Microbial seed coating?

Definition of PBM seed coating

In the last years, the application of microorganisms as alternatives to chemical treatments in agricultural products and pastures has increased against various stresses (Deaker et al. 2012). Seed coating is the application of exogenous onto the seed external to boost seed form and handle characteristics such as seed size and weight and delivery of energetic compounds (e.g., plant growth regulators, micronutrients, and microbial inoculants), consequently protecting the seeds from phytopathogens and enhancing germination and plant growth (Pedrini et al. 2017). It is well established from a variety of studies that seed coating with PGPB (such as Pseudomonas sp., Bacillus sp.), AMF, andTrichoderma was an effective and suitable strategy that could introduce PBM into the rhizosphere and provide them to plant roots and other tissues (Rocha et al. 2019b). In comparison to traditional seed treatments, seed coating for different crops was a promising tool that causes a reduced use of inoculum (Afzal et al. 2020). Seed coating with PBM could protect plants against pathogens and improve seed germination against environmental stresses (e.g., drought and salinity) and agrochemicals (e.g., pesticides, growth regulators, and mineral fertilizers) (Ma 2019, Rocha et al. 2019a). Generally, different equipment and methods are used in seed coating to attain good application uniformity and adherence (Jeffs 1986). The use of appropriate seed coating equipment and methods stand can improve plant establishment and seedling vigor under environmental stresses (Ma 2019, Pedrini et al. 2017).

Ingredients of seed coating

The materials used in seed coating include a binder, filler, carriers, and active ingredients (Fig. 2), which assist to release a suitable amount of PBM in physiologic conditions (Ma 2019). Binders are polymers such as the natural and syntactic origin, which ensure the adherence and cohesion of the material on the seed surface and keep the ingredients active (Afzal et al. 2020). The Arabic and xanthan gum can be applied as binders to develop the survival of bacteria, rhizobia, and AMF applied to seeds (Jambhulkar et al. 2016).
The fillers are generally static powders (such as bentonite, calcium carbonate, talc, diatomaceous earth, sand, and wood dust), which can be single or mixed to modify seed shape, size, and weight (Amirkhani et al. 2016, Ma 2019, Pedrini et al. 2017). Nowadays, biochar and chitosan are used as fillers in microbial seed coating (Głodowska et al. 2016). In seed coating, the carriers as one of the factors affecting inoculants should be compatible with these materials and also have the ability to retain sufficient moisture for the growth and survival of inoculants (Jambhulkar et al. 2016). In seeds coated with bio-treatments, some materials such as vermiculite, perlite, etc., are used as carriers, which have high water holding capacity, non-toxicity to seeds, and the capability to stick to the seed external (Ma 2019) and also can assure seedling emergence and quality and the survival of PBM on the seeds (Jambhulkar et al. 2016).
Active ingredients are different from those used in processes of seed coating. The most common active ingredient is protectants, including fungicides, pesticides, insecticides, nematicides, predator deterrents, and herbicides (Yarzábal and Chica 2019), which is used to promote germination and emergence, growth and yield by decreasing predation and putridity by pathogens (Yang et al. 2014). However, sometimes these protectants can negatively affect the germination rate (Qiu et al. 2020). Nutrient amendments, namely macronutrient (e.g., N, P, and K) and micronutrients (Bo, Cu, Mn, Mo, and Zn) applications in seed coating positively affect germination, growth and yield of plants, and also reduce the negative effects of osmotic stress (Pedrini et al. 2017). The most common application of symbiotic microorganisms into coatings involves the inoculation of rhizobia for legumes. The rhizobia-friendly coating formulizations along with the election of desiccation-resistant bacteria modified the survival of symbiotic microorganisms and the beneficial storage life (Scott 1989). To attract and hold water close to the seed, soil hydrophilic materials have been extensively used in seed coating. In addition, the soil surfactant applies within the seed coating materials to enhance water availability to seeds and seedlings in water-repellent soil (Serena et al. 2012).
A range of components (such as PBM, amino acid, chitosan, and soy flour) can be used in the processes of coating seeds of crop and vegetable species in order to stimulate germination and growth, improve stress resistance and establishment, disease reduction, restoration efficacy of native seeds, and protect the finite resource and enhance business for seed technology (Pedrini et al. 2017). The incorporation of fluorescent colorants and magnetic powder into coatings has been expanded to meliorate the traceability of seed batches via the supply chain (Pedrini et al. 2017).

Machines of seed coating

In general, three major kinds of seed coating tools containing a fluidized bed, rotary coater, and rotary pan are used to procreate five kinds of seed coatings, namely dry coating, seed dressing, film coat, entrustment, and seed pellet.
The rotating pan was the first device applied for seed coating, consisting of a circular and usually sloping container rotated by a motor. The seeds were placed in a pan, while the container was spinning, liquids were sprayed on the seeds with a nozzle, and powders were added via a blower or hand spraying (Afzal et al. 2020, Pedrini et al. 2017). The round pan is widely used in the different seed coating methods (Accinelli et al. 2018, Oliveira et al. 2017).
The fluidized or spouted bed apparatus is a cylindrical apparatus that causes the rotation of seeds by airflow through the spray nozzle that atomizes the coating liquid towards the suspended seed mass. This process is used for film coating and surface incrustation, but it is not possible for pelleting.
The rotary coater is an apparatus used in the pelleting and film coating, and it includes a cylindrical drum with a concave disk at the base. Its rotation leads the seed mass to whirl in a regular flow along the drum wall. Usually, a smaller rotating disk attaches to the drum lid and suspends in the middle of the drum. It is accountable for atomising and spraying (Pedrini et al. 2017). In the seed coating industry, these systems are standard in seed treatment. However, nowadays, considering seed coating commercialization and industrial, a lot of information is not disclosed regarding tools and details of seed coating methods.

Types of seed coating

Dry powder coating

Dry powder coating is a method in which seeds are placed in a dry powder and mixed. Also, dry powder can be utilized for bacterial or fungal treatments followed by drying (hydration/dehydration) (Afzal et al. 2020). There is a rotating brush made of stainless steel which sieves a powder material using a dosing sieve (Afzal et al. 2013). It has been reported that talc and graphite are the most common dry powders (Badua et al. 2019). The dosage for dry coating powders used onto seeds is extended with their adherence to seeds and ranges from 0.06 to 1.0% of seed weight (Afzal et al. 2020).

Seed dressing

Seed dressing coating is a method that uses a low dosage of active ingredients to create a thin layer around the seed. In this method, the active materials especially chemical protectants can be used in a wide range (Kimmelshue et al. 2019). The most common equipment in seed dressing is the rotary coater. The rotary coater places the liquids onto a spinning disc and atomizes onto seeds that are spinning inside a metal cylinder, then discharges the freshly treated seeds. The dosage of liquid seed treatment formulations typically ranges from <0.05 to 1.0% by weight (Afzal et al. 2020).

Film coating

Film coating is modeled based on the industries of pharmaceutics and confectionary (Afzal et al. 2020) (Fig. 3a). In this method, the seed size does not change, and a small layer (less than 10% of the seed weight) of coating materials such as pigments, fungicides, and polymers are placed around the seed (Ma 2019). During this process, the shape and size of the seed do not change and its application creates successful sowing in the field and protects the environment (Afzal et al. 2020). Nowadays, film coating has been considered an effective and reliable tool to improve crop productivity in the seed industry.

Encrusting

Encrusting is the process of coating seeds by adding liquids and solid particulates to complete seed coverage (Fig. 3b). In this method, the original seed shape is retained, and seedling emergence is improved (Afzal et al. 2020, Pedrini et al. 2017).