Effect of Plant Growth-Promoting Rhizobacteria on Yield, Yield Components and Qualitative Traits of Soybean Cultivars (Glycine max) in Different Irrigation Regimes

In recent decades, sustainable agricultural management, protection of soil living organism’s community and the efforts to use biological solutions for plant nutrition and society health has been considered. Among soil microorganisms, plant growth-promoting rhizobacteria (PGPR) are the most promising, including all bacteria inhabiting the rhizosphere and the rhizoplane, which able to stimulate plant growth and yield. The modes of action of PGPR are clearly diverse and not all bacteria possess the same mechanisms. These mechanisms vary from changes in hormonal content, production of volatile compounds, increasing of nutrient availability and enhance abiotic stress tolerance such as the water deficit stress. Therefore, the objective of the present study was to evaluate the effects of the PGPR on the yield and yield components of soybean under different irrigation regimes.

The field experiment was conducted during 2016 at the research farm of Razi University, Kermanshah, Iran (34°, 19´ N, 47°, 50´ E and altitude 1320 m). A split plot factorial experiment was conducted based on Randomized Complete Block Design (RCBD). Main plots had three irrigation regimes in which irrigation was cut based on the soybean stages (I1: water deficit stress from mid pod development stage to maturity stage; I2: water deficit stress from seed filling development stage to maturity stage; and I3: optimum irrigation in all development stages) and sub-plots were composed of plant growth-promoting rhizobacteria (PGPR) (B1: no bacteria; B2: Bacillus subtilis; and B: Bacillus licheniformis) and soybean cultivar (TMS, M9 and Kosar). The experimental plots were irrigated based on furrow method. I3 treatment were irrigated every 7 days until the end of the growing period while in the I1 and I2 treatments, the plots were irrigated every 7 days until the start of the water deficit stress. In order to inoculate with the PGPR, the soybean seeds were plunged in a 1:10 (V:V) solution of liquid culture and distilled water respectively, for 10 minutes. All seeds were oven-dried at 30° C for 5 h. Finally, the seeds by PGPR were inoculated by Bradyrhizobium japonicum before sowing and cultivated immediately at 4 to 5 cm soil depth. About 1.5 m2 harvested at the physiological maturity stage. The evaluated traits were the total dry weight, seed yield, 1000 seed weight, pod per plant, seed per pod, oil percentage and protein percentage.

The results indicated that the water deficit stress reduced the total dry weight, seed yield, 1000-seed weight, pod per plant, seed per pod, oil percentage and protein percentage of soybean. The PGPR application improved all measured traits of soybean in all irrigation regime treatments. The highest seed yield (380.9 g m-2) and total dry weight (1082.8 g m-2) were observed in the optimum irrigation and B. subtilis treatment for TMS cultivar and the highest 1000-seed weight (136.2 g) and protein percentage (33.2) also were related to the optimum irrigation and B. licheniformis treatment for TMS cultivar. The lowest total dry weight (828.1 g m-2), seed yield (134.2 g m-2), 1000-seed weight (84.8 g) and protein percentage (21.4) were related to the water deficit stress from mid pod development stage to maturity stage and no bacteria treatment for Kosar cultivar. The results also indicated that the effects of interaction between irrigation regimes, application of PGPR and cultivars on evaluated traits were not significant. 

It seems that the PGPR via increasing of the root system and more uptake of water in the rhizosphere could promote the soybean yield and yield components. Nevertheless, as the results showed, the more effects of the PGPR were observed in the I2 treatment compared to other treatments. The PGPR actually could promote the yield and yield components of soybean in the mid water deficit stress.