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Evaluation of Bacillus spp. as plant growth-promoting rhizobacteria (PGPR) in broccoli (Brassica oleracea var. italica) and lettuce (Lactuca sativa)

Abstract:

Lettuce and broccoli are valuable agricultural products in Ecuador whose cultivation demands considerable quantities of mineral nutrients, usually obtained from chemical fertilizers. As the use of plant growth-promoting rhizobacteria (PGPR) has shown to be a biological alternative for cropping these vegetable species, several strains of Bacillus spp. were evaluated through laboratory and field tests. Biological nitrogen fixation was performed by a qualitative assessment in a free nitrogen culture medium; the indoleacetic acid production was carried out in a Tryptic Soy Broth (TSB) medium by quantifying its concentration using Salkowski’s reagent; phosphate solubilization was established on an NBRIP medium and the transformed phosphorus concentration was quantified with the Mo-Blue reagent. The data obtained showed a significant difference between the treatments (p < 0.01) where B. megaterium and B. licheniformis showed a higher ability to fix nitrogen, produce auxins, and solubilize phosphate. Field tests achieved, on the one hand, an increase in height of 26.7 % and 13.7 % in lettuce and broccoli, respectively, with a weekly application of B. licheniformis. On the other hand, a weekly application of B. megaterium significantly increased the dry matter content, length, and root weight in lettuce as well as in broccoli. All comparisons were made against a control. These results show that the strains identified in this research could be considered as growth-promoting microorganisms and are a biological alternative to chemical fertilizers.