Two field experiments were conducted during the two successive growing seasons of 2011 and 2012 to evaluate the effect of ten inoculation treatments with mixtures of some plant growth promoting rhizobacteria (PGPR); i.e., Paenibacillus polymyxa (P), Azospirillum brasilense (A) and some isolates of growth promoting rhizobacteria; i.e., Pseudomonas synxantha (P1) and Bacillus subtilis 2 (B2) on Ocimum basilicum plant growth and herb yield, and to assess to what extant inoculation treatments reduce the demand of basil plant to nitrogen chemical fertilization. Inoculated plants were received only 75% of the recommended N fertilizer dose (45 kg N/ fad), while control plants were received 100% of the recommended N fertilizer dose (60 kg N/ fad) without PGPR inoculation.
Results indicated that inoculation basil plants with bacterial inoculants mixture containing (A+P1+B2) or (A+P1) under the effect of 75% N dose resulted in the tallest plants bearing the highest number of branches/ plant and increased herb fresh and dry weights/ plant and/ fad comparing to control plants and all other inoculation treatments. Also, plants treated with these two the abovementioned inoculation treatments resulted in essential oil %, essential oil yield/ plant and/ fad statistically exceed or equal to that produced from control plants.
Respecting essential oil components, 75% N+ (A+P1) treatment increased β-pinene and camphor percentages, while 75% N+ (A+ P1+B2) treatment increased linalool % comparing to their percentages in oil extracted from control plants. In addition, Methyl-chavicol %, which is described as a carcinogen and limits the application of basil oil in therapy, was decreased under the effect of these two treatments.
The caused improvement in plant growth, herb yield and essential oil yield and its properties in plants received 75% N and inoculated with inoculant mixture containing (A+P1) or (A+P1+B2) was associated with increases in percentages of total carbohydrates, N, P and K in their tissues. Also, inoculation with all PGPR under this study significantly reduced dehydrogenase enzyme activity, but significantly increased nitrogenase enzyme activity in the soil rhisosphere of inoculated basil plants.
Conclusively, it could be conclude that inoculation of Ocimum basilicum plant with inoculant bacterial mixture containing equal portions of (Azospirillum brasilense + Pseudomonas synxantha) or (Azospirillum brasilense+ Pseudomonas synxantha + Bacillus subtilis 2) can be compensate the demand of nitrogen mineral fertilization and can improve plant growth, herb yield, essential oil yield and medicinal properties of herb and its essential oil as compare to herb and oil of 100% N fertilized plants without PGPR inoculation.