Evaluating role of plant growth promoting rhizobacteria (PGPR) to increase flower production, flower periodicity and essential oil contents in Rosa damascena Mill.

Abstract

The role of plant growth promoting rhizobacteria for improving yield and essential oil contents in Rosa damascena Mill. was evaluated in this study. Plant growth promoting rhizobacteria (PGPR) were isolated from the rhizosphere of Rosa damascena in first experiment, fifteen distinctive and fast growing rhizobacterial colonies (strains) were chosen to inoculate the Rosa cuttings by dipping in them. Results of the pot experiments revealed that most of the isolates showed growth promoting activities in Rosa cuttings except the isolates B5 which was proved to be growth limiting strain as it gave 20-40% less result for all parameters. All the other strains enhanced cutting growth characters up to 30- 60% compared to control. Strains LSI19, F6 and LC4 were proved to be superior strains in all growth attributes. Therefore, these three strains (LSI19, F6 and LC4) were selected for further studies. Three separate field experiments were laid out in factorial arrangement under randomized complete block design (RCBD) with four replications. Total eight inoculum combinations were made from three PGPR strains (alone and in different combination) for each experiment. Second experiment comprised of root application of eight combinations of PGPR treatments in the form of liquid inoculum along with zero, half of recommended and recommended dose of nitrogen and phosphorus. Maximum plant growth and production was observed in all PGPR treatments at NP @ 60 and 20 g/m2 only the stem diameter was found maximum when PGPR treatments were applied along with NP @ 30 and 10 g/m2. Combination of PGPR strains perform better than individual strains when applied at all three levels of nitrogen and phosphorus. Results regarding essential oil contents showed that PGPR combination of F6, LC4 and LSI19 at NP fertilization @ 30 and 10 g/m2 provided the highest concrete and absolute oil percentage but increasing levels of NP ultimately reduced oil contents. Third experiment comprised of foliar application of similar PGPR combinations, in which PGPR treatments were applied at three different growth stages (on new shoots after pruning, after flower bud formation and at flower burst). Results exhibited that plant growth and flower yield attained maximum values in triple inoculum of F6, LC4 and LSI19. Application of PGPR strains on new shoots after pruning (GS1) showed much better results regarding reproductive growth and flower yield than other growth stages while vegetative growth characters were enhanced by application of PGPR strains after flower bud formation (GS2). Last experiment was designed to assess the PGPR combinations by two methods of application (root and foliar). The results showed that plant growth, biomass of flowers, N contents in shoots and P contents in leaves and shoots were significantly improved by root application of PGPR strains while flower yield and N contents in leaves were improved with foliar application of PGPR strains. PGPR inoculation by different methods (root and foliar) significantly affected oil yield and chemical composition of essential oil. Overall results for all studies reflected that yield of Rosa damascena was enhanced with the application of PGPR inocula either single or in combination. However, co-inoculation performed better than individual inoculation.