![]()
1
Role of Additives in Improving Efficiency of Bioformulation for Plant Growth and Development
G. P. Brahmaprakash, Pramod Kumar Sahu, G. Lavanya, Amrita Gupta, Sneha S. Nair and Vijaykumar Gangaraddi
Contents
1.1 Introduction
1.2 Additives in Bioformulations
1.3 Additives for Liquid Inoculants
1.4 Additives for Alginate-Based Inoculants
1.4.1 Advantages of Encapsulation
1.4.2 Additional Potential Beneficial Features (Bashan et al. 2014)
1.4.3 Major Drawback of Polymeric Inoculants (Bashan et al. 2014)
1.4.4 Role of Additives in Alginate-Based Bioformulations
1.4.5 Skim Milk Powder as an Additive in Bioinoculant Formulations
1.4.6 Other Additives in Polymer Entrapped Bioinoculants
1.5 Additives in Carrier-Based and Other Inoculants
1.6 Future Potential of Additives in Bioinoculant Industry
Acknowledgements
References
1.1 Introduction
In the last few decades, awareness on the use of biologicals has increased among the farming community. The ill-effects of excessive agrochemicals on soil, plant and human health (Seneviratne and Kulasooriya 2013; Arora and Mishra 2016) are visible. In plant protection, it is impractical to expect complete replacement of toxic chemical inputs, due to the fact that ‘being toxic’ is the trait which is desirable to control deleterious pathogens. Therefore, efforts are being made to explore potential use of biological inputs in the agriculture production system in order to maintain ecosystem sustainability. The major part of toxic agrochemicals can be replaced by rather safer alternatives such as bioagents, metabolites, newer molecules etc. The success and replacement rate depend mainly on the fitness of applied bioagents. Efforts are being taken to enhance on-farm performance and fitness of bioagents.
Biofertilizers are composed of live or latent microbes that upon application to crops have beneficial impacts like enhancing plant growth, suppressing pests, ameliorating abiotic stress etc. (Compant et al. 2010; Tan et al. 2011; Lavanya et al. 2013, 2015a; Maji and Chakrabartty 2014; Glaeser et al. 2016; Sahu et al. 2016a, 2017a, b; Meena et al. 2017). Microbial inoculants have several distinct impacts on plants which make them a suitable alternative for partial substitution of harmful agrochemicals. Apart from enhancing plant growth parameters, the concern about bioinoculant use is increasing from a sustainability point of view (Brahmaprakash and Sahu 2012).
There are several factors affecting the quality of applied bioinoculants, such as crop, cultivar, soil type, cultural practices, temperature, salinity, moisture availability, humidity, organic matter content, rhizosphere competence, agrochemicals etc. (Brahmaprakash and Sahu 2012; Sahu et al. 2016b; Sahu and Brahmaprakash 2016; Brahmaprakash et al. 2017; Nair et al. 2017; Meena et al. 2017). Despite the fact that some of the factors are difficult to control, improving performance of bioinoculants is the need of the hour, and several aspects such as exploration, strain improvement, formulating technique, delivery technique, etc. are being standardized for it. One of the major thrust areas for improvement is formulation, i.e. the physico-chemical environment of inoculum. Several additives have been tested in order to improve the physico-chemical environment of inoculants during storage and application (Arora and Mishra 2016; Surendragopal and Baby 2016; Yadav et al. 2017).
Additives also act as cell protectants, which encourage higher survival during storage and tolerance to adverse climatic conditions (Krishan Chandra et al. 2005). Polymers are being used as major additives owing to their high water activity and restricted heat transfer (Mugnier and Jung 1985). The performance of bioinoculants was reported to increase by the addition of various additives. Working with cowpea rhizobia, Girisha et al. (2006) observed that use of poly vinyl pyrrolidone (PVP) as an osmoprotectant resulted in a longer shelf life as compared to an inoculum without PVP. In any formulation there are some additives reported to be added in the inoculant in order to improve survival, tolerance and performance. Surendragopal and Baby (2016) reported that use of 15mM trehalose supported a higher population of Azospirillum and 2.5% poly vinyl pyrrolidone (PVP) supported a higher population of phosphate solubilizing microbes (PSB). In a liquid formulation of Bacillus megaterium and Azotobacter, addition of amendments 2% PVP K-30, 0.1% CMC and 0.025% polysorbate 20 were reported to enhance survival to 480 days of storage (Leo-Daniel et al. 2013).
1.2 Additives in Bioformulations
Two major issues that concern microbial formulations are loss of viability during storage and stability of the product over a wide range of temperatures. Success of a biofertilizer depends on overcoming these problems and developing enhanced high-end inoculants. In this regard, the additives are becoming very crucial to develop formulations with higher shelf life.
Additives are substances that protect the cells and provide longer shelf life along with giving tolerance to adverse conditions (Surendragopal and Baby 2016). A good additive should be non-toxic and of a complex chemical nature, that could prevent a formulation from rapid degradation in the soil. Additives in bioformulation ensure longer shelf life, proper spreading upon application and better adherence to seed surfaces thereby leading to enhanced plant growth and tolerance to abiotic stresses. A sufficiently long shelf life of the inoculants maintaining its biological traits is a major challenge in any bioformulation (Bashan et al. 2014). Addition of additives in bioformulations have been shown to increase viability, increase cell densities overcoming biotic/abiotic stresses and improve physiological activity preferential cell growth leading to improved performance in field.
1.3 Additives for Liquid Inoculants
Maintaining standard minimum microbial population in a bioformulation without any significant contamination is major challenge for the biofertilizer industry (Xavier et al. 2004). In the Indian context, there were abundant carrier-based bioformulations available which have reduced shelf life, contamination, variability in performance, etc. (Bhattacharyya and Kumar 2000), whereas in liquid inoculants, these problems are less persistent. Also, liquid inoculant formulation does not face problems of processing as in the case of solid carrier-based formulation. Composition, sustainability at ambient temperature and maintenance of bioactivity in the desired duration are significant criteria which determine the quality and cost-effectiveness of a liquid biofertilizer (Tabrizi et al. 2017).
The use of various broth additives to liquid inoculant formulations can extend the protection to bacterial cells from abiotic stresses and enhance their establishment in the host (Mugilan et al. 2011). Additives can be polymers (polyvinylpyrrolidone, poly ethylene glycol, sodium alginate, gum arabic etc.), adjuvants (carboxymethylcellulose, xanthan gum, carrageenan etc.) and surfactants (polysorbate 80, 40 and 20) (Leo-Daniel et al. 2013). The sticky nature of polymers may help the cells to easily adhere to seed and their viscous nature may help to slow the drying process of the inoculant after its application to the seed. Surfactants and adjuvants function as emulsifiers and stabilizing agents.
The selection of the ideal polymer is based upon several properties such as complex chemical nature, solubility in water and non-toxicity which can reduce the rate of degradation of microorganisms in the soil (Yadav et al. 2017). The polymers used in liquid inoculants protect inoculants against desiccation and sedimentation, which is a property indicating cell death (Sivasakthivelan and Saranraj 2013). A common polymer used in liquid biofertilizers is PVP. Addition of stabilizing polymers such as PVP reduces protein precipitation and cell coagulation thus maintaining their cellular structure leading to improved biological integrity (Deaker et al. 2004). Liquid formulation of cowpea Rhizobium prepared with an osmoprotectant poly vinyl pyrrolidone (PVP) had a recorded higher shelf life than those formulations without PVP amendment (Girisha et al. 2006). PVP at 1% has also been shown to support survival of saline tolerant PGPR strains till six months of storage period without causing any significant loss of population (Karunya and Reetha 2...
