Yield Enhancement by Application of Plant Growth Promoting Rhizobacterial (PGPR) Inoculants in Dryland Wheat (Triticum aestivum L.)

The explosion of world population in recent decades has caused excessive application of chemical fertilizers in agricultural systems; resulting in critical environmental and health issues. Integrated Nutrient Management (INM) method or biofertilizers are considered as logical strategies to reduce the rate of chemical fertilizers. . Biofertilizers consist of various types of microorganisms in soil which are in close relation with plant roots and are called Plant Growth Promoting Rhizobacteria (PGPR). Several mechanisms have been postulated to explain how PGPR benefit the host plant, which could be classified into four categories a) The ability to produce plant growth regulators or phytohormones such as indole acetic acid (IAA), cytokines, and gibberellins; b) Enhancing a symbiotic N2 fixation; c) Solubilizing inorganic phosphate and mineralization of organic phosphate and/or other nutrients; d) Antagonistic effect against phytopathogenic microorganisms by production of siderophores, the synthesis of antibiotics, enzymes, and/or fungicidal compounds, and competition with detrimental microorganisms. Therefore, PGPR are commonly used as inoculants for improving the growth and yield of agricultural crops, however screening for the selection of effective PGPR strains is very critical. This study focuses on the screening of effective PGPR strains on the basis of their potential for plant growth promoting activity under water stress conditions in Iran's cold temperate and warm dryland areas.

Experiments were carried out in randomized complete block design (RCBD) with four replications in 2009-2010 cropping year. Treatment included fifteen PGPRs with a control (without inoculation) in six dryland agricultural research stations; including Maragheh, Ghamloo (Kurdistan), Sararood (Kermanshah), Gachsaran, Khodabandeh (Zanjan), Shirvan Chardavol (Ilam). Soil samples collected from 0-25 cm depths before planting time and was characterized through determination of soil available P, K, Fe, Mn, Zn and Cu, soil texture, organic carbon, pH, EC and calcium carbonates equivalent. Dryland wheat Azar2 cultivar was cultivated with 350 seed per m2 in 5 to 7 cm soil depth. Moreover,plant traits such as grain, straw and biological yields, TKW (1000-kernel weight), number of spikes per m2, number of seed per spike, harvest index (HI), plant height and spike length, were measured. All research data was analyzed via GenStat14 statistical software.

The results showed that, the plant growth promoting bacteria (PGPR) had increased the grain yield of dryland wheat in the Maragheh, Kermanshah, Kurdistan, Ilam, Zanjan and Gachsaran regions. This increase in most experimental dryland stations has been significant statistically. The maximum grain yield increase in Maragheh, Kermanshah, Kurdistan, Ilam, Zanjan and Gachsaran were in the order amount of 19.6 (treatment no. 2), 45.1 (treatments no's 4 and 12), 12.4 (treatment no 8), 18.4 (treatment no 11) 10.2 (treatment no. 11) and 11.6 (treatment no. 11) kg.ha-1, which was 382 kg.ha-1 for the best average plant growth promoting bacteria in all the study regions. With the use of GGE biplot method, the inoculating treatments of plant growth promoting bacteria had 2 groups, including the high yielding groups with treatments (2, 3, 6, 8, 9 and 10), and low yielding treatments (1, 4, 5, 7, 11, 12. 13, 14, 15 and 16).The study locations were separated in 3 groups, first group including; Gachsaran and Kurdistan regions, second group including; Maragheh, zanjan and Ilam regions, and the third group including; Kermanshah region. The suitable treatments were the treatment numbers 9 and 10 for the first group, treatment numbers 2, 3, 8 and 6 for the second group and treatment numbers 7, 4 and 5 for the third group. Among the mentioned treatments, the most suitable treatments for first, second and third groups are treatment number 9, treatment number 2 and number 5, respectively.. According to these results, if the first aim is introducing the growth promoting bacteria for all the regions, this strain is the inoculating bacteria number 9. In the second order, the treatment numbers 13 and 10had the closest conditions to the estimated ideal treatment.

Therefore, for the plant growth promoting bacteria, we can use them in reducing the effects of environmental stresses, and conducting non-environmental stresses on dryland conditions, as well as increasing in grain yield of dryland wheat.