Effect of foliar and soil application of plant growth promoting bacteria on kale production and quality characteristics

•Fresh and dry weight were increased by bacillus subtilis with soil application.•Photosynthetic rate was increased by bacillus subtilis with foliar application.•P. megaterium and B. subtilis led to kales of increased greenness and yellowness.•Application techniques did not affect firmness and total phenolic compounds. Novel production strategies have been focusing more and more towards organic, sustainable, and environmentally friendly cultivation systems. Kale is becoming popular among consumers that are increasing constantly their demand; thus, kale cultivations are gaining a significant portion among other leafy crops. Plant growth promoting bacteria (PGPB) are increasingly applied in different cultivation species and cultivation systems, in order to increase plant productivity and improve the quality of the harvested product. A pot experiment was conducted in a greenhouse following a completely randomized design (CRD) with ten replications and two main factors. The first factor was the application method of PGPB (foliar or soil) and the second the use of PGPB (4 treatments and control). The aim of this study was to explore the influence of PGPB (Azotobacter chroococcum, Bacillus subtilis, Priestia megaterium and the mix Azotobacter chroococcum with Bacillus subtilis), as well as of the application method on plant growth, physiology, fresh weight, dry weight and quality of kale. According to the results, nearly all PGPB that were used, had positive effects on the fresh and dry weight, photosynthetic rate and plant height of the plants. Bacillus subtilis with soil application increased the fresh weight up to 33%, the dry weight up to 42% and the plant height up to 14.6%. Moreover, the photosynthetic rate of kale plants was increased by Bacillus subtilis with foliar application up to 89%. The use of P. megaterium and B. subtilis increased greenness (lower a-values) and yellowness (higher b-values) of kale leaves compared to all the other treatments.