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).