**13. Interaction effect of** *Rhizobium* **and PGPR on growth of French bean**

An application of PGPR together with *Rhizobium* improved the growth and seed production of beans [62]. Plant growth parameters, physiological parameters and quality parameters of French bean were influenced due to consortium comprising of efficient rhizobial and PGPR isolates [69]. The plant growth parameters were significantly improved due to inoculation of rhizobial isolates and PGPR isolates could be attributed to plant growth promoting abilities of PGPR isolates and; nitrogen fixing and PGP traits of rhizobial isolates. Selected rhizobial isolates RD20–3 (R1) and RK3–1 (R2) being GA producer and ammonia producer, stimulate plant growth through plant growth hormone production and protecting plants against phytopathogens. Both isolates were also capable of utilizing highest number of carbohydrates which depicts their ability to proliferate in diverse soils having varying carbon sources. Selected PGPR isolates NAG-K3 (P1) and CRC-J2 (P2) being IAA, GA producer and P solubilizer; and IAA, GA producer, Zn solubilizer and HCN producer, respectively improved plant growth through beneficial effects on seed germination, differentiation, proliferations and division of root cells and through biocontrol activity against harmful fungus. Phosphorus solubilizing capability of isolates increased the availability of phosphorus and through Zn solubilizing ability improves Zn nutrition of plants. Higher root growth and phosphorus availability renders the plant to uptake more amount of other nutrients also from soil and also increase the nitrogen fixing potential of rhizobial isolates which makes more amount of atmospheric nitrogen available to the plants to increase their vegetative growth, chlorophyll content, no. of trifoliate leaves, leaf area, root acid phosphatase activity and nitrogen and phosphorus status in plants. Higher root proliferation due to more differentiation and division of root cells increases the surface area which resulted higher water and nutrient absorption from soil. Higher uptake of nitrogen and phosphorus improved the starch synthesis and carbohydrate portioning in plant, resulted highest root and shoot length; dry biomass of root and shoot; and shoot: root ratio; when rhizobial (R1) and PGPR (P2) isolates applied conjointly at 100 kg N ha−1 application. N being the chief constituent of amino acids and protein; and P as constituent of phospholipid, both are the building blocks of protoplasm which forms the body structure of plant. Thus N and P improves the body structure and increase number of trifoliate leaves and leaf area of plants and various physiological parameters vis-à-vis dry biomass accumulation in plants. There are two types of phosphatases on the basis of pH as acid and alkaline phosphatase, because maximum activity occurs at low pH (6.5) and high range of pH (11.0), respectively. Acid level phosphatases secretion can vary with crop sp. [106]. Thus there is differential interspecific genetic variation in root enzyme secretion and acid phosphatase activity [107]. Co-inoculation of rhizobial isolates and PGPR isolates significantly improved root acid phosphatase activity in plants converting organic phosphorus compounds to inorganic-P. Concurrent exudation of organic acids and phosphatase by phosphate solubilizing microorganisms could enhance P solubility, by releasing bound organic phosphates and its mineralization by escalating the rate of hydrolytic cleavage [108].

As the main constituent of chlorophyll and an element of porphyrin ring, N content supplied by symbiotic nitrogen fixation and external application significantly improved the synthesis of chlorophyll 'a', chlorophyll 'b', chlorophyll 'a':'b' ratio in plant, when rhizobial and PGPR isolates applied in combination. Chlorophyll 'a' is always greater than chlorophyll 'b' and chlorophyll 'a' plays an important role in

#### *Characterization of* Rhizobium *and Plant Growth Promoting Rhizobacteria from French Bean… DOI: http://dx.doi.org/10.5772/intechopen.100592*

photosynthesis through absorbing light energy and converting it into chemical energy while chlorophyll 'b' as an accessory pigment absorb more light energy and transfers it to chlorophyll 'a' for photosynthesis. The chlorophyll 'a': chlorophyll 'b' ratio could be a useful indicator of N partioning within a leaf because this ratio is positively correlated with the ratio of PSII cores to light harvesting chlorophyll-protein complex.

Nitrogen and phosphorus uptake was significantly improved in plants could be attributed to higher N availability through symbiotic nitrogen fixation and high P availability through root acid phosphatase activity and phosphorus solubilizing ability of PGPR isolates. Significantly higher amount of fixed nitrogen in shoots, roots and grains of plant might be attributed to more nodulation and higher root growth due to phosphorus and nitrogen supplementation. The higher amount of nitrogen fixed in French bean due to combined inoculation of rhizobial isolate RD20–3 and PGPR isolate NAG-K3 over solitary inoculation of individual isolate is attributed to the higher phosphorus availability made due to P-solubilization and root acid phosphatase activity. The enhanced P-availability facilitates the more ATP synthesis which is required as a source of energy for carrying out the N2-fixation by an enzyme nitrogenase. Number of pods plant−1, pod yield, grains pod−1 and grain yield was significantly increased due to inoculation of French bean with rhizobial isolates and PGPR conjointly at 100 kg N ha−1 might be attributed to higher content of nitrogen and phosphorus in plant body which help in pod formation and grain formation in plants. Nitrogen as a chief constituent of protein and phosphorus also helps in protein synthesis, resulted highest protein content and protein yield plants receiving rhizobial and PGPR isolates both, at 100 kg N ha−1 application. Co-inoculation of rhizobial isolate RK3–1 (R2) and PGPR isolate CRC-J2 (P2) significantly reduced all plant growth parameters, physiological parameters, quality parameters, grain yield and nutrient uptake in plants compared to application of RK3–1 (R2) and CRC-J2 (P2), alone [69] which depicts the non-synergistic interaction between these rhizobial and PGPR isolates. Thus these two isolates RK3–1 (R2) and CRC-J2 (P2) may not be used as consortium to improve plant growth.

Co-inoculation of *Rhizobium* and PGPR showed a better nodulation which resulted higher shoot dry biomass and seed yield production. Beneficial effects of PGPR on symbiotic efficiency of rhizobia nodulating legume crops have also been reported by various scientists [64, 109]. According to Samavat et al. [110] the significant correlation has been observed between nitrogen absorption and improvement in growth of plant roots and shoots, as a result of *Rhizobium* and *Pseudomonas* interaction. Lucas Guarcia et al. [111] reported that the co-application of rhizobacteria and *Rhizobium* might have effects on their symbiotic relation with the host legume, depending on the applied isolates. Co-application of *Rhizobium* and *Pseudomonas* improved nodulation, leaf chlorophyll content and other growth factors under greenhouse conditions [110]. The increased chlorophyll content in plant leaves as the result of bacterial isolates co-inoculation could be due to the increased plant nutrition and photosynthesis [112]. An application of PGPR together with *Rhizobium* improved the growth and seed production of beans [62]. Mishra et al. [113] reported that combined application of *Rhizobium* + PSB + PGPR improved plant height and number of pods significantly. An over view of the combined dual inoculation of *A. chlorophenolicus* and *Enterobacter;* and triple inoculation of strain *B. megaterium*, *A. chlorophenolicus* and *Enterobacter* gave significantly higher plant height.

#### **14. Conclusion**

This study revealed the presence of efficient multi-trait rhizobial and PGPR isolates in French bean rhizosphere. Those rhizobial isolates which nodulate the plant

under controlled conditions may be authenticated as *Rhizobium* and, having plant growth promoting traits may increase plant height, nodule number, dry biomass, chlorophyll content, grain yield, nutrient uptake and protein yield after inoculation. PGPR isolates from French bean rhizosphere possessed the efficient pant growth promoting traits *viz;* IAA production, GA production, P solubulization, Zn Solubilization, HCN production, ammonia production and siderophore production may be considered effective for improving plant growth, physiological and quality parameters after inoculation in plants. Inoculation of *Rhizobium* with higher levels of N application may have a positive influence on plant height, dry biomass production, leaf area content, chlorophyll content, grain yield, protein content and water soluble carbohydrate content whereas negative effect on nodule number and dry biomass of nodule. Co-inoculation of *Rhizobium* and PGPR improved the plant growth, physiological and quality attributes of French bean. Microbial consortium comprised of *Rhizobium* and PGPR may not only enhance growth and yield of French bean but may also reduce inorganic fertilizer application.
