The objective of modern seed-coating technology is to uniformly apply a wide range of active components (ingredients) onto crop seeds at desired dosages so as to facilitate sowing and enhance crop performance. There are three major types of seed treating/coating equipment: dry powder applicator, rotary pan, and pelleting pan with the provisions to apply dry powders, liquids, or a combination of both. Additional terms for coatings produced from these types of equipment include dry coating, seed dressing, film coating, encrustments, and seed pelleting. The seed weight increases for these different coating methods ranges from 5000% (>100,000-fold range). Modern coating technology provides a delivery system for many other materials including biostimulants, nutrients, and plant protectants. This review summarizes seed coating technologies and their potential benefits to enhance seed performance, improve crop establishment, and provide early season pest management for sustainable agricultural systems.

Biostimulant seed coating formulations were investigated in laboratory experiments for their potential to increase maximum germination, germination rate, germination uniformity, and seedling growth of red clover (Trifolium pratense L.) and perennial ryegrass (Lolium perenne L.) seeds. Red clover and perennial ryegrass seeds were coated with different combinations of soy flour, diatomaceous earth, micronized vermicompost, and concentrated vermicompost extract. Coated and non-coated seeds of red clover and perennial ryegrass were evaluated for germination and growth after 7 and 10 days, respectively. Red clover seed was maintained at a constant 20 °C with a 16/8 h photoperiod, whereas for perennial ryegrass seed, the germinator was maintained at 15/25 °C, with the same photoperiod as red clover. Coated treatments significantly improved germination rate and uniformity with no reduction in total germination, compared to the non-treated controls in red clover. In contrast, for perennial ryegrass, the total germination percentage of all coated seeds was reduced and displayed a delayed germination rate, compared with the non-treated controls. Shoot length, seedling vigor index, and dry weight of seedlings of coated seed treatments of both crops were significantly higher when compared to controls for both species. In addition to growth metrics, specific surface mechanical properties related to seed coating quality of seeds of both species were evaluated. Increasing the proportion of soy flour as a seed treatment binder in the coating blend increased the integrity and compressive strength of coated seeds, and the time for coatings to disintegrate. These data show that seed coating technologies incorporating nutritional materials and biostimulants can enhance seedling growth and have the potential to facilitate the establishment of cover crops in agriculture and land reclamation.

Seed treatments are used globally on a wide range of field, vegetable and ornamental seeds, for efficient early season control of insects and diseases. However, specific seed-treatment compounds may be phytotoxic and this phytotoxicity is most acute in laboratory germination tests. Several strategies have been developed to alleviate seed-treatment phytotoxicity that include spatial separation of the pesticide from the seed. This can be accomplished by the application of the active compounds at the end of pelleting or by using a two-pellet system, termed ‘smart-pill technology’. Another approach is to detoxify or adsorb the agrochemical in a standard germination test by applying a peat medium over the seeds in a roll towel or blotter test. Many new seed-treatment chemicals have systemic activity, and the efficacy of these systemic seed treatments depends on the ability of these applied chemical compounds to be absorbed, and then transported in the developing plant. The present article describes seed-coat permeability to systemic seed treatments, examined by monitoring the movement of fluorescent tracers into intact seeds during imbibition. Two moderately lipophilic, fluorescent tracers have been used – rhodamine (ionic) and coumarin (non-ionic) – which differ mainly in electrical charge. Seed-coat permeabilities of particular species have been grouped into three categories: (1) permeable to both tracers; (2) selectively permeable to only coumarin; and (3) non-permeable to both tracers. The ability of a particular compound to diffuse through the seed coat is related to the chemical nature of the seed-covering tissues and the physico-chemical properties of the compound applied.

Seed coat permeability was examined using a model that tested the effects of soaking tomato (Solanum lycopersicon) seeds in combination with carbon‐based nanomaterials (CBNMs) and ultrasonic irradiation (US). Penetration of seed coats to the embryo by CBNMs, as well as CBNMs effects on seed germination and seedling growth, was examined. Two CBNMs, C₆₀(OH)₂₀ (fullerol) and multiwalled nanotubes (MWNTs), were applied at 50 mg/L, and treatment exposure ranged from 0 to 60 minutes. Bright field, fluorescence, and electron microscopy and micro‐Raman spectroscopy provided corroborating evidence that neither CBNM was able to penetrate the seed coat. The restriction of nanomaterial (NM) uptake was attributed to the semipermeable layer located at the innermost layer of the seed coat adjacent to the endosperm. Seed treatments using US at 30 or 60 minutes in the presence of MWNTs physically disrupted the seed coat; however, the integrity of the semipermeable layer was not impaired. The germination percentage and seedling length and weight were enhanced in the presence of MWNTs but were not altered by C₆₀(OH)₂₀. The combined exposure of seeds to NMs and US provided insight into the nanoparticle‐seed interaction and may serve as a delivery system for enhancing seed germination and early seedling growth.

A novel delivery method for the application of bio-based biostimulants as seed coatings was developed using different sources of liquid and powder forms of vermicompost and soy flour. Micronized vermicompost (MVC) and soy flour (SF) were mixed in different combinations as dry seed coating blends and applied using rotary pan seed coating equipment. The physical properties of coated seeds were measured, and as binder concentration increased, coating strength increased. The rates and percentages of germination of the newly developed coating formulations of SF+MVC did not decrease the germination parameters and were not significantly different than the control. However, the SF, SF with concentrated vermicompost extract, and SF + MVC from dairy manure increased the seedling vigor index by 24, 30, and 39 percent, respectively, compared to the control. Plant biometric parameters and nitrogen uptake per plant were also significantly higher for SF and SF+MVC coated seeds than the control, in a greenhouse environment. This is the first seed coating study to show an enhancement of plant growth with vermicompost, and vermicompost in combination with a plant-based protein that serves as a dry seed coating binder and biostimulant, respectively. Seed coatings developed in this study can serve as a model for development of the delivery systems of seeds for the application of bio-based biostimulants to enhance early plant growth.

The seedcoat permeability, uptake, and transport of model fluorescent tracers were investigated in snapbean ( Phaseolus vulgaris ), pepper ( Capsicum annuum ), tomato ( Solanum lycopersicum) , onion ( Allium cepa ), cucumber ( Cucumis sativus ), and lettuce ( Lactuca sativa ) seeds. Nine fluorescent tracers and one vital stain were selected to represent a diversity of physicochemical properties (lipophilicity, electrical charge, etc.) and to simulate behavior of applied seed treatments. To study seedcoat permeability, tracers were applied to seeds as dry powders, and treated seeds were sown in moistened sand at 20 °C and removed after 18 to 24 h, a time before visible germination. Imbibed seeds were dissected and fluorescence (staining) was observed in embryos with a dissecting microscope under ultraviolet (365 nm) or visible radiation. Seedcoat permeability of species to solutes was grouped into three categories: 1) permeable—snapbeans; 2) selectively permeable—tomato, pepper, and onion; and 3) non-permeable—cucumber and lettuce. Systemic tracers that failed to permeate seedcoats during seed imbibition were taken up by roots or hypocotyls after visible germination.