Effects of plant growth promoting rhizobacteria (PGPR) and humic acid on yield and yield components of basil (Ocimum basilicum L.) under drought stress in Qom region

Drought is one of the most limiting factors on plant production and both the grain yield and yield components of medicinal plants are influenced by it. Today, factors stated as being capable of improving stress resistance are included in the research requirements and organic methods are given the priority. In order to achieve these aims, the effects of plant growth promoting rhizobacteria (PGPRs) and humic acid on the yield and yield components of basil (Ocimum basilicum L.) under drought stress conditions were evaluated.
An experiment was carried out during 2013 in the Northeast of Qom region, as a split factorial based on complete randomized blocks design in 3 replications. Irrigation interval on 3 levels (irrigation after 40, 70 and 100 mm evaporated water from a class A evaporation pan) as the main factor, humic acid at 2 levels (non-application and application) and PGPR using methods on 4 levels (non-application and application by seed inoculation, irrigation water and seed inoculation irrigation water) as factorial were considered as subfactors. Humic acid and PGPR applications in irrigation water were undertaken twice at the 4- and 12-leaf formation stages. The humic acid consumption volume was 3 liters per hectare for each irrigation treatment. The PGPR sources applied were Azotobacter (Azotobacter crococcum) and Azospirillum (Azospirillum brasilense).
The results demonstrated that drought stress had a significant decreasing effect on all experimented traits; however, PGPR and humic acid had a positive effect on evaluated characteristics in both stress and non-stress conditions (irrigation after 40 mm). According to the findings, with the exception PGPR × humic acid, all double and triple interaction effects were significant on seed and biological yield of basil. In this case, the highest seed yield (2336.67 kg ha-1) and biological yield (6867.01 kg ha-1) were observed in the 40 mm irrigation, humic acid and PGPR (seed inoculation irrigation water) treatment. The triple interaction effect of experimented factors was meaningful on 1000 seed weight. In this case, the highest amounts of the aforementioned traits (2.09 g) belonged to the integrated use of PGPR sources and humic acid in the optimal irrigation method (irrigation at 40 mm evaporation) and resulted in a 26.31 percent increase compared to the control treatment. Based on the research findings, drought stress improved the shoot essential oil content of basil; however, application of anti-stress agents by providing most of the plant requirements under both normal irrigation and water deficit stress conditions eventually made and stored higher essence content.
The drought stress created by increasing irrigation intervals, reduced significantly all yield and yield components of basil. Overall, the PGPR and humic acid application decreased adverse effects of water deficit stress, although the role of PGPR was greater than that of humic acid. The effect of experimental factors on the shoot essential oil percentage was in contrast and, under these circumstances, the role of drought stress in the consequential changes in essence content was greater than the total impact of the stimulating factors, namely PGPR and humic acid.