Several techniques have been used to facilitate sowing, and to improve seedling establishment and growth under a range of environmental conditions. These techniques, generally described as seed enhancements, are performed to seeds before sowing (Halmer, 2004). Seed enhancements may be defined as postharvest treatments that improve germination or seedling growth, or facilitate the delivery of seeds and other materials required at the time of sowing (Taylor et al., 1998). Enhancements include seed priming, seed conditioning, and seed coating. Seed coating technology is concerned with uniform application of materials onto the seed surface and is expressed as the percent build-up or percent weight increase. Thus the seed coating provides delivery of desired quantities of materials, and an opportunity for reduced application rate per hectare by reducing the need to treat the seed furrow or bulk soil.
Three coating techniques have been routinely used for vegetable crop seeds: film coating, encrusting, and pelleting (Taylor, 2003). Film coating is a liquid application method containing a binder and other components dispersed in water resulting in a continuous deposition of materials and minimizing product dust-off without obscuring the seed size and shape. The increase in weight or build-up is relatively small and ranges from 0.5% to 10%. Encrusting and pelleting involve the application of solid particles as fillers in addition to the binder resulting in a larger weight increase than film coating. Pelleting was originally developed to allow for the precision planting of irregularly shaped seeds by mechanical means. Pelleted seed weight varies by crop species and application rates of materials. Seed weight from pelleting can increase from 200% to ≥5000%. By contrast, seed coating or encrusting is a minimal application of inert materials resulting in a smaller seed size increase of just 20% to 200% (Taylor, 2003).
Seed coatings and seed treatments may be grouped into different classes based on their mode of action or properties including: plant protection, environmental stress reduction, or plant growth enhancement. Selective treatments may be used for early season pest management to protect the seeds and seedlings from fungal and insect attacks (Taylor et al., 2001) or act as an herbicide safener (Rushing et al., 2013). Coating materials can influence the microenvironment during germination by holding water around the seed (Scott, 1989) or providing a source of oxygen. For instance, calcium peroxide may be applied as a dry powder during pelleting that produces hydrogen peroxide after sowing to release oxygen to the germinating seeds (Hill, 1999). Seed coatings to enhance seedling and plant growth can provide micro- and macronutrients (Farooq et al., 2012), growth regulators (Halmer, 2004), or other biostimulants.
A plant biostimulant is any substance and/or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance, and/or crop quality traits (Du Jardin, 2015). Biostimulants are a broad group of mostly natural ingredients that contain one or more components such as microbial inoculants, humic acids, fulvic acids, protein hydrolysates, and seaweed extracts (Calvo et al., 2014). Of particular interest to the present research are protein hydrolysates and amino acids that can act as biostimulants. Foliar applications of amino acids have increased plant height, FW and DW, and N content (Shehata et al., 2011), as well as, reduced salinity damage (Sadak et al., 2015). Biostimulants are generally applied to plants at low dosages to enhance plant growth and development. However, biostimulants are not classified as fertilizers and are thus not applied to provide plant nutrients. Biostimulants have no direct action against pests, and therefore do not fall within the regulatory framework of pesticides (Biostimulant Coalition, 2016; Calvo et al., 2014; EBIC, 2014). Most of the research on biostimulants has been conducted using foliar applications while little work has been conducted on biostimulants as seed treatments. Combining the attributes of biostimulants and applying them to seeds via coating may have tremendous potential to enhance early plant growth and development.
The specific objective of this research was to determine the effect of a plant-derived biostimulant, soy flour, on coated broccoli seed. Characteristics measured were seed germination, seedling and plant growth, and N content. An added advantage of soy flour is that it is inexpensive and commercially available in most parts of the world.
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