The influence of plant growth-promoting rhizobacteria on growth and enzyme activities in wheat and spinach plants

A pot experiment in a greenhouse was conducted in order to investigate the effect of different N-2-fixing, phytohormone-producing, and P-solubilizing bacterial species on wheat and spinach growth and enzyme activities. Growth parameters and the activities of four enzymes, glucose-6-phosphate dehydrogenase (G6PD; EC 1.1.1.49), 6-phosphogluconate dehydrogenase (6PGD; EC 1.1.1.44), glutathione reductase (GR; EC 1.8.1.7), and glutathione S-transferase (GST, EC 2.5.1.18) were determined in the leaves of wheat (Triticum aestivum L., Konya) and spinach (Spinacia oleracea L.), noninoculated and inoculated with nine plant growth-promoting rhizobacteria (PGPR: Bacillus cereus RC18, Bacillus licheniformis RC08, Bacillus megaterium RC07, Bacillus subtilis RC11, Bacillus OSU-142, Bacillus M-13, Pseudomonas putida RC06, Paenibacillus polymyxa RC05 and RC14). Among the strains used in the present study, six PGPR exhibited nitrogenase activity and four were efficient in phosphate solubilization; all bacterial strains were efficient in indole acetic acid (IAA) production and significantly increased growth of wheat and spinach. Inoculation with PGPR increased wheat shoot fresh weight by 16.2%-53.8% and spinach shoot fresh weight by 2.2%-53.4% over control. PGPR inoculation gave leaf area increases by 6.0%-47.0% in wheat and 5.3%-49.3% in spinach. Inoculation increased plant height by 2.2%-24.6% and 1.9%-36.8% in wheat and spinach, respectively. A close relationship between plant growth and enzyme activities such as G6PD, 6PGD, GR, and GST was demonstrated. Plant-growth response was variable and dependent on the inoculant strain, enzyme activity, plant species, and growth parameter evaluated. In particular, the N-2-fixing bacterial strains RC05, RC06, RC14, and OSU-142 and the P-solubilizing strains RC07 and RC08 have great potential in being formulated and used as biofertilizers.