**4. Classification of bio-inoculants for tree crops**

*Sustainable Crop Production*

health by controlling diseases.

**2. Rhizosphere and microbial interaction**

nodule of legume and actinomycete plants [4].

scanty in developing countries like India.

**3. Bio-inoculants**

wildlife. They also reduce the atmospheric temperature and the impact of green-

Various types of microorganisms inhabit air, water and soil. They play an important role in restoring the physical, chemical and biological property of soil. Rhizosphere ecology and microbial interactions are responsible for key environmental processes, such as the bio-geo chemical cycling of nutrients, organic matter and maintenance of plant health and soil quality [1]. Among the microbial population, both beneficial and harmful bacteria as well as fungal species were found, but

Rhizosphere is the physical location in soil where plants and microorganisms interact. The interest in the rhizosphere microbiology derives from the ability of the soil microbiota to influence plant growth and vice versa. The presence of microorganisms in the rhizosphere will increase root exudation and it was found that 5–10% of the fixed carbon was exudates from the root under sterile condition, on the introduction of beneficial microorganisms, root exudation rate increases by 12–18%. The interaction between bacteria and fungi associated with plant roots may be beneficial, harmful or sometimes neutral for the plant, and effect of a particular bacterial species may very as a consequence of soil environmental conditions [3]. The beneficial microbes can be divided into two major types based on the living nature; free living (that live in soil) and symbiotic relationship with the plant root

Bio-inoculants are beneficial microorganisms for nutrient management, plant growth and are eco-friendly and natural inputs providing alternate source of plant nutrients, thus increasing farm income by providing extra yields and reducing input cost also. Bio-inoculants increase crop yield by 20–30%, replace chemical N and P by 25%, stimulate plant growth, enhance soil biodiversity, restore natural fertility and provide protection against drought and some soil borne plant pathogens. The role of bio-inoculants has already been proved extensively in enhancing the mineralization processes of organic matter and helping the release of nutrients, utility of soil organic matter contents and cations exchange capacity [5] and therefore, bio-inoculants are gaining importance in agriculture for the past few decades. However, the scientific exploitation of bio-inoculants in horticulture and forestry is

the microbial population was low when compared to rhizosphere soil [2].

Plant growth and productivity is generally regulated by the availability of soil nutrients. One of the major efforts to increase the plant productivity is through management of nutrients, which can be achieved by application of fertilizers. However, application of chemical fertilizers is not eco-friendly and economically viable in current scenario. Other alternative method is supplement of bioinoculants (bio-fertilizers) for sustainable development of horticulture and forestry crops. Bio-inoculants are plant growth promoting beneficial microorganisms such as the species of *Azospirillum, Azotobacter, Bacillus, Ecto and Endo-mycorrhizal* fungi*, Frankia, Pseudomonas, Rhizobium*, *Trichoderma,* etc. Such microorganisms accelerate certain microbial process to augment the extent of availability of nutrients in the form, which can be assimilated by plants and also maintain the plant

house gases by maintaining low levels of carbon dioxide.

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#### **5. Plant growth promoting rhizobacteria (PGPR)**

Plant growth promoting rhizobacteria are group of bacteria that actively colonize roots and increase plant growth and yield [6]. It enhances plant growth and productivity by synthesizing phytohormones, increasing the availability and facilitating the uptake of nutrients by decreasing heavy metal toxicity in the plants, antagonizing the plant pathogens [7]. The mechanisms by which PGPR promote growth are not fully understood [8], against phytopathogenic microorganisms by production of siderophores, the synthesis of antibiotics, enzymes and fungicidal compounds [9] and also solubilization of mineral phosphates and other nutrients [10].

#### **5.1 Azospirillum**

*Azospirillum* species are free-living N2-fixing bacteria commonly found in soils and in association with roots of agriculture, horticulture and forestry species [11]. *Azospirillum* are known to act as plant growth promoting rhizobacteria (PGPR) and stimulate plant growth directly either by synthesizing phytohormones or by promoting improved N nutrition through biological nitrogen fixation (BNF). PGPR also produce several the growth promoting substances including IAA, GA3, Zeatin and ABA [12]. Presently there are seven species have been identified in this genus, *A. amazonense* [13]*, A. brasilense, A. lipoferum,* [14]*, A. doebereinerae* [15]*, A. halopraeferens* [16]*, A. irakense* [17] *and A. largimobile* [18]*.*

Applications of plants with Azospirillum have promoted plant growth of agronomically important field crops by 10–30% in the field experiment [19, 20]. Nursery experiments proved that the inoculation of tree cops with *Azospirillum* could result in significant changes in various growth parameters, particularly shoot and root growth, biomass, nutrient uptake, tissue nitrogen content, leaf size of several shola tree species [21] and *Casuarina equisetifolia* [22, 23]*, C. cunninghamiana* Mig. [24]*,* 

*Moringa oleifera* [25]*, Acacia nilotica* [26]*, Azadirachta indica* [37]*, Delonix regia* [28]*, Erythrina indica* [29]*, Feronia elephantum* [30]*, Jatropha curcas* [31]. Two years old *Casuarina equisetifolia* plants treated with bio-inoculants in field condition improve the growth of plants by 90% over uninoculated control [32]. *Azospirillum lipoferum* treated with *Jatropha curcas* under field conditions has increased the shoot length by 44.85% and primary and secondary root length by 39.3 and 37.5% respectively. Similarly, the root and shoot biomass also increased by 24.01 and 15.04% leaf area by 28.57% increase over control and the other *Azospirillum* species such as *A. brasilense*, *A. haloference* and *A. amazonense* [33]. The stimulatory effect exerted by *Azospirillum* has been attributed to several mechanisms including secretion of phytohormones (auxins and gibberellins), biological nitrogen fixation, and enhancement of mineral uptake of plants [8] due to the ability of synthesis of in vitro phyto-hormones such as IAA, gibberellins, cytokinin [34, 35] and produced by ethylene [36].

#### **5.2 Effect of bio-inoculants and biochemical changes of tree crops**

Plants inoculated with *A. brasilense* were always characterized by a higher chlorophyll concentration. Inoculation of crops caused a statistically significant increase of chlorophyll content in the case of oats in 1996 (15%) and wheat in 1997 (15%). Chlorophyll appeared to be a sensitive indicator of inoculation effect, which was also supported by Bashan et al. [37]. *A. lipoferum* inoculated *Jatropha curcas* seedlings has increase in level of chlorophyll a, b and carotene and such increase was maximum by 31.98, 14.5 and 18.9% and protein content (37.35%) amino acid (26.33), lipids (8.9) and carbohydrates (9.37) when compare to control plant under field conditions [31]. The total chlorophyll and soluble protein content was found to be higher in the *Moringa oleifera* seedlings inoculated with *A. brasilense* [25].

#### **5.3 Pseudomonas**

The genus *Pseudomonas* is one of the most diverse gram-negative non-spore forming, motile, rod shaped bacteria with an important metabolic versatility and pathogenicity [38]. Morphologically this genus is straight or slightly curved rods and produced yellowish green pigment in King's B. Medium. Plant growth promoting rhizobacteria consisting of primarily *Pseudomonas fluorescens* and *P. putida* were identified as important organisms with ability for plant growth promotion and effective disease management properties. The population density of fluorescent pseudomonas in the rhizosphere in usually reduced by AM fungi colonization [34, 39, 40]. Many strains of genus *Pseudomonas* possess the capability to promote plant growth [41], due to their 1-aminocyclopropane-I carboxylate deaminase activity, indole acetic acid (IAA) and siderophore production [42], PGPR can exert a beneficial effects on plant growth by suppressing soil borne pathogens [43], improving mineral nutrition [44] and phytohormone synthesis [45].
