Review
Application of Ultrasound, Microwaves, and Magnetic Fields Techniques in the Germination of Cereals
2019 Volume 25 Issue 4 Pages 489-497
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2019 Volume 25 Issue 4 Pages 489-497
Cereal germination involves a series of complex physiological and biochemical processes that are influenced by multiple internal and external factors. Previous studies have shown that certain non-thermal processing techniques can be used to effectively induce a range of physiological and biochemical changes in cereal seeds, thereby increasing the digestibility and bioavailability of cereals and enhancing the nutritional value, flavor, and quality of cereal sprouts. Here, we provide an overview of the principle of promoting cereal germination to enrich physiologically active components review the effects of treating cereal seeds with three new non-thermal processing technologies, namely, ultrasound, microwaves, and magnetic fields, on the contents of enzymes and various other seed components during germination. Finally, we summarize the advantages and disadvantages of applying the above techniques to cereal germination.
Germination is the most energetic and intense period of seed growth. In cereals, the process of germination is associated with various material transformations, during which macromolecular substances such as starch and proteins are broken down, the amino acid compositions are modified, and certain constituents, including reducing sugars, restrictive amino acids, vitamins and minerals, are increased in contents (Li et al., 2014; Yang et al., 2001). Furthermore, the anti-nutritional substances such as phytic acid and trypsin inhibitor are reduced in concentration or eliminated, digestibility and bioavailability are increased, and the nutritional value is enhanced, thereby improving the flavour and quality of cereal food (Bau et al., 1997; Ding et al., 2018c; Ghorpade et al., 1986; Vasagam and Rajkumar, 2011).
Cereals are among the most important economic crops grown worldwide, with an annual output of more than 2.5 × 1012 kg (Tuan et al., 2018). The contents of the functionally active components, such as proteins, γ-aminobutyric acid (GABA), free amino acids, flavonoids, and phenols, are significantly increased in germinated cereals and the subsequent seedlings, whereas the contents of unsaturated fatty acids, carbohydrates, and minerals are reduced compared with non-germinated cereals seed and seedlings. Germinated cereals are, thus, considered to have unique nutritive value and health functions (Ding et al., 2018c; Farooqui et al., 2018; Guo et al., 2011; Sugawara et al., 2018). Germination of cereals has been used to soften the kernel structure and to increase the nutrient contents and availability, which is favoured by an increasing number of consumers in the fields of foods, pharmaceuticals, health products, and cosmetics (Farooqui et al., 2018). As a food product, germinated cereals can be cooked fresh or used as value-added health ingredients after the dry grains have been processed powder (Dziki and Gawlik-Dziki, 2019).
Seed germination is characterized by a series of complex physiological and biochemical processes (Farooqui et al., 2018; Tuan et al., 2018) that are affected by multiple internal factors, including seed vigour, temperature, and time, as well as certain external stimulations such as those relating to hormones, visible light, ultraviolet light, ultrasound (US), microwaves (MW) and magnetic fields (MFs) (Raso and Barbosa-Cánovas, 2003; Ross et al., 2003; Xia et al., 2018). Controlling germination is an effective method for improving the nutritional value and flavour of food products (Ding et al., 2018b; Ding and Feng, 2019). Among the new processing technologies is non-thermal processing, which includes the use of ultra-high voltage, high voltage pulsed electric fields, ionizing radiation, and pulsed MFs. Compared with the thermal treatments, non-thermal techniques are not limited to changing the physical and chemical properties of cereals, because improvements in nutrients and functional components and the reductions in anti-nutrition factors can also be achieved by inducing related biochemical transformations (Xia et al., 2018). Non-thermal technologies, such as US, MW, and MFs, also stimulate the germination of cereal seeds and produce certain specific effects that trigger a series of physiological and biochemical changes and have accordingly attracted increasing attention from researchers (Pittman, 1977; Pittman et al., 1979; Raso and Barbosa-Cánovas, 2003). The application of US in biological technology has been widely reported as an effective tool that can be used to regulate and control biological processes, including fermentation, enzymatic transformation (Hashemi et al., 2018), seed germination, and plant growth (Mota et al., 2018). Furthermore, the literature indicates that MW and MF treatments of seeds results in superior germination rate (GR), seedling growth (SG), bud length, and root length compared with unexposed seeds (Srikanth et al., 2018; Wang et al., 2018). The “cavity effect” characteristic of US, MWs, and magnetization associated with MFs all affect the growth and metabolism of plants (Răcuciu, 2011; Xia et al., 2018).
In arid and semi-arid areas, soil salinization is a major threat to agriculture and under such adverse conditions, the use of treatments that can mitigate the effects of salinization stress represents a promising solution for ensuring stable crop production under such adverse conditions (Costa et al., 2018). These physical treatments often change only certain physiological processes in seeds, so as to increase seed vitality and promote plant development (Srikanth et al., 2018). The appropriate and effective treatment of seeds can increase GR and germination energy (GE), promotes growth, and shortens the maturation period of crops. Furthermore, the non-thermal biological effects of MW, like electromagnetic fields (EMFs), and MFs, can increase seed GE and GR, promote SG and seedling quality, enhance the metabolic function of crops, induce the stress resistance in seedlings, and increase crop yield (Ponomarev et al., 1996; Shine et al., 2011).
Although at present, US, MW, and MFs treatments are widely used as supplementary extraction and drying methods, few studies examined the application of these measures to cereal germination (Arballo et al., 2018; Li et al., 2016; Sugawara et al., 2018). The aim of this review is to present the current state of research on the use of US, MW, and MFs for cereal germination.
Principle US as a high-efficiency, non-toxic, and environmentally friendly method of physical stimulation, has been used to promote GR, SG, and the production of useful active compounds (Ding et al., 2018a; Ding et al., 2018c; Kai, 2015). Treatment of seeds using US involves placing the seeds in a US device and selecting different field patterns by adjusting the frequency, power, time, and temperature to alter the physiological activity of the seeds (Chen et al., 2013). The most common interaction mechanism in this regard is acoustically induced cavitation, which has both thermal and chemical effects. Most of the reported effects of US on plants are associated with local oscillations that produce highly heterogeneous acoustic fields within cells. In response to US treatment, gaseous channels develop between the cells of higher plant tissues as a consequence of pulsations, the characteristics of which depend on a number of factors, including channel width, cell wall thickness, swelling pressure, and the elastic modulus of the cell wall. Multiple physical and mechanical effects occur when the seeds within a liquid medium are subjected to US. Large particle surfaces are subject to erosion (as a result of cavitational collapse in the surrounding liquid) or particle size reduction (fission due to collision between particles or collapse of cavitation bubbles formed on the surface) (Mason et al., 1996). The local oscillations caused by US produced several types of steady perturbations within cells, including small-scale acoustic streaming, radiation force and torque, and local force and pressure fields. These mechanical disturbances act on the physical structure and integrity of the cell (Miller, 1983). The mechanical effect of US, namely the shear stress caused by the eddy current arising from shock waves, can potentially accelerate the entry of material into seeds (Yaldagard et al., 2008b). Furthermore, US oscillations cause seed coat fragmentation, which promotes the hydration of dormant seeds. This treatment not only enhances the water uptake by seeds but also effectively modifies the structure of enzyme molecules via using its cavitation and mass transport, thereby promoting enzyme catalysis and accelerating SG (Miyoshi and Mii, 1988; Yaldagard et al., 2008a; Yaldagard et al., 2008b). However, this hydration does not have a negative effect on seeds (Yaldagard et al., 2007a; Yaldagard et al., 2007b). In some cases, alternative mechanisms, such as the generation of heat, transverse-wave excitation (in cell walls), or other (as yet undetermined) processes may be important (Miller, 1983). Moreover, the rate of mutagenesis due to high-intensity US is expected to be low, owing to the high temperature or cavitation of mutagenic chemicals (sonochemistry) (Miller, 1983).
Effects of UW treatment on the germination of cereals It has been demonstrated that using US power to treat barley (Hordeum vulgare L.) seeds for 5–15 min at 20%–90% of the 460 W increased the final GR of seeds by 4.2% to 6.5%, whereas the germination period (GP) was shortened by 30%–45% compared with untreated seeds (Yaldagard et al., 2008a). Cavitation generated by US shatters seed coats and thereby markedly reduces the resistance to water diffusion. This not only promotes the passage of water molecules through cell walls and increases the mass transfer rate of the target components, but also enables the embryo to freely absorb excess water, thereby promoting embryo development and growth (Yaldagard et al., 2008a). Assays of α-amylase activity have confirmed that both US power and time influence the enzymic activity of seeds compared to untreated seeds, and that the contribution rates were 48.46% and 45.27%, respectively (Yaldagard et al., 2007b; Yaldagard et al., 2008b). US treatment of 40–60 kHz has also been demonstrated to significantly increase the uniform coefficient, vigour index (VI) and germination index (GI) of the wheat seedlings compared with those of untreated seedlings, and this treatment was found to be beneficial in terms of root and shoot growth and the increased the total weight of the bud seedlings. It is also possible that a brief exposure to US stimulates cell division (Goussous et al., 2010; Mirshekari, 2015; Paparella et al., 2015; Wang and Wang, 2017). A further study has shown that US treatment (25 kHz, 16 W/L, 5 min) of long-grain de-hulled rice (Oryza sativa L.) seeds increases GR and sprout length (Ding et al., 2018a). The seed contens of GABA, riboflavin (vitamin B2), o-phosphoethanolamine, and glucose-6-phosphate were found to be significantly increased after germinating red rice (Oryza sativa L.) seeds for 72 h, and were subsequently further increased after US treatment at different stages, which may be attributed to the activation of endogenous enzymes (Ding et al., 2018c).
Effects of US treatment on the composition of cereals US treatment of wheat (Triticum aestivum L.) seeds at 50–60 Hz for 5 min has been found to increase the chlorophyll content in the resulting seedlings by 1.6 times compared with the seedlings that developed from untreated seeds (Mirshekari, 2015). Moreover, US treatment (45 kHz and 160 W) of wheat seeds (cultivar Xiaoyan. No. 22) exposed to heavy metal ions such as lead and cadmium confirmed that US treatment not only promotes the biosynthesis of chlorophyll and proteins in seedlings but also significantly reduces the concentrations of malondialdehyde (MDA) and the superoxide radical (O2•−) and electrical conductivity. This result indicated that US treatment can effectively reduce the toxicological effects of heavy metals (Chen et al., 2013). At a US power of 280 W, temperature of 30 °C, and treatment time of 30 min, the content of total flavonoids in tartary buckwheat (Fagopyrum tataricum Gaertn.) seedings (6 d) reached 9.46 g/100 g, which was 228.07% and 69.71% that in seeds and the control seedlings, respectively (Wang and Wang, 2017). In order to adapt to the high-temperature stress environment induced by US, seeds may synthesize large amounts of flavonoids by regulating gene expression, inducing rapid metabolism of cells, and modifying the activity of related enzymes. In addition, US treatment for 5 and 30 min was shown to increase the GABA content in 72 h-germinated soft white wheat by 10.26% and 30.69%, respectively, probably by promoting the activity of glutamate decarboxylase (Ding et al., 2018b), whereas during germination, the glucose content in wheat samples increased by 4%–37% and 227%–357% after germination for 5 and 15 h, respectively (Ding et al., 2018b).
Effects of US treatment on the enzymes of cereals It has been demonstrated that US treatment of wheat seeds can alleviated the stress induced by exposure to lead and cadmium by increasing the activities of catalase (CAT), superoxide dismutase (SOD), and glutathione reductase. The results indicated that treatment enabled plants to eliminate the harmful effects of reactive oxygen species (ROS) in cells by enhancing the activity of antioxidant enzymes (Chen et al., 2013). US stimulation has also been found to increased the activity of α-amylase in barley seeds and it was increased by 7.14% after treatment with 100% cavitation intensity for 15 min (Paparella et al., 2015; Yaldagard et al., 2008b; Ding et al., 2018a), which could be attributed to an increase in casein penetration into carrier gels resulting from surface cavitation effects. It is also believed to be associated with the channeling of more microfluidic reagents to the surface (Hughes and Nyborg, 1962). In this regard, it is possible that endosperm modification, including ultrasonic degradation of starch, leads to an increase in enzymatic reaction rates (Yaldagard et al., 2008a). US vibration has also been shown to alter the structure of cell walls and promote the release of enzymes from the cell walls to enhance biochemical metabolism (Hughes and Nyborg, 1962; Yusaf, 2015), thereby increasing the content of bioactive substances and the antioxidant activity in seeds.
Effects of US treatment on the antioxidant and antibacterial properties of cereals US treatment not only enhances the antioxidant activity of seeds but also increases the resistance to metal ions, such as those of lead and cadmium, and saline-alkali conditions (Chen et al., 2013). The DPPH (2, 2-diphenyl-1-picryl-hydrazyl-hydrate) radical scavenging activity of tartary buckwheat sprouts treated with US has previously been shown to be 86.47% greater than that of the control sprouts (Wang and Wang, 2017). Furthermore, US treatment has been demonstrated to reduce the total number of bacteria on the surface of cereal seeds and enhance seed antibacterial properties (Chiu and Sung, 2014; Kouchebagh et al., 2015). Total inhibition of aflatoxin (produced by Aspergillus spp.) and citrinin (produced by Penicillium spp.) has been reported after exposure to US for at least 6 min at 60 °C when applied at a power ranging between 20 and 39 W/cm2. If the acoustic power is sufficiently high (>60 W/cm2), the cavitation effect of US cause cell rupture, leading to microbial inactivation (Schmidt et al., 2018). The protective effect of US in this regard can be attributed to at least three different effects: inhibition of electrolyte leakage and lipid peroxidation, thereby maintaining cell membrane integrity and enhancing the activity of antioxidant enzymes; enhancing cell division and biochemical syntheses; and enhancing the decomposition rate of starch and protein.
Although US treatment can promote the enhancement of enzyme activity and growth of crops, as a type of energy, high-intensity US treatment of seeds and seedlings may cause cell rupture and enzyme inactivation (Xia et al., 2018). Thus, US treatment tends to be applicable only under certain conditions (Goussous et al., 2010), and may additionally be crop plant dependent. Moreover, although numerous studies have investigated the effects of US treatment, the full spectrum of effects on seeds remains to be determined. For example, the effects of US on mycotoxins has rarely been studied (Schmidt et al., 2018). Accordingly, further studies are necessary to examine the mechanism underlying the effects of US on other active substances of seeds and increases in crop yield (Hozayn et al., 2015).
Principle MW radiation produces non-thermal effects that are biological in nature. When interacting with the biological tissues, MW radiation alters the charge density on the surface of the cell membranes and the electric potential difference on either side of the membrane, thereby affecting the activity of ion channels, which in turn promotes a series of physiological changes in plants (Goussous et al., 2010; Vian et al., 2016). It has been reported that MW energy generates an extensive network of fissures in seed coats, and that there are more brittle fissures on the whole seed coats (Tran, 1979). As a consequence of the absorption of MW energy by plant seeds, the internal temperature of cells increases rapidly and the internal cell pressure exceeds cell wall expansion capacity, thereby resulting in cell rupture and the freely outflow of effective cell components. By destroying the structure of plant cells and cell membranes, MW radiation hereby increases the capacity of intracellular soluble matter to penetrate through the thin biofilm (Kouchebagh et al., 2015). Studies have shown that the polar molecules in plant cells absorbed electromagnetic energy, thereby altering biological macromolecular structure and affecting the physiological and biochemical characteristics of plant cells (Kouchebagh et al., 2015). At a frequency of 300 MHz, MW can loosen seed coat membranes, decrease their permittivity, and increased the electric conductivity. When these polar molecules are oriented to a sufficiently high range of alternating electric field intensity, the dielectric constant decreases and the strength of the molecular field increases, leading to a more constrained arrangement (Anand et al., 2009; Olsen et al., 1966). Changes in the electrophoretic peaks of certain proteins indicates that these effects occur only in response to certain exposure times and are repeated at 6-MHz intervals. This pattern is most likely to be attributable to harmonics, and thus the underlying mechanisms may be related to molecular resonance (Olsen et al., 1966).
Effects of MW treatment on the germination of cereals It has been shown that low-power MW irradiation modulated with 1 kHz square wave is beneficial with regards the seed GR, seedling vigour, plant height, root length, and material dry weight of wheat (Triticum aestivum L.). Compared with the control group, the seed germination, seedling activity, plant height, root length, and percentage biomass of most samples decreased with an increase of MW power and irradiation time. Moreover, with a decrease in power density, more favorable seedlings growth was observed than at higher power density (Ragha et al., 2011). In a study by Abu-Elsaoud, a NGM-2001 MW oven was used to heat six types of wheat seeds for 1 to 240 s, which increased seed germination vigour, GR, and relative germination coefficient, with the GR reaching 100% (Abu-Elsaoud, 2015). During elongation growth, cells can partially repair damage at the membrane level. In this regard, a study has shown that the GR of tartary buckwheat (Fagopyrum tataricum Gaertn.) seeds at 7 days after treatment with MW (600 W, 10 s) was two times that of the control group (Wang et al., 2018). Such growth enhancement could be attributed to the breakdown of sugars in cereals by irradiation, which mobilizes nutrients for the embryo development (Hamada, 2007). Therefore, appropriate MW treatment can promote seed germination.
Effects of MW treatment on the composition of cereals Studies have shown that MW radiation at a wavelength of 2.85 cm and frequency of 10.525 GHz for 15 min can increase the content of proteins and amino acids in wheat seeds (cultivar Sakha 61) (ungerminated) and stimulate the synthesis of proline, but reduces the contents of sugars, nucleic acids and phenolic compounds (Hamada, 2007). MW pretreatment at a power of 126 mW·cm2 for 10 s has been demonstrated to alleviate the growth inhibition of wheat seeds (cultivar Wenmai No. 006) under cadmium stress. Moreover, the content of MDA has been found to be reduced, whereas the concentrations of hydrogen peroxide (H2O2) and O2−, ascorbic acid, glutathione, carotenoid and chlorophyll were all increased (Qiu et al., 2013a; Qiu et al., 2013b). However, the relationship between MW treatment and signal molecules (including H2O2 and calcium) in the MW-induced cadmium tolerance of wheat seedlings needs to be further studied. Other results have indicated that MW (600 W, 30 s) treatment can promote the contents of total flavonoids, reducing sugars, and soluble proteins in tartary buckwheat, and that the content of free amino acids reached 11 mg/g after 5 d following MW treatment at 800 W for 10 s (Wang et al., 2018).
Effects of MW treatment on the enzymes of cereals Appropriate MW treatment has been found to stimulate the activity of enzymes to promote germination and the accumulation of active substances in cereal seeds (Gaurilčikienė et al., 2013; Qiu et al., 2013b; Wu et al., 2017). Studies have also indicated that MW radiation treatment under suitable conditions can enhance the activity of antioxidant enzymes, including SOD, peroxidase (POD), CAT, ascorbate peroxidase (APX), and glutathione peroxidase (GSH-Px) (Qiu et al., 2013a). Similarly, MW pre-treatment has been demonstrated to be effective in promoting an increase in α-amylase activity (Chen et al., 2009). Low-power MW treatment of oat (Avena sativa L.) seeds was found to promote the activities of nitrate reductase (NR) and glutamine synthetase (GS) in leaves and inhibited the activity of proteolytic enzymes and ribonuclease (Wu et al., 2017). It has also been found that MW pre-treatment for 5–25 s can increase the activity of α-amylase activity and the intensity of photon emission, with the highest α-amylase activity (up to 0.88 mg/g·min−1) being recorded following treatment for 10 s (Chen et al., 2009). In general, it thus appears that MW treatment at certain powers can effectively activate various enzymes during seed germination (Wang et al., 2018).
Effects of MW treatment on the antioxidant and antibacterial properties of seeds The inhibitory mechanism of MW on microorganisms is assumed to be based primarily on the effect of internal heating on the molecular motion of a sample, which leads to the denaturation of proteins, enzymes, and nucleic acids (Schmidt et al., 2018). However, under optimized MW conditions, microbial growth on cereals can be completely inhibited without damaging the quality of cereal germination (Chemat and Khan, 2011). A study has found that MW treatment of cereals (120 s, 2450 MHz, 1.25 kW) can effectively inhibite the growth and spore germination of aflatoxin-producing fungi (Aspergillus spp.) (Schmidt et al., 2018). Appropriate MW radiation can also enhance plant metabolism under cadmium stress, by increasing cadmium tolerance in wheat seedlings and increased the antioxidant activity of seeds (Qiu et al., 2013a; Qiu et al., 2013b). Appropriate MW treatment has also been found to enhance the ability of wheat seedlings (cultivar Zhengmai No. 9023) to scavenge free radicals induced by saline osmotic stress, thus enhancing resistance to salt stress (Chen et al., 2009). The protective effect of MW treatment is assumed to be based on at least three mechanisms. Firstly, it enhances the activity of SOD, POD, and CAT, which are considered to be the key enzymes in suppressing the function of abiotic stress pathways. Secondly, it serves to increase the concentration of glutathione and ascorbic acid, which can eliminate free radicals and reduce oxidative stress. Thirdly, it increases the concentration of nitrous oxide (NO: a plant signal), because NO, which can react with free radicals such as O2− and H2O2.
A previous study has examined the effects of strong MW electric fields (SMEFs) of 80–100 kW at 2.6, 5.7, and 9.3 GHz for treatments of 5, 10, and 20 min, respectively, and the effects of SMEF on the germination and sterilization of wheat seeds were studied. The results indicated that the SMEFs had no significant effect on the GE or germination of winter wheat seeds, whereas the number of abnormally germinated seeds increased, and the GI and VI decreased. It has also been found that the higher the seed humidity, the greater is the negative effect was on GI and VI. Low-power MW radiation of 160–640 W has also been found to result in the death of wheat seed-borne pathogens such as Phaeosphaeria nodorum (the causal agent of Stagonospora glume blotch), Pyrenophora tritici-repentis (the causal agent of tan spot) and Fusarium spp. (the causal agents of seedling and mature plant foot and root rots and Fusarium head blight), thereby enhancing the antibacterial properties of seeds (Gaurilčikienė et al., 2013).
In a study of the insecticidal and GR effects of MW treatment on wheat (Triticum aestivum L.) seeds, it was found that the GR of seeds decreased significantly in response to 800 W MW treatment for 20 and 25 s, and that MW at this intensity affected the vitality of the seeds. Therefore, it is assumed that the effects of MW radiation are dependent on cell moisture content, and that MW treatment at an intensity of 800 W has deleterious effects (Wang and Li, 2011).
Higher MW output power and longer MW treatment time leads to a higher energy of molecular absorption, which may destroy the function and physiological balance of cereal seed cells, thus inhibiting the SG. In this regard, studies have shown that SMEFs irradiation of wheat seeds decreases seed GI and VI, increases the number of abnormal seeds, and reduces the infection of the wheat germ, thereby increasing the weight and number of ear grains (Wang and Li, 2011).
As a consequence of internal heating, MW treatment generally showes little potential in cereal sterilization due to the result of heat-induced sample damage (Schmidt et al., 2018). Moreover, poor uniformity and high energy consumption mean that MW heating is not conducive to large-scale production. Nevertheless, further studies should be conducted on the effects of low-power MW conditions on physiological and biochemical indices, and it is considered that studies of MW-induced changes can still make an important contribution in agriculture, industry, and animal husbandry (Wu et al., 2017; Manickavasagan et al., 2007).
Principle MFs are generated via several means, including shielding (ferromagnetic metal plates with high permeability are wrapped around the experimental area to make them concentrate in the metal) and compensation (involving the use of a system of the Helmholtz rings were used). The effect of MFs on plants is dependent on the size and nature of the MFs. Treatment with low magnetic flux density has been shown to have a positive effect on plant growth (Vashisth and Nagarajan, 2010). Such treatment stimulates various reactions, not only affecting the organism itself but also directly affecting the surrounding environment, such as water or growth medium (Galland and Pazur, 2005). Given that the energy of weak MFs is insufficient to break chemical bonds, other physical mechanisms are necessary to trigger biological reactions. The possible mechanistic modes of MFs treatment include (1) the torque of ferromagnetic particles, (2) modulation of the chemical reaction rate (radical-pair mechanism), (3) modulation of transmission rate and binding by the “ion cyclotron resonance” mechanism, and (4) the paramagnetic behavior of electrons or ions (Galland and Pazur, 2005). It has previously been found that numerous free radicals are formed in the biofilm of the seed coat and internal cells. High concentrations of free radicals increased the permeability of the membranes and enhanced the seed coat penetrability, thereby accelerating the rate at which water and oxygen entere seeds (Pittman et al., 1979). These changes are assumed to alter the electrostatic balance of the plant system at the level of the cell membrane, which is the main site associated with the inhibition or promotion of plant growth (Radhakrishnan, 2012). This study also found that MFs modified the activity of Ca2+/calmodulin-dependent cyclic nucleotide phosphodiesterase and cytochrome C oxidase (Radhakrishnan, 2012). Further studies have also shown that MFs can modify biological functions by altering hormone concentrations and inducing DNA synthesis or transfer (Kornarzyński et al., 2018). However, our understanding of the complex mechanisms and sites of interaction between weak MFs and biological systems is still incomplete and deserves further in-depth study.
Effects of MFs treatment on the germination of cereals A fixed MFs has been shown to have a positive effect on plant seeds, increase GR, and promote growth (Carbonell et al., 2000; Galland and Pazur, 2005). At the initial germination stage, root formation has been found to increase by nearly 25% and the 6-d-old seedlings were observed to have increased in length by 40% when 30 mT MFs was used to treat wheat seeds for 15 min (Martinez et al., 2009). Furthermore, studies have indicated that the uniformity coefficient and seedling VI of wheat seeds treated with MFs increased significantly compared with those of control seeds (Mirshekari, 2015). Other study have shown that a constant MFs can increase the seed GR and VI and significantly increase the whole plant and ear weight of plants grown from treated wheat seeds (Kouchebagh et al., 2015). Determination of α-amylase activity has indicated that MFs have a certain effect on this enzyme, which may contribute to the observed enhancement of seed germination (Katsenios et al., 2016). In addition, in maize (Zea mays L.), it has been reported that fixed MFs enhance seed germination and early growth (Srikanth et al., 2018), whereas MFs treatment (40 mT) of seeds for 5 min has been shown significantly increase the yield (Siyami et al., 2018). These responses may be attributable to the multiple effects of MFs with regards to repairing cell membrane damage, restoring cell barrier function, enhancing seed germination, activating enzyme systems, and contributing to an increase in seed GR (Vashisth and Nagarajan, 2010).
Effects of MFs treatment on the composition of cereals It has been reported that appropriate MFs treatment of wheat seeds can increase the chlorophyll content of seedlings and also increased the contents of nitrogen, phosphorus, and potassium in seedling leaves (Mirshekari, 2015). In addition, it has been shown that the treatment of wheat seeds with an MFs of 50 mT/0.5 h for 0.5, 1, and 2 h increased the protein content of wheat seeds to 10%, 14% and 8%, respectively (Hussein et al., 2012). Moreover, the enhancement of plant growth has been proposed to be attributable to the energy released by the reunion of north and south magnetic monopoles (Hussein et al., 2012). Furthermore, the content of reducing sugars in winter wheat seeds treated with MFs has been found to be significantly increased within 72 h of germination compared with the control seeds (Pittman and Ormrod, 1970), which has been attributed to enhanced amylase activity catalyzing the hydrolysis of starch into maltose (Katsenios et al., 2016). Moreover, it has been found that MFs of 0–800 mT reduced the MDA content, the concentration of O2−, and the conductivity of electrolyte leakage in wheat seeds (cv. Xiaoyan No. 22) subjected to lead and cadmium stress (Chen et al., 2017).
Effects of MFs treatment on the enzymes in cereals Studies have shown that MF treatment of wheat seeds promotes the activity of α-amylase (Katsenios et al., 2016), with different MFs intensities having different inductive effects on the same enzyme. Moreover MFs of the same strength have been found to have different induction effects on different enzymes. For example, the specific activity of phenylalanine ammonia-lyase (PAL) reached its highest value in response to treatment with MFs at an intensity was 0.3 T, whereas the highest value of the enzyme activity of chalcone isomerase (CHI) occurred at an MFs intensity of 0.2 T. Furthermore, at an MFs intensity of 0.3 T, the activity of PAL increased by 18.5% compared with that in response to 0.2 T, whereas CHI activity showed a corresponding increase of 12.9%, thereby indicating that MFs have a stronger induction effect on PAL activity than on that of CHI (Zhou et al., 2012). Other studies have shown that MFs can contribute to the enhancement of CAT, SOD, POD, and glutathione reductase activities in wheat seeds (Chen et al., 2017). Treatment with MFs has been found to have positive effects on protein biosynthesis, cell replication, photochemical activity, respiration rate, enzyme activity, nucleic acid content, and growth cycle, which variously contribute to morphological and developmental changes, regulation of ion transport, and metabolic changes, including carbon metabolism and the synthesis of compatible solutes (Vashisth and Nagarajan, 2010).
Effects of MFs treatment on the antioxidant and antibacterial properties of seeds The MFs treatment can aslo produce a variety of non-genetic phenotypic effects, which have been shown to contribute to the prevention and amelioration of disease, enhanced resistance to insect pests, and increased the yields (Borthwick et al., 1952). It has been found that treatment of wheat with MFs significantly reduces the negative effects of both cadmium- and lead-induced stress, thereby enhancing resistance to the toxicity associated with these heavy metals (Chen et al., 2017). Given that the catabatic effect of MFs can reduce oxidative stress and maintain membrane integrity. MFs application at both 4 and 7 mT was found to promote the early growth of wheat and bean seedlings, irrespective of the determinantal osmotic and toxic effects of salt (Cakmak et al., 2010). Similarly, it has been reported that MFs with a period of 2.2 and 19.8 s at a magnetic flux of 2.9–4.6 mT has an positive impact plant defense against pathogens, salt tolerance, and senescence (Atak et al., 2007).
There are multiple applications of non-thermal techniques such as US, MW, and MFs for the treatment seeds in order to induce germination and enhance the contents of biologically active components. These non-thermal treatments can be used to enhance seed GR and GE, promote rooting, and increase the leaf area of seedlings; in addition, these techniques can modify the activities of SOD, POD, and CAT enzymes in plants to augment resistance/tolerance to various biotic and abiotic stresses, and thereby enhance the biological yield. Nevertheless, these treatments need to be optimized to ensure effective responses, for although mild treatments can be used to enhance enzyme activities and crop growth, high-strength US, MW, and MFs treatments of seeds and seedlings can cause cell disruption and enzyme inactivation. Accordingly, in order to gain high levels of desirable bioactive substances and yields, considerably more research on the conditions and underlying mechanisms of the aforementioned technologies will be necessary to enable their routine application in promoting cereal germination in agricultural production.
Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No. 31772025), the Anhui Natural Science Foundation (Grant No. 1808085MC93) and the Natural Science Foundation of Higher Education Institutions of Anhui Province (Grant No. KJ2016A061).
