2020 Volume 26 Issue 3 Pages 319-328
Buckwheat is a kind of medicinal and edible crops. Suitable treatment methods can be used to promote germination, shorten germination time, improve the activity of metabolic hydrolase and synthase, and induce the synthesis and enrichment of flavonoids such as rutin. In this paper, the effects of environmental factors (water, temperature, light), ultrasonic, electric field, magnetic field, microwave, laser, and metal ions on the seed germination of buckwheat, the activity of enzymes related to the growth of sprouts, and the nutrient composition of sprouts were reviewed, and mechanisms of action of these treatments on seed germination and sprout growth of buckwheat was discussed. The purpose of this study was to provide technical reference for the development and utilization of buckwheat seeds.
Buckwheat, especially Tartary buckwheat (Fagopyrum tataricum), is rich in flavonoids such as rutin and quercetin and a variety of trace elements (Bonafaccia et al., 2003), which has good antioxidant (Ishiguro et al., 2016) anti-inflammatory, anti-cancer, and anti-tumor effects (Ren et al., 2001; Kim et al., 2007). Modern medicine has proved that long-term consumption of Tartary buckwheat can soften blood vessels, lower blood sugar, blood lipid, blood pressure and enhance human immunity (Bonafaccia et al., 2003; Qu et al., 2013). As a food raw material with a variety of functional ingredients, buckwheat has been widely used in gluten-free bread (Costantini et al., 2014), Tartary buckwheat tea (Guo et al., 2017), and buckwheat vegetables (Kim et al., 2004). Germination treatment is an effective method to improve the bioactive ingredients of buckwheat seeds (Zhou et al., 2015). Nowadays, many researchers are enthusiastic about the various biological effects of ultrasonic wave (Xia et al., 2020), electromagnetic field (Massah et al., 2019), microwave (Wang et al., 2018) and laser (Podlesna et al., 2015) on crop seeds and sprouts during germination, and it will be of great significance if these techniques are properly applied to promote the germination of buckwheat.
During germination, vibrant buckwheat seeds jump from resting state to dynamic state with frequent physiological activities, and the respiration is enhanced, the number and type of enzymes are significantly increased and the enhancement of enzyme activity enables metabolism to proceed at a higher level, which allows a large number of enzymatic reactions to be initiated for biological transformation (Park et al., 2017). Germination can not only improve the digestibility and bioavailability of protein and starch and reduce or eliminate the content of toxic, harmful and anti-nutritive substance in buckwheat seeds (Zhang et al., 2015b), but also can improve the content of some biological active substances such as γ-aminobutyric acid (GABA) (Hao et al., 2016), free amino acids and flavonoids (Zhou et al., 2015), especially rutin in buckwheat sprouts, which has a more significant effect and can be multiplied (Liu et al., 2018). So the nutritional value and biological activity of buckwheat could be significantly improved by germination.
Physical and chemical treatments on buckwheat seeds or sprouts, which has various degrees of impact, can improve the germination capacity, germination potential and enhance the activity of superoxide dismutase (SOD), peroxidase (POD) and other enzymes, so as to resist adverse stress, accelerate metabolism and photosynthesis and promote the production of secondary metabolites (Jeong et al., 2018; Wang et al., 2018). However, some chemical methods are controversial due to some problems such as large dosage of agents, poor degradation and environmental hazards.
In recent years, some new physical technologies, including ultrasonic wave, laser, electric field, magnetic field and microwave, are more popular because of their simple operation, good efficiency, low cost and environmental protection. These methods have good biological effects and show great advantages in improving the germination rate, growth activity, enhancing the activity of enzymes related to growth and metabolism and enriching the bioactive substances of buckwheat. Therefore, many researchers have developed great enthusiasm for these new methods (Wang et al., 2018; Zhou et al., 2012). And the application of these methods has profound significance for the enrichment of bioactive components and the increase of yield of buckwheat. In this paper, the effects of different methods on the seed germination of buckwheat, the activity of enzymes related to the growth of sprouts and the nutrient composition of sprouts were reviewed, and mechanisms of action of these treatments on seed germination and sprouts growth of buckwheat was discussed to provide reference for the development and utilization of buckwheat seeds (especially Tartary buckwheat seeds).
Changes of germination index and enzyme Plant seeds can recognize changes in a variety of environmental signals leading to primary and secondary metabolic regulation and produce significant changes to adapt to various environmental pressures, and water, temperature and light are important environmental factors that regulate the synthesis of secondary metabolites.
The water quality, temperature and light conditions had different effects on the germination of buckwheat. Studies have confirmed that slightly acidic electrolyzed water treatment can increase the activity of glutamic acid decarboxylase (GAD) and phenylalanine ammonialyase (PAL) in sprouts, but has no positive effect on germination rate and bud length (Hao et al., 2016). However, high available chlorine concentration had an adverse effect on sprout growth, but pH did not (Qiao et al., 2019).The germination of Tartary buckwheat should have a relatively suitable temperature range as common buckwheat (Wijngaard et al., 2005), and it has been reported that the activity of rutin glycosidase in bud seedling leaves increased after cold treatment (−5 °C for 5 min, the control group of 25 °C) of Tartary buckwheat (Suzuki et al., 2005). However, light with a certain intensity and time has a significant impact on plant growth (Lee et al., 2014). Researches have shown that the appropriate intensity of ultraviolet (UV), blue light, white light, green light, yellow light, red light and blue light with the combination of the UV treatment can significantly improve the activity of key enzyme [PAL and chalcone isomerase (CHI)] in the process of metabolism of flavonoids (common buckwheat, Nam et al., 2018; Tartary buckwheat, Thwe et al., 2014), increase the content of phenolics, flavonoids and rutin (Lee et al., 2014; Shin et al., 2018; Thwe et al., 2014), and improve the antioxidant activity in buckwheat sprouts. Moreover, the increase of the key enzymes activity related to flavonoids synthesis was positively correlated with the flavonoids content and antioxidant activity in common buckwheat (Nam et al., 2018).
Changes of nutrient content The synthesis of nutrients and active substances in buckwheat seeds would be affected by changes in germination conditions. For example, the contents of albumin, globulin and glutelin in buckwheat nuts increased under certain water shortage condition (30–35% of capillary water capacity) (Pszczółkowska et al., 2010). Hao et al. (2016) found that the content of GABA and rutin in the sprouts of Tartary buckwheat at the initial rooting stage could be increased by slightly acidic electrolytic water treatment (Table 1), and the content reached 143.20 mg/100 g and 739.9 mg/100 g, respectively, which were significantly higher than that in control of 117.01 mg/100 g and 573.7 mg/100 g, respectively. After that, the researchers confirmed that the increase of GABA and GAD in Tartary buckwheat sprouts could be achieved by electrolysis with appropriate slightly acidic water (Qiao et al., 2019).
| Different treatment | Seeds | Treatment condition | Major positive impact (induced by) | Reference |
|---|---|---|---|---|
| Slightly acidic electrolyzed water | Tartary buckwheat | pH 5.95 ± 0.1; ACC 20.25 ± 0.45 mg/L | GAD; PAL; GABA; rutin; Had no significant effect on GR and bud length | (Hao et al., 2016) |
| pH 5.0–6.5; ACC 10–30 mg/L | GABA (Increased along with the ACC); GAD (pH 5.7 best); GR and bud length were not affected by pH, but high ACC had inhibitory effect | (Qiao et al., 2019) | ||
| Cold | Tartary buckwheat | 4 °C for 4 d | Anthocyanins; TFC | (Li et al., 2015) |
| −5 °C for 5 min (bud seedling leaves) | Rutin glycosidase (1.9-fold); quercetin (4.92-fold) | (Suzuki et al., 2005) | ||
| 4 °C for varying periods with white fluorescent light | Maltose (20.6-fold); glutamic acid (2.08-fold); aspartic acid (2.75-fold); quinic acid (9.07 fold); cyanidin 3-O-glucoside (11.3-fold); cyanidin 3-O-rutinoside (6.3-fold); epicatechin (8.6-fold); catechin (5.3-fold) | (Jeon et al., 2018) | ||
| Light | Common buckwheat | Blue (460 nm); red (625 nm), (3.3 V, 1 W per module, 16 h photoperiod) | TFC; TPC; quercetin-3-O-robinobioside (blue, 2.9-fold; red 2.0-fold); rutin (blue, 2.8-fold; red, 2.0-fold) | (Nam et al., 2018) |
| Tartary buckwheat | Red (660 nm); blue (470 nm) white (380 nm); 50 µmol/m2, 16 h photoperiod | Catechin (9.6-, 43.1-fold under blue light higher than blue and white); cyanidin 3-O-rutinoside (2.8-, 10.6-fold under red light higher than white and blue) | (Thwe et al., 2014) | |
| Blue (15 W), UV (16 W, GL–9406), blue + UV, 12h | TFC (1.24–1.56-fold); rutin; quercetin; PAL (1.18–1.76-fold); CHI (1.20–2.74-fold) | (Ji et al., 2016) | ||
| Ultrasonic | Common buckwheat | 40 KHz, 480 W, 24 °C, 10–20 min | GR (reached 94%); TFC(reached 156.70 mg/g); TPC (180.8 µg gallic acid/mL) DPPH radical scavenging rate | (Zhang et al., 2015a) |
| Tartary buckwheat | 40 KHz, 280 W, 20 °C, 35 min | GR (1.71-fold) | (Wang and Wang, 2017) | |
| 40 KHz, 240 W, 15 °C, 35 min | RSC (1.40-fold) | |||
| 40 KHz, 280 W, 30 °C, 30 min | TFC (1.70-fold); DPPH scavenging rate | |||
| Electromagnetic field | Tartary buckwheat | 0.3 T, 30 min | PAL (1.19-fold); TFC (1.43-fold) | (Zhou et al., 2012) |
| 0.2 T, 30 min | CHI (1.13-fold) | |||
| Common buckwheat | 50 Hz, 30 mT, 8 s | initial GR of long-stored seeds | (Ciupak et al., 2007) | |
| Microwave | Tartary buckwheat | 600 W, 10 s | GR (2-fold); RSC (1.17-fold) | (Wang et al., 2018) |
| 600 W, 30 s | TFC (1.56-fold); DPPH radical scavenging rate | |||
| 800 W, 10 s | Free amino acid (1.72-fold) | |||
| Laser | Tartary buckwheat | 630 nm, 4 mW/m2, falling moment | GR (increased but not significantly) | (Ciupak and Gladyszewska, 2006) |
Note: ACC, available chlorine concentration; GR, germination rate; TFC, total flavonoids content; TPC, total phenol content; RSC, reducing sugar content; GAD, glutamic acid decarboxylase; GABA, γ-aminobutyric acid; PAL, Phenylalanine ammonia-lyase; CHI, Chalcone isomerase.
Studies have also confirmed that the biosynthesis of flavonoids such as anthocyanin and proanthocyanidins will be affected under environmental stress such as drought, low temperature and ultraviolet radiation (Luo et al., 2017; Suzuki et al., 2005). After cold treatment (Table 1) of Tartary buckwheat, most sugars and their derivatives were increased significantly, and the content of anthocyanin and procyanidin were also significantly increased (Li et al., 2015). However, the content of some amino acids (glycine and isoleucine) and their derivatives decreased (Jeon et al., 2018).
Light is more beneficial to the seed germination, seedling growth and flavonoid accumulation of buckwheat than dark conditions (Lee et al., 2014; Shin et al., 2018; Nam et al., 2018). Study has shown that the content of rutin, free amino acids and vitamin C in common and Tartary buckwheat sprouts under light condition was higher than that under dark condition (Kim et al., 2006). In addition, Luo et al. (2015) showed that the type of light source affected the distribution and content of flavonoids in cotyledon and hypocotyl, and the flavonoid content was significantly correlated with the expression of FtPAL, FtCHS, FtF3H and FtANS (the correlation coefficient was greater than 0.75), with the highest correlation with FtPAL (the correlation coefficient was 0.921). The content of total flavonoids in the hypocotyl in particular was only significantly correlated with the expression of Ft4CL (the correlation coefficient was 0.975). The combination of UV-B, LED blue, and red was more suitable for regulating the flavonoid content in Tartary buckwheat at the budding stage (Luo et al., 2015).
Mechanism of action Water environment, which is involved in seed infiltration, material transport, reaction media, energy metabolism and other functions, affects the metabolic rate of sprout growth and the activity of hydrolases and biosynthases, thus leading to the change of seed germination potential and germination rate, as well as the change of nutrient content in the germination process of sprout (Koller and Hadas, 1982). While temperature mainly affects the activity of various enzymes involved in metabolism, and too high temperature will lead to enzyme inactivation, growth metabolic disorder or even stop growth (Peterson et al., 2007). The structure and permeability of seed coat will be changed under high temperature, and the activity of key enzymes in the metabolism of phenylpropane will be reduced, thus hindering the synthesis of flavonoids such as rutin. In addition, the expression of FtMYB9 in seedling stage could be induced by cold stress and drought stress, and the overexpression of FtMYB9 enhances tolerance to cold stress and drought stress by activating some stress-related genes (Gao et al., 2017). The accumulation of several phenylpropanoid biosynthetic transcripts (one or more mature mRNAs for coding proteins formed by transcription of a gene) in cold-treated buckwheat were determined by Jeon et al. (2018), and the expression of most phenylpropanoid biosynthetic transcripts except FtDFR was promoted after cold treatment. The increase of flavonoids content under light treatment may be related to the activation of FtPAL, FtCHS, FtF3H, FtANS, and other genes (Luo et al., 2015), which can promote the activity of PAL, CHI, and other related enzymes in phenylpropane metabolic pathway. In addition, under light treatment, the improvement of DPPH free radical scavenging rate and antioxidant effect of Tartary buckwheat sprouts was realized by the increase of flavonoids content (Shin et al., 2018). Finally, studies have suggested that the increase of flavonoid content and related enzyme activity may be related to the enhancement of anti-stress system of Tartary buckwheat bud germination (Suzuki et al., 2005).
Changes of germination index and enzyme In recent years, ultrasonic wave, a kind of elastic mechanical wave with vibration frequency greater than 20 KHz, has been widely used in biological science field as a pollution-free physical treatment method. Ultrasonic wave, different intensity and time have different biological activity effects on organisms, can improve the hydration process of seeds, at the same time, promote the breaking of seed shell, reduce the time needed for seed germination, and facilitate germination (Miano et al., 2015). Studies have confirmed that the germination rate of common buckwheat seeds and the initial germination rate and final germination rate of Tartary buckwheat seeds can reach 94%, 88%, and 100% respectively after ultrasonic treatment (Table 1, Zhang et al., 2015a; Wang and Wang, 2017).
The growth of biological cells can be promoted and suppressed and even killed by ultrasonic treatment, which is related to the frequency, intensity, and treatment temperature of ultrasonic waves. The production of reactive oxygen species (ROS) is stimulated by ultrasonic treatment, forcing the expression of PAL in Taxus yunnanensis cell (Hasan et al., 2017), SOD, and POD in aged grass seeds of tall fescue and Russian wildrye (Liu et al., 2016) to increase, thus the production of secondary metabolites such as flavonoids is promoted, preventing the accumulation of ROS. Kwon and Park (2018) demonstrated that ultrasonic treatment of seeds could increase the alpha-amylase activity of barley malt. Moreover, Xia et al. (2020) showed that ultrasonic stimulation increased amylase activity, accelerated starch hydrolysis and increased reducing sugar content during the germination of wholegrain Oryza sativa L.. Currently, there are few studies on ultrasonic treatment (Wang and Wang, 2017; Zhang et al., 2015a) of buckwheat germination, and no studies have been conducted on the enzyme changes of buckwheat during the germination process by ultrasonic treatment.
Changes of nutrient content Wang and Wang (2017) showed that the reducing sugar content of Tartary buckwheat sprouts (4-day-old) was up to 11.24 g/100 g under certain ultrasonic conditions (Table 1). And under another ultrasonic condition, the content of total flavonoids in Tartary buckwheat sprouts (6-day-old) reached 9.46 g/100 g, which increased by 228.07% and 69.71%, respectively, compared with that of seeds and controls, and under this condition, DPPH free radical scavenging rate of sprouts reached 86.47%. According to the research of Zhang et al. (2015a), the quality of buckwheat sprouts (9-day-old) was improved, at the same time, rutin content and antioxidant activity was enhanced after ultrasonic pretreatment. And in a word, the best effect was achieved after 20 min treatment, with the total phenol content reaching 180.8 µg gallic acid/mL, the total flavonoids content reaching 156.701 mg rutin/g, and the DPPH free radical scavenging capacity reaching the maximum (0.197 mg trolox eq/g freeze-dried sample). All these results indicated that suitable ultrasonic treatment could effectively promote the increase of biological active components in Tartary buckwheat sprouts.
Mechanism of action Different physiological effects on organisms, such as thermal effects, mechanical effects and chemical effects, can be produced by different ultrasonic intensity and action time in a short period of time (Hughes and Nyborg, 1962). And changes in molecular structure, caused by ultrasonic vibrations, allow enzymes to be released from cell walls and improve metabolism (Goussous et al., 2010). After appropriate ultrasonic treatment, microflow is generated in the cell, which makes intracellular matrix eddy current, and the permeability of cell membrane and cell wall is increased, which promotes the substrate to enter the enzyme-catalyzed part and the product to enter the medium (Miano et al., 2015), thus improving the catalytic efficiency of enzyme and enhancing the metabolism capacity of cells. After ultrasonic treatment, the production of free radicals can be increased, which allows many biochemical reactions to occur in plant cells. In addition, cavitation (Hughes and Nyborg, 1962) and vibration of ultrasonic wave make the seed coat of plant seeds soften to increase membrane permeability, which is conducive to water absorption, and the cellulose layer becomes loose, which leads to the easier binding of enzyme molecules with substrates, thus enhancing the reaction rate of enzymes, so as to improve the germination rate of seeds and promote the growth of sprouts (Yaldagard et al., 2012). Therefore, it is of great significance to study the effect of ultrasonic treatment on the growth of buckwheat, especially Tartary buckwheat, seeds and its influencing mechanism.
High voltage electrostatic field, magnetic field and electromagnetic field, which are important physical technologies used in agriculture at present, are widely concerned by scholars because of their simple operation, rapid and environmental protection.
Changes of germination index and enzyme Additional electromagnetic field processing of plant seeds with appropriate intensity can improve the activity of related enzymes, increase the respiration rate of seeds during germination, shorten the germination time of wheat seeds (Cirkovic et al., 2017), promote the growth of roots (Arturo et al., 2010), and significantly improve the germination rate and fresh weight of wheat seeds (Massah et al., 2019). And high voltage electrostatic field and magnetic field treatment can significantly improve the seed germination vigor of wheat (Payez et al., 2013), soybean (Cakmak et al., 2010) and corn (Arturo et al., 2010). Pittman and Ormrod (1970) found that the magnetic treatment slowed the rate of respiration of winter wheat seeds before germination, but accelerated growth for the initial 16 h. However, electrostatic field and magnetic field are seldom used in the germination of buckwheat. For germination rate, Ciupak et al. (2007) showed that electromagnetic treatment did not increase the final germination percentage of buckwheat seeds. Zhou et al. (2012) found that the activity of PAL in Tartary buckwheat sprouts reached its highest after the magnetic field treatment of 0.3 Tesla (T), but the activity of rutin degradingenzymes (RDEs) was not affected by the condition. In addition, 0.2 T treatment of magnetic field intensity maximized the activity of CHI (Table 1). And these indicate that different magnetic field strengths have different induction effects on the same enzyme and the same intensity has different induction effects on different enzymes. The effects of electric and magnetic fields on the germination index and enzyme changes of Tartary buckwheat need to be further explored by researchers.
Changes of nutrient content Pittman and Ormrod (1970) found that the magnetic field treatment accelerated the increase of moisture and reducing sugar content in winter wheat during the initial 72 h of germination. And researchers (Yao and Shen, 2018) found that magnetic water treatment had certain effects on reducing starch and protein and increasing reducing sugar content during Tilia miqueliana M. seed germination. Moreover, magnetic field pre-treatment significantly increased the total nitrogen content in wheat sprouts (Katsenios et al., 2015). Therefore, it is of great significance to study the effect of magnetic field on the change of components during seed germination. However, the research on the effect of magnetic field on the germination of buckwheat is limited to Zhou et al. (2012). Zhou et al. (2012) found that after the magnetic field intensity of 0.3 T and germination for 5 days, the total flavonoids content in Tartary buckwheat sprouts significantly increased to 62.90 mg/g. This indicates that magnetic field stimulation has a certain induction effect on the accumulation of nutrients in Tartary buckwheat sprouts such as flavonoids. At present, there are few studies on the changes of nutrients in buckwheat affected by electric field and magnetic field, which need to be further explored by researchers.
Mechanism of action When plant seeds absorb energy from electric and magnetic fields, a large amount of heat is generated, resulting in rapid increase of intracellular temperature and changes in molecular structure of partial key fracture (Pittman et al., 1979), leading to changes in cell structure and biochemical characteristics, and increase of free radical concentration in biofilm and seed coat permeability (Zheng and Xu, 2010). As a result, the transportation of water, oxygen and other substances is accelerated (Pittman et al., 1979; Zheng and Xu, 2010), the germination potential and the germination rate of seeds are improved (Cirkovic et al., 2017), and the stress resistance of plants would be induced to protect against electromagnetic-induced immune function injury (Zhang et al., 2016). All in all, electric and magnetic fields with different properties and intensities have different effects on plant seeds. And compared with the constant magnetic field, electromagnetic fields with the same intensity and time have more significant effects on seed germination (Payez et al., 2013).
At present, the application of microwave extraction and microwave heating (Kishimoto, 2019), as well as the influence of microwave on seed germination of plant are more, but the influence on buckwheat seed germination is less.
Changes of germination index and enzyme Microwave, as a non-thermal processing method, can change the biological macromolecular structure of plant seeds (Deng et al., 2015), thus affecting the physiological and biochemical characteristics during seed germination. Some studies have suggested that microwave can promote the growth of sprouts but may have a delayed effect on germination rate (Monteiro et al., 2008). Other researchers believe that corriander seeds grow faster in microwave water than in normal water (Jaffer et al., 2017) and the length of primary shoot and root of rice seeds were promoted by microwave (Talei et al., 2013). However, Bigu-Del-Blanco et al. (1977) found that the germination rate of corn seeds was significantly inhibited after microwave irradiation, which may be related to the water loss in seeds caused by too long irradiation time. Moreover, the germination rate of quinoa was significantly decreased after microwave drying with 0.875–7 W/g (Maqueda et al., 2018). Wang et al. (2018) cultured Tartary buckwheat seeds irradiated with 600 W microwave for 10 s for 7 days had a higher germination rate (Table 1). However, the germination rate of Tartary buckwheat seeds irradiated with 800 W microwave for 30 s for 7 days was only 10%, which was significantly reduced by 87% compared with the control group. The researchers' differences in the effect of microwaves on seed germination rates may be due to the intensity and timing of their use. Suitable microwave power irradiation for a certain time can significantly improve the seed germination rate of Tartary buckwheat, possibly because microwave irradiation activates the activities of enzymes needed for the growth and development of Tartary buckwheat, such as CAT and SOD (Kowalski and Lukasiewicz, 2017; Wang et al., 2018). At present, there are few studies on the effects of microwave on various enzyme activities of Tartary buckwheat during the germination process (Wang et al., 2018), and there is still a lot of room for research.
Changes of nutrient content Previous studies were basically limited to the effect of microwave on seed germination characteristics, and the effect of microwave on nutrient changes during seed germination was limited to Wang et al. (2018), although these studies were urgently needed due to the importance of finding an economical and efficient method to promote the germination products of buckwheat. The study of Wang et al. (2018) showed that proper microwave treatment had positive effects on reducing sugar, total flavonoids, free amino acid content and DPPH radical scavenging rate in Tartary buckwheat (Table 1). Microwave irradiation may increase the total flavonoids content by stimulating the increase of PAL, CHI, and other enzymes related to phenylpropane metabolism (Zhou et al., 2012). However, too high microwave irradiation power or too long irradiation time may lead to inactivation of some enzymes closely related to metabolism, so as to reduce the germination rate or even inhibit the transformation of substances during germination (Wang et al., 2018).
Mechanism of action In fact, the mechanism of microwave effect on seed germination is not clear. After the radiation of microwave, a kind of electromagnetic waves, water, protein, nucleic acid, carbohydrate, fat, and other polar substances in plant cells absorb electromagnetic energy, and the bond between proteins, nucleic acid, and other biological macromolecules may be changed (Nawaz et al., 2018), which leads to the change of the morphological characteristics of plants (Bigu-Del-Blanco et al., 1977). It has been reported that microwave irradiation can cause cracks in seed coat (Tran, 1979) and soften seeds (Nelson, 1976). Therefore, seed germination can be promoted by microwave with appropriate irradiation intensity and irradiation time (Wang et al., 2018), while too high microwave intensity and too long treatment time will inhibit seed germination of Tartary buckwheat (Manickavasagan et al., 2007). However, the mechanism of the effect of microwave on seed germination is not clear and needs to be explored. When microwave is widely applied to buckwheat germination, it will provide a convenient and efficient means for improving buckwheat crops and enriching buckwheat flavonoids.
Changes of germination index, enzyme and nutrient content Recently, laser radiation has been applied to biological systems to activate growth processes. By pretreating plant seeds with appropriate laser, seed dormancy is broken (Parera and Cantliffe, 2010), and germination rate, seedling growth, and vigor index of pea seeds (Podlesna et al., 2015) and wheat seeds (Jamil et al., 2013) can be enhanced, at the same time, alpha-amylase and protease activities activity of wheat seeds can also be improved. Studies have confirmed that the germination rate, plant height, leaf surface area and evenness of sunflower seeds were improved under the irradiation of He-Ne laser, which accelerated seed maturation (Perveen et al., 2010). In addition, research shows that after He-Ne laser pretreatment, the concentration of the amylase and the content of indole-3-acetic acid (IAA) in pea seeds and seedlings were increased (Podlesna et al., 2015). However, Ciupak and Gladyszewska (2006) showed that the effect of laser treatment on the germination rate of buckwheat was not significant, although there was a certain degree of promotion. In conclusion, the research on laser treatment of buckwheat is limited to that of Ciupak and Gladyszewska (2006), and the promotion effect of laser on other seeds is expected to be applied to buckwheat.
Mechanism of action As an external physical factor, laser irradiation can promote the growth and development of seeds when the dose is appropriate. The mechanism may be that He-Ne laser irradiation changes the cell membrane function of seeds, and even damages or destroys the structure, leading to increased permeability, which leads to enhanced electrolyte extravasation and the destruction of seed structure (Podlesna et al., 2015).However, low doses of laser can also repair the membrane system of seeds (Bityurin et al., 1996), which can effectively promote seed germination.
Changes of germination index, enzyme and nutrient content Different metal ions have different effects on seed germination and growth. Firstly, selenium treatment can alleviate the damage of UV radiation to buckwheat seedlings and promote their growth (Breznik et al., 2005). Secondly, low concentrations of Al3+, Cu2+, and Zn2+ solutions can respectively stimulate the germination of Tartary buckwheat and improve the germination potential and germination rate (Wang et al., 2013). Wang et al. (2013) also showed that the treatment of Tartary buckwheat with Al3+, Cu2+, and Zn2+ solutions promoted the accumulation of flavonoids and the activity of PAL and CHI, whereas trypsin inhibitor activity decreased. In addition, proper amount of trace element water (30% mineral salt) can improve the content of copper, zinc, manganese, and iron and their antioxidant activity in common buckwheat sprouts, but the effect on the content of rutin is not obvious (Liu et al., 2007). Therefore, suitable metal ion treatment can effectively promote the germination of buckwheat.
Mechanism of action The influence of metal ions on the seed germination of Tartary buckwheat was promoted by low concentration (Wang et al., 2013), while high concentration inhibited or even poisoned it (Matsumoto, 2002), which was different due to different metal ions. A variety of effects of metal ions, such as regulating osmotic balance and increase cell permeability, as enzymes cofactors (Mitić et al., 2013), and form across the membrane potential (Kitasato, 2003), may promote or inhibit seed cell division and elongation, stimulate or inhibit the activity of related enzyme, protein synthesis, photosynthesis and respiration, which effect the growth and development. The mechanism of different metal ion treatment on different parts of buckwheat seeds during germination needs to be further studied.
In addition to the above, the addition of natural and chemical mutagenic agents can regulate plant growth (Guo et al., 2019). Natural plant hormones IAA and gibberellin (GA) improved common buckwheat growth indicators (plant height, root length, and fresh weight) (Park et al., 2017). And the content of rutin and total flavonoids in common buckwheat sprouts treated with 0.1–1.0 mg/L IAA, 0.1 mg/L GA and 0.5 mg/L GA, respectively, were increased to different degrees (Park et al., 2017). Moreover, the addition of exogenous L-phenylalanine (L-phe) (Seo et al., 2015) and sucrose (Jeong et al., 2018) not only promoted the synthesis of phenolic compounds (Seo et al., 2015), but also improved the nutritional content such as vitamins, tyrosine, flavonoids, and rutin, improved PAL activity and antioxidant effect in sprouts (Jeong et al., 2018). As a precursor of phenylpropane metabolic process, the addition of a certain amount of L-phe can promote the synthesis of phenylpropane metabolic reactant and contribute to the accumulation of flavonoids (Seo et al., 2015). All in all, IAA, GA, L-phe, and sucrose are useful for promoting the germination of buckwheat seeds and improving the content of flavonoids in sprouts, which may be related to some important pathways such as the presence of certain signal factors that stimulate specific genes and generate related enzymes, which promote the metabolism of phenylpropane (Burbulis and Winkel-Shirley, 1999; Jeong et al., 2018).
Germination is a process of hormone metabolism and the increase of the types, quantity and activity of various enzymes. Different treatments have different effects on seed germination, which may be related to increasing seed transdermal permeability, activating enzyme activity necessary for seed germination, and regulating hormone metabolism. All in all, the germination potential and germination rate of seeds, the activity of various hydrolytic enzymes, synthases, antioxidant enzymes and other related enzymes in the process of plant metabolism, and the content of nutrients in sprouts can be promoted by appropriate treatment conditions, such as water, temperature, light, ultrasonic, electric field, magnetic field, microwave, laser, and metal ions. But excessive intensity of treatment can also lead to cell rupture, enzyme inactivation and metabolic disorders and other adverse phenomena. Nevertheless, there is still a long way to go before emerging technologies are applied to promote the germination of buckwheat to obtain bioactive substances with high yield and high quality, and the mechanism of these technologies and the enrichment mechanism of bioactive ingredients in seeds still need to be further explored.
Acknowledgements This work 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).
