiment was conducted to study the effect of Azotobacter nitrogen fixation bacteria in combination with Pseudomonas putida phosphate solubilizing bacteria on reducing the effects of irrigation water shortage on soybean cultivars under field conditions. This research was carried out as a factorial experiment in a randomized complete block design with three replications in Tarbiat Modares University in 2015. Factorial combinations of four irrigation regimes (15% (control), 30% (mild stress), 45% (moderate stress) and 60% (severe stress) of available soil moisture depletion) and four soybean seed inoculation groups (control or without bacteria, inoculum with Azotobacter, inoculation with Pseudomonas putida, and inoculation with both bacteria) were considered. Drip irrigation (T-tape) was applied The row length in each experimental plot was 6 m, 50 cm apart. The distance between the plots and between the repetitions was 1 and 3.5 m, respectively. Plant to plant distance within each row was 8 cm. The irrigation schedules were based on soil moisture discharge of field capacity at the root zone of soybean with a depth of about 30 cm. Results The results of this study showed that main effects of irrigation regimes and biofertilizer were significant for all traits except grain number per pod. The highest pod number per plant, grain yield, phytohormones such as indole acetic acid, gibberellins, cytokinins and ABA were obtained from combined application of Azotobacter and Pseudomonas putida. The highest pod number per plant, grain number per pod, grain yield, phytohormones such as indole acetic acid, gibberellins and cytokinins were observed in non-stressed control, while the highest ABA was found in severe drought stress. Azotobacter alone, Pseudomonas putida alone, and the combined application of both bacteria increased 25%, 30% and 42% seed yield, respectively, in comparison with control. Conclusion In general, it can be concluded that these microorganisms are able to increase yield and yield components, the phytohormones such as indole acetic acid, gibberellins and cytokinins, and plant resistant under water deficit conditions and decrease severe yield losses.