*2.2.1. Seed priming*

Magnetic field exposure increases the germination of non-standard seeds and also improves their quality. Magnetic field influences the initial growth stage of the plants after the germi‐ nation [35]. In recent years, work on magnetic-treated water revealed that plant growth and

Plasma application in agriculture and medicine is a recent advancement [37–40]. The agricul‐ tural aspects include seed germination and plant growth. Many researches report that germination and growth enhancement mechanism is affected by use of plasmas with several gases as aniline, cyclohexane and helium [41, 42]. To enhance seed development and plantgrowth microwave plasma, magnetized plasma and atmospheric plasma are adopted treat‐ ments [43, 44]. The effect of gases is much commonly studied in plasmas treatments. Various reports revealed that the quality of plant development controlling thiol groups is diversified by redox reaction persuaded by the active oxygen species of water vapour plasma [45].

Non-thermal plasma radiations are applied in agriculture as alternative to scarification, stratification and priming helped to improve the plant growth [46]. Plasma helps to attain zero seed destruction, no chemical use and environment friendly treatments to seeds [41, 46, 47]. Plasma treatment improves seed quality and plant growth [43, 48]. Seed exposure to plasma also resulted in alterations of enzymatic activity [45] and caused sterilization of seed surface

Plasma chemistry can tune seed germination by delaying or boosting with application of plasma-treated deposits on seed surfaces [41]. The recent important plasma-related investi‐ gation includes the practice of microwave discharges [43] and low-density radio frequency (RF) discharges [49, 50]. The discharge of atmospheric pressure and the discharge of coplanar barrier have been assessed in recent studies [41, 48]. The investigation of various seed germi‐ nation patterns was implemented on different seeds including wheat, maize, radish, oat, safflower and blue lupine [43, 46, 48, 50]. Safflower seeds expressed 50% greater germination rate when treated with radio frequency plasma for 130 min with argon [46]. Soybean seeds were treated with cold plasma treatment with 0, 60, 80, 100 and 120 W for 15 s and found positive effects of cold plasma treatments on seed germination and seedling growth of soybean

With recent advancements in agriculture, gamma radiations can improve plant characteristics such as precocity, salinity tolerance, grain yield and product quality in suboptimal environ‐ ment depending upon the level of irradiation [52]. Second, gamma radiation can also sterilize agricultural products to prevent pathogen infestation thus increasing conservation time

The biological effects of radiations is based on chemical interaction with biomolecules and water to produce free radicals that can manipulate biomolecules and induce cell to switch on antioxidant system [54] that prepared the defensive shield against upcoming stresses [55, 56].

seed germination were improved by priming [36].

50 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

*2.1.2. Plasma seed treatments*

[47].

[51].

*2.1.3. Radiation seed treatments*

during storage and trading [53].

Seed priming is a pre-sowing approach for influencing the seedling development by stimu‐ lating pre-germination metabolic activities prior to the emergence of radicle and improvement in the germination rate and performance of plant [16, 17]. Seed priming is a controlled hydration process in which seeds are dipped in water or any solution for a specific time period to allow the seed to complete its metabolic activities before sowing and then re-dried to original weight [15, 16].

Priming treatments include osmopriming by polyethylene glycol (PEG) or a salt solution [59], hydropriming [16, 60], solid matrix priming in which seeds are soaked in inert medium of known matrix potential [63] and hormonal priming [62]. A balance of water potential between osmotic medium and seed is necessary for conditioning, and different non-penetrating agents such as organic solutes and salts are used for this purpose [63]. Furthermore, these priming treatments show positive response only at sub-optimal or supra-optimal field conditions such as drought [64], excessively high or low temperatures [60, 65] and salinity [59].

## *2.2.1.1. Hydropriming*

Hydropriming is a controlled hydration process that involves seed soaking in simple water and then re-drying to their initial moisture [59, 63]. No chemical is used during this technique but some cases of non-uniform hydration causes uneven germination [66]. Among the different seed enhancement techniques, hydropriming could be a suitable treatment under salinity stress and drought-prone environments [67].

Hydropriming as a risk free, simple and cheap technique has become popular among farmers, with promising effects in the context of extensive farming system [68]. Hydroprimed seeds produced healthy seedlings, which resulted in uniform crop stand, drought resistance, early maturity and somewhat improved yield.

## *2.2.1.2. Osmopriming*

Osmopriming involves seed hydration in an osmotic solution of low water potential such as polyethylene glycol or a salt solution under controlled aerated conditions to permit imbibition but prevent radical protrusion [67]. For osmopriming, mostly polyethylene glycol or salt solution is used to regulate water uptake and to check radicle protrusion [64]. Most commonly used salts for osmopriming are potassium chloride (KCl), potassium nitrate (KNO3), sodium chloride (NaCl), magnesium sulphate (MgSO4), potassium phosphate (K3PO4), calcium chloride (CaCl2) and potassium hydrophosphate (KH2PO4). All these salts provide nutrient like nitrogen to the germinating seed, which is required for the protein synthesis during the germination process. However, these salts rarely cause nutrient toxicity to the germinating young seedlings [63]. Osmopriming induced more rapid and uniform germination and resulted in decreased mean germination time.

#### *2.2.1.3. Hormonal priming*

Plant-growth hormones or their derivatives contained by several products are indole-3-butyric acid (IBA), an auxin and kinetin type of cytokinin. Cytokinins play a vital role in all phases of plant development starting from seed germination up to senescence [70]. Priming with optimum concentration of cytokinins has been reported to increase germination, growth and yield of many crop species [16]. Gibberellic acid (GA3) is known to break seed dormancy, enhance germination, hypocotyl growth, internodal length, and cell division in the cambial zone and increase the size of leaves. GA has stimulatory effect on hydrolytic enzymes, which speed up the germination and promote seedling elongation by degrading the cells surrounding the radicle in cereal seeds [69, 71].

Various naturally occurring growth promoting substances such as moringa leaf extract, chitosan, sorghum water extract and seed weed extract [62, 65] are commonly used for seed priming. Moringa (*Moringa oleifera* L.) as a natural source of plant-growth regulators contains cytokinins as zeatin [72]. In addition, moringa leaf extracts contain higher concentrations of various growth enhancers such as ascorbates, phenolic compounds, K, and Ca. Priming maize seed with moringa leaf extract reduces mean germination (MGT) and T50 with increased germination index and germination count that ultimately improved seedling growth by increasing chlorophyll content, amylase activity and total sugar contents under chilling conditions [62]. Moringa leaf extract diluted up to 1:36 with water was applied on various field crops and 35% increase in the yield of sugarcane, sorghum, maize, turnip and bell pepper was observed [72]. Nonetheless, moringa leaf extracts being low cost can be a viable option for improving the productivity of resource poor farmers.

#### *2.2.1.4. Nutrient priming*

The application of micronutrients with priming can improve stand establishment, growth and yield; furthermore, the enrichment of grain with micronutrients is also reported in most cases [73]. Many researchers proved the potential of nutrient priming in improving wheat, rice and forage legumes. Among micronutrients, Zn, B, Mo, Mn, Cu and Co are highly used as seed treatments for most of the field crops [74–76].

Seed treatment with micronutrient is a potentially low-cost way to improve nutrition of crops. Farmers have responded in South Asia in a positive way in the seed treatment, which is a simple technique soaking seeds in water overnight before planting [77]. Seed priming with zinc salts is used to increase growth and disease resistance of seedlings.

#### **2.3. Biological seed enhancements**

#### *2.3.1. Bacterial seed agents*

like nitrogen to the germinating seed, which is required for the protein synthesis during the germination process. However, these salts rarely cause nutrient toxicity to the germinating young seedlings [63]. Osmopriming induced more rapid and uniform germination and

52 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

Plant-growth hormones or their derivatives contained by several products are indole-3-butyric acid (IBA), an auxin and kinetin type of cytokinin. Cytokinins play a vital role in all phases of plant development starting from seed germination up to senescence [70]. Priming with optimum concentration of cytokinins has been reported to increase germination, growth and yield of many crop species [16]. Gibberellic acid (GA3) is known to break seed dormancy, enhance germination, hypocotyl growth, internodal length, and cell division in the cambial zone and increase the size of leaves. GA has stimulatory effect on hydrolytic enzymes, which speed up the germination and promote seedling elongation by degrading the cells surrounding

Various naturally occurring growth promoting substances such as moringa leaf extract, chitosan, sorghum water extract and seed weed extract [62, 65] are commonly used for seed priming. Moringa (*Moringa oleifera* L.) as a natural source of plant-growth regulators contains cytokinins as zeatin [72]. In addition, moringa leaf extracts contain higher concentrations of various growth enhancers such as ascorbates, phenolic compounds, K, and Ca. Priming maize seed with moringa leaf extract reduces mean germination (MGT) and T50 with increased germination index and germination count that ultimately improved seedling growth by increasing chlorophyll content, amylase activity and total sugar contents under chilling conditions [62]. Moringa leaf extract diluted up to 1:36 with water was applied on various field crops and 35% increase in the yield of sugarcane, sorghum, maize, turnip and bell pepper was observed [72]. Nonetheless, moringa leaf extracts being low cost can be a viable option for

The application of micronutrients with priming can improve stand establishment, growth and yield; furthermore, the enrichment of grain with micronutrients is also reported in most cases [73]. Many researchers proved the potential of nutrient priming in improving wheat, rice and forage legumes. Among micronutrients, Zn, B, Mo, Mn, Cu and Co are highly used as seed

Seed treatment with micronutrient is a potentially low-cost way to improve nutrition of crops. Farmers have responded in South Asia in a positive way in the seed treatment, which is a simple technique soaking seeds in water overnight before planting [77]. Seed priming with

zinc salts is used to increase growth and disease resistance of seedlings.

resulted in decreased mean germination time.

*2.2.1.3. Hormonal priming*

the radicle in cereal seeds [69, 71].

*2.2.1.4. Nutrient priming*

improving the productivity of resource poor farmers.

treatments for most of the field crops [74–76].

Plant-growth-promoting rhizobacteria (PGPR) are free-living, soil-borne bacteria, which when applied to soil, seeds or roots promote the growth of the plant or reduce the inci‐ dence of diseases from soil-borne plant pathogens. PGPR can influence plant growth either directly or indirectly through fixation of atmospheric nitrogen, solubilization of phospho‐ rus and zinc and producing siderophores, which can solubilize/sequester iron, synthesize phytohormones, including auxins, cytokinins and gibberellins to stimulate plant growth, and synthesize ACC-deaminase enzyme by modulation of ethylene level under stress conditions [78, 79].

Among various genera of PGPR endophytes are good priming agents because they colonize roots and create a favourable environment to develop and function with their hosts—symbiotic partner.

Biopriming is a new technique of seed enhancement integrating biological (inoculation of seed with beneficial organism to protect seed) and physiological aspects (seed hydration) to promote plant growth, development and suppression of diseases. It is used as an alternative approach for controlling many seed- and soil-borne pathogens. Seed priming with beneficial microorganisms (bacteria and fungus) often result in more rapid growth and increase plant vigour and may be useful under adverse soil conditions. Besides diseases control, the appli‐ cation of PGPR as a biopriming agent for biofertilization is an attractive option to reduce the use of chemical fertilizers [80, 81]. PGPR that have been tested as co-inoculants with rhizobia include strains of the following rhizobacteria: *Azotobacter* [82], *Azospirillum* [83], *Bacillus* [84], *Pseudomonas* [85, 86], *Serratia* [86] and *Streptomyces* [87].

#### *2.3.1.1. Role of a bacterial biopriming agent in plant-growth promotion*

The Plant growth promoting bacteria (PGPB) are a heterogeneous group of beneficial micro‐ organisms present in the rhizosphere, on the root surface or inside plant tissues, and are able to accelerate the growth of plants and protect them from biotic and abiotic stresses [88–90]. Beneficial effects of biopriming have been reported in several vegetable seeds [91]. Priming of tomato seed with beneficial bacteria improved the rate of germination, seedling emergence and growth of plant [92]. The beneficial response of biopriming on seed germination and seedling vigour in chilli was reported [93]. Similarly, improvement in okra growth and yield was reported up to 60% when seeds were bioprimed with *P. fluorescens* culture [94]. In experiments where lettuce plants were treated with *Bacillus* strains, it was observed that after two weeks the tissues of roots and shoots contained a greater amount of cytokinin than control plants [95, 96]. The accumulation of cytokinins was associated with a 30% increase in plant biomass

#### *2.3.1.2. Role of a bacterial biopriming agent in plant disease control*

Seed enhancement by biopriming agents involves coating/soaking the seed with one biological agent or microbial consortium, then incubating the seed under optimum (temperature, moisture) conditions.


**Table 1.** Observed effects of plant-beneficial bacteria in regard to plant-growth promotion and stress tolerance.

Biopriming of seeds with different bacterial strains particularly rhizobacteria have been shown to be effective in suppressing disease infection by inducing a resistance mechanism called 'induced systemic resistance' (ISR) in varied agronomic and horticultural crops [97]. Among various bacterial genera, *Bacillus* and *Pseudomonas* spp. are ubiquitous rhizosphere inhabitant bacteria that are the most studied biopriming agents reported as disease suppressing in plants [98]. Priming seeds of many crops with biological control agents (BCA), *Bacillus subtillus* and *Pseudomonas fluorescens* are the most effective approach for controlling seed and root rot pathogens [99, 100] and as a substitute for chemical fungicides without any risk to human, animal and the environment.

#### *2.3.1.3. Seed enhancement by alleviating abiotic stresses using biopriming*

Seed priming with beneficial microorganisms may promote plant growth and increases abiotic stress tolerance in arid or semiarid areas [101]. PGPB are adapted to adverse conditions and protect plants from the deleterious effects of these environmental stresses, thus increasing crop productivity [102]. Bioprimed seeds with *Enterobacter* sp. P-39 showed maximum improve‐ ment in germination and seedling growth of tomato under osmotic stress [103]. **Table 1** shows the selected examples of beneficial response of biological inoculants for enhancing growth and yield of various crops under normal and stress conditions.

#### *2.3.2. Fungal seed agents for biopriming*

*2.3.1.2. Role of a bacterial biopriming agent in plant disease control*

54 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

**Bacterial strain Target plant Condition Proposed**

Faba bean (*Vicia faba*)

Wheat (*Triticum aestivum*)

Rice (*Oryza sativa*)

Sunflower (*Helianthus annuus*)

Cucumber (*Cucumis sativus*) and Tomato (*Solanum Lycopersicum*)

Onion (*Allium cepa* L.)

Chinese cabbage (*Brassica rapa* subsp. *pekinensis*)

(Festuca arizonica Vasey)

moisture) conditions.

*Rhizobium leguminosarum* bv. *Viciae*

*Pseudomonas* spp. NUU1 and *P. fluorescens* NUU2

*Pseudomonas fluorescens* MSP-393

Pseudomonas putida GAP-P45

Rhizobium and Pseudomonas species

Pseudomonas chlororaphis isolate TSAU13

*T. Harzianum* T22 *Rifai* KRL-AG2

*Piriformospora indica*

Neotyphodium Arizona fescue

Seed enhancement by biopriming agents involves coating/soaking the seed with one biological agent or microbial consortium, then incubating the seed under optimum (temperature,

> Green house and field

experiment

Maize (*Zea mays*) Pot experiment decreases in osmotic

Pot

Field experiment **mechanism** 

osmolytes

Production of exopoly saccharides,

potential, and increase in osmoregulant (proline) production, maintenance of relative and selective uptake of K ions.

Green house Antibiosis Stimulated shoot

Axenic trial Osmotic adjustment through physiological responses

> expression of diverse stressrelated genes

conductance

Pot experiment Involved in

**Table 1.** Observed effects of plant-beneficial bacteria in regard to plant-growth promotion and stress tolerance.

Green house Regulate stomatal

biofilm

Improved nitrogenase activity and production of IAA

Auxin production

Green house Production of

**Effects References** 

[19]

[149]

[150]

[152]

[153]

[20]

[154]

[155]

[151]

Increased nodulation and nitrogen fixation under drought and salinity stress

Stimulated the shoot and root length and dry weight

increased the survival, plant biomass, and root adhering soil/ root tissue ratio of sunflower seedlings under drought stress

Promote plant growth and increased relative water contents

growth, dry matter and the fruit yield of tomato and cucumbers under saline conditions

Increased germination %age, shoot length and seedling fresh weight under saline conditions

Promotes root and shoot growth, and promotes lateral root formation

Increased relative growth rates, High W.U.E and biomass yield under drought

Increased plant growth and vigour

> In this approach, beneficial bacterial and fungal agents are exploited for the purpose of biopriming of seeds to enhance growth, yield and mitigation of biotic and abiotic stresses. It is an environmental friendly, socially accepted approach and also offers an alternative to the chemical treatment methods gaining importance in seed, plant and soil health systems. Seed biopriming enhanced drought tolerance of wheat as drought-induced changes like photosyn‐ thetic parameters and redox states were significantly improved by *Trichoderma* sp. under stress conditions over control. Very recently, Junges et al. [84] compared the potential of biopriming (*Trichoderma* and *Bacillus* spp.) with commercial available products Agrotrich plus® and Rhizoliptus® for enhancing growth and yield of beans. Results revealed that biopriming with spore or bacterial cell suspensions promoted bean seedling growth compared to other techniques.

#### **2.4. Seed coating and pelleting**

Seed film coating, pelleting, priming and inoculation are globally practiced seed treatments [104] used with the objectives of enhancing plantability, distribution, germination and storage of seeds. These techniques aim to apply adhesive films, fungicides, herbicides, growth promoters and biological agents [3, 91, 105]. Seed coating is carrier of chemical materials to support seedling growth [106]. Compounds such as growth regulators, inoculants, micronu‐ trients, fungicides, insecticides and other seed protectants are applied to the pellet to enhance seed performance [107].

Seed coating demands uniform application of inert material over the seed surface. This also helps to protect the seed from soil and seed-borne pathogens [17]. Pharmaceutical industry uses seed polymer coating for a constant application of numerous materials to seeds. The commercially available plasticizers, polymers and colourants (commercially they are readily available to be used as liquid) are applied as film formulations [108]. However, the exact composition of coating material is a carefully guarded secret by the companies who develop them. Usually, coating material contains binders, fillers (e.g., polyvinyl alcohol, gypsum and clay) and an intermediate layer (e.g., clay, polyvinyl acetate and vermiculite). Seed agglom‐ eration is an alternate coating technology with the purpose to sow multiple seeds of the same seed lot, or multiple seeds of different seed lots, varieties or species [109].
