Introduction:

In India, chemical fertilizers are extensively used for increasing the agricultural productivity but they are imposing harmful effect on the environment and also they are expensive. Nowadays biofertilizers are creating growing level of interest since these are ecofriendly and are helping in sustainable agriculture (Roychowdhury et al., 2014).

Phosphatic biofertilizer is one of the important groups required for the solubilization of fixed phosphorus in the soil. Phosphorous is a major essential plant nutrient that plays an important role in plant growth and development. Most of the soils are deficient in phosphorus containing only 0.05% of total phosphorus of which only 0.1% is available to plants (Achal et al., 2007). Large amount of phosphatic fertilizers are added in regular basis for maintaining the phosphorus deficiency but majority of this fertilizers get fixed in the form of calcium salt in calcareous soils and iron and aluminum salts in acidic soils and remain unavailable to the plants (Gyaneshwar et al., 2002). Phosphate-solubilizing microorganisms (PSMs) have the capacity to solubilize the fixed phosphorus into plant accessible form in environment-friendly manner (Gyaneshwar et al., 2002). These microorganisms are isolated from different soil types in India (Vikram et al., 2007) and are concentrated more in the rhizosphere where they are metabolically more active (Vazquez et al., 2000). Phosphate solubilizing microorganisms include bacteria (Oves et al., 2013), fungi (Khan et al., 2010) and actinomycetes (Hamdali et al., 2012). Phosphate solubilizing bacteria constitute 1-50% and phosphate solubilizing fungi 0.1 - 0.5% (Chen et al., 2006). PSMs have the potential as biofertilizers. Inoculation in plants results in improved plant growth and yield of several crops.

Phosphate Solubilization Mechanisms

Phosphate solubilizing microbes inhabiting different soil types are capable of converting the insoluble forms of phosphorus to soluble form (Khan et al., 2013). These microbes have potential to avail accessible form of phosphorus to plants from both inorganic and organic sources by solubilizing (Wani et al., 2007) and mineralizing the complex forms (Ponmurugan and Gopi, 2006) respectively. Several strategies for the solubilization of phosphorus are adapted by these organisms but the principal mechanism is by the production of low molecular weight organic acids (Goldstein, 1995). Li et al (2015) reported the production of oxalic acid during calcium phosphate, magnesium phosphate, aluminium phosphate and iron phosphate solubilization and tartaric acid during rock phosphate solubilization by Aspergillus niger strain An2. Various other studies have reported the production of organic acids during phosphate solubilization such as citric, gluconic and oxalic acids by Aspergillus niger FS1, Penicillium canescens FS23, Eupenicillium ludwigii FS27, Penicillium islandicum FS30 (Mendes et al., 2013), Oxalic, malic, citric, succinic and fumaric acids by Aspergillus awamori S19 (Jain et al., 2012), lactic, maleic, malic, acetic, tartaric, citric, fumaric and gluconic acids by Aspergillus flavus, A. candidus, A. niger, A. terreus, A. wentii, Fusarium oxysporum, Penicillium sp., Trichoderma isridae and Trichoderma sp. (Akintokun et al., 2007), citric and gluconic acids by Penicillium rugulosum (Reyes et al., 2001). The organic acids further release fixed phosphorus either by reducing the soil pH or by chelation of heavy metal ions such as Ca, Al and Fe (Awasthi et al., 2011). Scervino et al. (2010) reported the importance of quality of organic acids for the solubilization of phosphate rather than the total amount of acids produced by the phosphate solubilizers. Some of the inorganic acids such as HCl (Kim et al., 1997), nitric and sulphuric acids (Dugan and Lundgren, 1965) are also helpful in solubilizing the insoluble phosphorus but are less effective than organic acids. Siderophores (Vassilev et al., 2006), phenolic and humic substances (Patel et al., 2008) produced by the microorganisms are also involved in the solubilization of phosphate. The role of phosphatases (Aseri et al., 2009) and phytases (Vassilev et al., 2007) secreted by these microorganisms for releasing soluble phosphorus from organic phosphorus compounds have also been reported.

Distribution of Phosphate Solubilizing Fungi

There are several soil microorganisms beneficial for plants growing in an ecosystem, but are not well studied. Pandya and Saraf (2010) reported the presence of fungi in nature, their diversity, their interaction with plant and their role in plant growth promotion. There are wide ranges of fungi capable of solubilizing insoluble phosphorus. The phosphate solubilization efficiency of fungi is greater than bacteria (Akintokun et al., 2007) and the predominant fungal genera are Aspergillus and Penicillium (Wakelin et al., 2004; Xiao et al., 2011). Phosphate solubilizing fungi occupies wide range of habitats.

Aspergillus niger

Aspergillus niger was reported from saline soil (Sanjotha et al., 2011), rhizosphere soil of chickpea (Yadav et al., 2011), forest soil (Das et al., 2012), sugarcane and sugar beet rhizosphere (Mahamuni et al., 2012), arecanut husk waste (Naveenkumar et al., 2012), rhizospheric soils of green gram (Kannahi and Umaragini, 2013), and Himalayan soil (Rinu et al., 2013).

Other Aspergillus sp.

Other species of Aspergillus were also reported from various habitats such as A. clavatus and A. melleus from agricultural soil (Chakraborty et al., 2010), A. awamori from rhizosphere of sugarcane and sugar beet (Mahamuni et al., 2012), A. nidulans, A. terreus and A. ustus from saline soil (Singh et al., 2012), A. flavus and A. fumigates rhizosphere soil (Priya et al., 2013), A. glaucus and A. sydowii from Himalayan soil (Rinu et al., 2013).

Other Fungi

Rhizopus and Aphyllophorales from rhizosphere of melon (Coutinho et al., 2011), Mortierella sp. from Hawaiian soil (Habte and Osorio, 2012), Alternaria alternate, Curvularia pallescens, Penicillium oxalicum, P. rubrum and Trichoderma viridae from rhizosphere of sugarcane and sugarbeet (Mahamuni et al., 2012), Cylindrocarpon obtusisporum, C. didynum, Paecilomyces marquandii, and Penicillium janthinellum from coffee plants (Posada et al., 2012), Galactomyces geotrichum from phosphate mines (Yingben et al., 2012), Emericella nidulans from vermicompost ( Bhattacharya et al., 2013), and Fusarium oxysporum from rhizoshere soil of green gram (Kannahi and Umaragini, 2013) have also been reported from different habitats.

PSF as Bioinoculant for Plant Growth

Inoculation of PSF in plants results in improved plant growth and yield of several crops such as cereals, legumes, fibers, vegetables and other crop plants. Richa et al. (2007) reported the efficiency of Aspergillus tubingensis and A. niger to improve the growth of maize (Zea mays) by nursery experiment. Inoculation of Aspergillus niger, A. melleus and A. clavatus in soyabean showed an increase in root phosphorus content (Chakraborty et al., 2010). A significant increase in phosphorus in shoot tissues of bamboo (Dendrocalamus strictus) was reported when A. tubingenesis and arbuscular mycorrhizal (AM) fungi were co-inoculated (Giridher Babu and Reddy, 2010). Kapri and Tewari (2010) recorded increase in fresh and dry weight of shoot and root, root and shoot length of chickpea inoculated with Trichoderma sp. under glasshouse conditions. Malviya et al. (2011) reported increase in various parameters of groundnut when inoculated with Aspergillus niger and Penicillium notatum. Increased growth and yield of wheat was reported when inoculated with Penicillium oxalicum (Singh et al., 2011). Dry matter content and yield of tomato was increased when inoculated with A. niger (Anwer and Khan, 2013). Inoculation of Penicillium chrysogenum and Aspergillus sp. significantly increased the biomass of Dalbergia sissoo grown under pot culture (Dash et al., 2013). Number of leaves, dry weight of shoot and total phosphorus content of Amaranthus cruentus L. was increased when inoculated with Aspergillus niger (Reena et al., 2013). Gudino Gomezjurado et al. (2015) reported the inoculation effect of Haematonectria ipomeae CML 3249 and Pochonia chlamydosporia var. catenulate CML 3250 on corn and cowpea. Hong-yuan et al. (2015) studied the inoculation effect of A. niger 1107on Chinese cabbage.

CONCLUSIONS:

The use of microbial inoculants has been found to be economical and nature friendly. Therefore, phosphate solubilizing fungi can be adapted as biofertilizer for solubilizing the fixed phosphorus in soil without causing any harmful effects. Limited studies have been done in the agronomic effectiveness of biofertilizers, so more efforts are required for exploring new and better means of its application for the crop plants. The commercialization and large scale field trials are very essential for the potentiality and popularity of biofertilizers. Thus, strains with multiple growth promoting activity should be explored that could be applied under diverse agroecosystems for better growth and yield of plants.

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Received on 28.05.2015 Modified on 12.06.2015

Research J. Science and Tech. 7(3): July- Sept. 2015; Page 141-145

DOI: 10.5958/2349-2988.2015.00019.4