Recent developments in physical invigoration techniques to develop sprouts of edible seeds as functional foods

Hassan, Sadia; Zeng, Xin-An; Khan, Muhammad Kamran; Farooq, Muhammad Adil; Ali, Amjad; Kumari, Ankita; Mahwish,; Rahaman, Abdul; Tufail, Tabussam; Liaqat, Atif

doi:10.3389/fsufs.2022.997261

REVIEW article

Front. Sustain. Food Syst., 23 September 2022
Sec. Sustainable Food Processing
Volume 6 - 2022 | https://doi.org/10.3389/fsufs.2022.997261

Recent developments in physical invigoration techniques to develop sprouts of edible seeds as functional foods

Sadia Hassan¹^†

Xin-An Zeng^2,3,4^*

Muhammad Kamran Khan⁵

Muhammad Adil Farooq⁶^*

Amjad Ali⁷

Ankita Kumari^2,3,4

Mahwish⁸

Abdul Rahaman^2,3,4^*^†

Tabussam Tufail⁹

Atif Liaqat⁶

¹Department of Nutrition and Dietetics, The University of Faisalabad, Faisalabad, Pakistan
²School of Food Science and Engineering, South China University of Technology, Guangzhou, China
³Guangdong Key Laboratory of Food Intelligent Manufacturing, Foshan University, Foshan, China
⁴Overseas Expertise Introduction Centre for Discipline Innovation of Food Nutrition and Human Health (111 Centre), Guangzhou, China
⁵Faculty of Life Sciences, Institute of Home and Food Sciences, Government College University, Faisalabad, Pakistan
⁶Faculty of Engineering and Technology, Institute of Food Science and Technology, Khwaja Fareed University Engineering and Information Technology, Rahim Yar Khan, Pakistan
⁷Department of Agriculture and Food Technology, Karakurum International University, Gilgit, Pakistan
⁸Institute of Home Sciences, University of Agriculture, Faisalabad, Pakistan
⁹Institute of Diet and Nutritional Sciences, The University of Lahore, Lahore, Pakistan

For nutritional security, the availability of nutrients from food sources is a crucial factor. Global consumption of edible seeds including cereals, pulses, and legumes makes it a valuable source of nutrients particularly vitamins, minerals, and fiber. The presence of anti-nutritional factors forms complexes with nutrients, this complexity of the nutritional profile and the presence of anti-nutritional factors in edible seeds lead to reduced bioavailability of nutrients. By overcoming these issues, the germination process may help improve the nutrient profile and make them more bioavailable. Physical, physiological, and biological methods of seed invigoration can be used to reduce germination restraints, promote germination, enhance early crop development, to increase yields and nutrient levels through sprouting. During sprouting early start of metabolic activities through hydrolytic enzymes and resource mobilization causes a reduction in emergence time which leads to a better nutritional profile. The use of physical stimulating methods to increase the sprouting rate gives several advantages compared to conventional chemical-based methods. The advantages of physical seed treatments include environment-friendly, high germination rate, early seedling emergence, uniform seedling vigor, protection from chemical hazards, and improved yield. Different physical methods are available for seed invigoration viz. gamma irradiation, laser irradiation, microwaves, magnetic field, plasma, sound waves, and ultrasonic waves. Still, further research is needed to apply each technique to different seeds to identify the best physical method and factors for seed species along with different environmental parameters. The present review will describe the use and effects of physical processing techniques for seed invigoration.

Introduction

Edible seeds including cereals, pulses, and legumes are globally used as a staple food and possess several functional properties like antioxidant, antidiabetic, anticancer, and antitumor effects. They are a rich source of vitamins, minerals, and fiber and also contain enough amount of bioactive components such as phenolics, carotenoids, lignin, β-glucan, inulin, resistant starch, sterols, and phytates. According to several studies, the controlled germination process is a valued technique to improve the nutritional and medicinal values of edible seeds (Hayat et al., 2014; Verspreet et al., 2015; Özer and Yazici, 2019) (Figure 1). Sprouts are germinated seeds of cereals, pulses, and legumes that grow into seedlings, and are characterized by nutrient bioavailability and their profile including phenolic profile, antioxidant profile, vitamins, minerals, along with other micro and macronutrients. As compared to the un-sprouted grains the sprouted grains are considered an important functional ingredient due to having major nutritional, textural, and tasteful, advantages. The edible sprouts can help to provide essential nutrients, maintain health status, and to prevent disease. Currently, interest in the use of sprouted grains as functional ingredients and food is growing with increased interest from food researchers, nutritionists, producers, and consumers (Aloo et al., 2021; Pires et al., 2021).

FIGURE 1

Figure 1. Schematic diagram of the germination process.

The germination process can improve the levels of simple sugars, free amino acids, and organic acids through the catabolism of macronutrients like carbohydrates, protein, and fatty acids (Wang et al., 2005; Shi et al., 2010; Benincasa et al., 2019). It can also decrease different anti-nutritional factors and indigestible components, like lectin and protease inhibitors (Saithalavi et al., 2021). Moreover, through germination secondary metabolites such as vitamin C and polyphenols can accumulate in edible seeds (Toro et al., 2021).

Sprouting is the crucial phase in the plant at which fast germination and seedling emergence are vital aspects that can be stimulated in seeds (Sharififar et al., 2015; Lai et al., 2016). Germination stimulation has been achieved through several methods such as fertilizers, light, seed scarification, seed stratification, salinity, temperature, humidity, and regulatory hormones (Rifna et al., 2019b). As a substitute for chemical invigoration techniques for plant growth stimulation, the use of physical methods attracts more and more attention. The physical techniques can improve food quality without imparting any safety concerns, thus the applications of these methods have increased to affect plant growth and germination (Bose et al., 2018).

Now sprouted grains are becoming popular and consumed all over the world. To increase the nutritional value of foods and to improve the sensory properties of sprouted grains flour is used in different products. Despite several efforts to explore the utilization of sprouted grains, limited products have been produced and introduced into the market. Thus, it is required to conduct more research and incorporate seed sprouts into different food industries to introduce them into the food chain (Liu et al., 2017; Yilmaz et al., 2020).

Therefore, different physical methods have been developed to facilitate the germination process and to improve the plants' growth and production. This paper gives a brief overview of such physical processing technologies including gamma irradiation, laser irradiation, microwaves, magnetic field, plasma, sound waves, and ultrasonic waves. Additionally, mechanisms of seed germination promotion by processing treatments and how they impact germination have also been discussed.

Physical seed invigoration techniques

Throughout the transient activation of the pre-germination metabolic activities, seed priming is a well-known and established way to improve seed quality (good nutritional composition along with reduction of seed dormancy, the breakdown of the complex of anti-nutrients, and the release of nutrients and their improved bioavailability) (Chakraborti et al., 2021). Under adverse environmental circumstances (abiotic stresses like water deficit, high salinity, high temperature, submergence, etc.) seed priming has arisen as a constructive way of improving seed vigor, germination synchronization, and seedling growth (Marthandan et al., 2020). A variety of priming techniques is available, and some of them are properly categorized such as hydro-priming and osmo-priming. As compared to osmo-priming and chemical-based conventional treatments, physical processing techniques have shown several advantages. Due to their less damaging effects on the environment like anthropogenic changes in the soil, water, and atmosphere, recently the use of physical methods for plant growth stimulation is becoming more popular (Table 1) (Bilalis et al., 2012; Rifna et al., 2019a). In chemical-based methods, the required chemical compounds are directly injected into the cell while in physical methods, energy is introduced into the cell which generates conditions for different transformations at a molecular level (Govindaraj et al., 2017). Different positive biological changes can be introduced in plants without influencing their biology through the application of various physical factors. These physical techniques decrease the on-farm pollution of raw materials, minimize the requirement for fertilizers, and can also be used for the disinfection of seeds (Table 2) (Bera et al., 2021). All biological activities depend on the exchange of energy between the cell and the environment. Improving the germination and yield of crops by using energy is an advanced area in research. Energy treatment stimulates the enzymatic reactions leading to the initiation of physiological and biochemical changes. All these changes are an indication of plant growth and development processes which eventually improve the quality and yield of produce (Govindaraj et al., 2017).

TABLE 1

Table 1. Different physical techniques used for the seed invigoration.

TABLE 2

Table 2. Advantages and limitations of novel processing techniques.

Ultrasound seed processing

Ultrasonic waves (US) are mechanical waves having a frequency higher than 20 kHz and cannot be detected by a human audition system. This technology has been successfully used in different mass transfer processes of food including drying, extraction, osmotic dehydration, desalting, and hydration (Miano et al., 2016; Asfaram et al., 2019). Nowadays to break the seed dormancy ultrasonic waves have attracted the researcher's attention as being a safe, easy, and time-saving technique (Ramteke et al., 2015; Liu et al., 2016). In recent years the mechanism of ultrasonic wave activity on seed germination in different plant species has been explored as mentioned in Table 3. In ultrasonic treatment, seeds are placed into an ultrasonic wave emitting apparatus in which water is used as a medium (Nazari and Eteghadipour, 2017). For seed germination, oxygen availability and water uptake are the essential parameters, so ultrasonic waves alter the seed's characteristics through which these factors become available more efficiently (Liu et al., 2016).

TABLE 3

Table 3. Effect of ultrasonic waves on seed germination (%).

Ultrasonic waves in water lead to cavitation, a phenomenon creating micro-bubbles in water that exerts mechanical pressure on the seeds. Mechanical pressure exerted by the cavitation process further causes fluidity of the cell wall and the formation of cracks and micropores in it (da Silva and Dobránszki, 2014; Rifna et al., 2019a). A study conducted on mung beans showed an increase in their porosity after ultrasonic treatment. Seeds become more porous for water and oxygen exchange due to the production of micro-pores and micro-cracks. It was demonstrated that ultrasound technology improved the hydration process of mung beans, reducing the total process time by almost 25% [increasing the water absorption rate to ~44%] (Miano et al., 2016). In studies, conducted by Yaldagard et al. (2008) and Sharififar et al. (2015) it has been revealed that ultrasonic treatment increases the hydration process in seeds, therefore leading to an increase in enzymatic reactions especially related to alpha-amylase. Thus, the starch hydrolysis conducted by alpha-amylase has resulted from an increment in seed germination speed and percentage (Yaldagard et al., 2008; Sharififar et al., 2015). A schematic presentation of ultrasonic wave treatment in seeds is shown in Figure 2.

FIGURE 2

Figure 2. Schematic diagram of the ultrasound seeds processing.

Through several studies, it has been investigated that ultrasound is a promising method to break seed dormancy and enhance germination. A summary of the findings regarding the effects of ultrasound treatment on the seed germination percentage of different edible seeds has been provided in Table 3. Yaldagard et al. (2008) reported a 6% increase in germination percentage of barley (Hordeum vulgar L.) seeds after ultrasound waves' treatment as compared to control. Application of sonication treatment on Norway spruce (Picea abies L.) Karsten seeds increased germination by 22% (Rîşca and Fártáiş, 2009). Goussous et al. (2010) showed that ultrasonic waves' application to chickpeas (Cicer arietinum), wheat (Triticum aestivum), and watermelon (Citrullus vulgaris) increased their germination percentage by 36, 2, and 2%, respectively, in comparison to control. Another investigation conducted by Aladjadjiyan (2011) revealed a 4 and 6% increase in the germination of wheat (Triticum aestivum) and lentils (Lens culinaris, Med.), respectively. According to an investigation by Wang et al. (2012) on switchgrass (Panicum virgatum L.) seeds, sonication enhanced germination by up to 23.2%.

The application of ultrasonic waves on sunflower (Helianthus annuus L.) seeds, enhanced their germination maximum by up to 43.38% (Machikowa et al., 2013). Another in vitro study conducted on snail medick [Medicagoscutellata (L.) Mill] seed indicated that ultrasound increased germination up to 63.3% (Nazari et al., 2014). In a study conducted on peas (Pisum sativum), as compared to control, sonication treatment caused a 13.1% increase in seed germination. The operating parameters were time and temperature at specific input power. Pea seeds were subjected to an ultrasonication treatment of 40 kHz for 1 min at 25 celsius (Chiu and Sung, 2014). A similar study conducted by Sharififar et al. (2015) showed that ultrasound treatment applied to big saltbush (Atriplex lentiformis), cumin (Cuminum cyminum), and caper beans (Zygophyllum eurypterid) significantly increased their germination percentage up to 28, 36, and 35.7%, respectively, in comparison to control treatment. Overall in agreement with the positive effects of ultrasonic waves studied in most cases of tested species, it's clear that ultrasonic waves can affect seed germination positively.

Microwave seed treatment

The microwave component of the electromagnetic spectrum includes radiation having a frequency within the range of 300 MHz to 300 GHz and wavelength ranging between 1 m down to 1 mm. Now it is recognized that microwave radiation (MWs) after absorption as non-ionizing electromagnetic radiation causes different changes in biological systems which are mostly thermal and non-invasive (Bera et al., 2021). MWs can induce various biological changes depending on different factors such as field strength, frequencies, waveforms, modulation, and duration of exposures (Vian et al., 2006). Mostly the effect of MWs on humans and animals was extensively studied and addressed, while there is a very small number of studies related to the effect of MWs on plants (Jayasanka and Asaeda, 2013). Most of the available work described the effect of radiations having the 2.45 GHz frequency, which is absorbed in living cells through water molecules (Creţescu et al., 2013). After absorption into living cells and tissues, MW radiations induce ionic movement, and dipole rotation leading to the deformation of the electron orbits which finally causes a fast and selective heating process (Mullin, 1995; Rifna et al., 2019a). The schematic diagram of the experimental set-up for microwave treatment is given in Figure 3.

FIGURE 3

Figure 3. Experimental set-up for microwave exposure.

MWs treatment can also result in electronic transitions between different rotational sublevels. In the organic molecules between vibrational levels transitions mostly occur in near Infrared (IR) regions (750 to 1,300 nm) of the electromagnetic spectrum, while between rotational levels occur in far IR regions and near microwave regions (1 mm to 1 m) (Al Mashhdani and Muhammed, 2016). Intermediated frequency levels of radiation (2,450 MHz) applied to seeds showed higher enzymatic reactions and increased growth rates. Still, the mechanism of MWs is not as yet fully understood but according to Rajagopal (2009) exposure to microwaves, 2.45 GHz and 650 W for 30 s are enough to ensure a high germination rate. In several studies, lethal level MWs have been used for preventing the growth of weeds in the soil while non-lethal level MWs treatments have been widely used for seed decontamination before sowing (Scialabba and Tamburello, 2002; Knox et al., 2013; Sahin, 2014). Application of MWs caused heating of soil up to 80°C which results in suppressed germination of the weed. Thus, in greenhouses from horticultural/ornamental plant nurseries, MWs appeared as an effective non-chemical alternative method for weed management (Velázquez-Martí et al., 2006).

There is very limited literature about the use of MWs radiations as seed stimulation treatment in a few plants, affecting their germination performance. Through different studies, it was investigated that application of 2.45 GHz MWs radiation has no major influence on seed germination, but in some plant species including wheat, green gram, moth bean, and Bengal gram, it showed a beneficial effect on biomass accumulation and growth (Jakubowski, 2010; Talei et al., 2013). A summary of the findings of the effects of microwave treatment on seed germination of the mentioned plant species has been provided in Table 4. The effect of microwave irradiation at 935.2–960.2 MHz with intensities of 0.07–0.15 mW/cm² on maize grains was studied and revealed a clear increase in germination and seedlings development (Khalafallah and Sallam, 2009). Aladjadjiyan (2010) conducted an experiment stating that microwave pretreatments with frequency 2.45 GHz for 5, 10, 15, 20, and 25 s and seeds showed enhanced germination parameters as compared with controls. Best results were obtained with an exposure time of 30 s and output power of 450 W giving 10% longer shoot length and 7% root length than the control one. Ragha et al. (2011) used the low power MWs (frequency range of 8.5–10.27 GHz) having non-thermal intensity having a frequency of 1 kHz and studied their effect on the wheat (Triticum aestivum), Bengal gram (Cicer arietinum), green gram (Vigna radiate), and moth bean (Vigna Aconitifolia). Effects of different parameters like frequency (8.5 to 10.27 GHz), power (−1.0 to 3.5 dBm), exposure time (12 to 28 min), and power density (1.5 to 5.5 cm) were studied to evaluate their effect on germination. As compared to control the different treatments induced stimulating effects on germination %, seedling vigor, and biomass % of plants including wheat, green gram, moth bean, and Bengal gram, especially when used with low levels of power, exposure time, and power density while high frequency stimulated seed germination as compared to control (Ragha et al., 2011).

TABLE 4

Table 4. Effect of microwaves waves on germination parameters.

In an experiment effect of MWs using a wireless router (WLAN: 70 mWm⁻²) and mobile devices (GSM: 100 mWm⁻²) was studied on three different aromatic plant seeds including parsley (Petroselinum crispum L. cv. Plained Leaved), celery (Apium graveolens L. cv. Pascal Giant), and dill (Anethum graveolens L. subsp. hortorum cv. Common) (Soran et al., 2014). Different plant parameters were studied regarding MWs effect including leaf structure, essential oil content, and emission of volatile compounds. The results exhibited that WLAN frequency MWs appeared to be more harmful than GSM-frequency MWs, and the MWs treatments caused both structural and chemical alternations (Soran et al., 2014). Like other radiation treatments, the efficiency of MWs application depends on different parameters such as plant species, growth stage, exposure duration, frequency, and power density (Jayasanka and Asaeda, 2013). Different studies in this area revealed that MWs treatment showed a positive effect on some plants while negatively influencing other plants; which recommended that the influence of MWs is related to radiation frequency, exposure duration, and environmental circumstances (Khalafallah and Sallam, 2009).

Magnetic fields for seed processing

The use of magnetic fields (MFs) also showed positive responses regarding the rate of germination, growth, and crop yield along with the decreased incidence of pathogenic diseases. In a study annual medics and dodder seeds were treated with an electromagnetic field which shows a significant effect on germination rate. Annual medics seeds were treated at 80 μT for 10 min and 30 min, and 128 μT for 10 min while dodder seeds were treated at 88 μT for 12 h in a wet state, and 128 μT for 24 h in dry seeds (Balouchi and Sanavy, 2009; Araújo et al., 2016). The exposure of MFs depends on flux density and duration of exposure which defines its dose and MFs dose influences the germination, seedling growth, and yield (da Silva and Dobránszki, 2016). The schematic representation of the experimental MFs setup is shown in Figure 4. The flux density of the magnetic field changes with the static or alternating magnetic fields, thus increasing the germination percentage and affecting the preliminary growth stages (Hozayn et al., 2019). In a study, magnetically treated water was used as a hydro-priming technique which as result enhanced the germination rate and plant growth (Morejon et al., 2007). Along with plant growth, the application of MFs also influenced the enzymatic activities, phytochemical reactions, and respiration process (Carbonell et al., 2000; Martinez et al., 2000; Rifna et al., 2019a). MFs treatment results in speeding up the plants' growth, root development, and protein biosynthesis (Kordas, 2002).

FIGURE 4

Figure 4. The schematic representation of the experimental magnetic fields.

The mechanism behind perceiving MFs and then regulating of signal transduction pathway is still not understood. However, according to Ahmad et al. (2007), the mechanism of MFs signaling is facilitated by blue light photoreceptors which are known as cryptochromes. Chloroplast has paramagnetic characteristics therefore, MF treatment induces metabolic reactions in the seed which stimulate the germination (Aladjadjiyan and Ylieva, 2003). Another work performed by Racuciu et al. (2008) showed that the application of a magnetic field enhanced the enzymatic activities. According to Copeland and McDonald (2012), the efficiency of MFs stimulation is assessed by two factors germination energy and germination capacity. Higher germination energy frequently leads to stronger radicle development and increased biomass percentage.

For several years in research studies influence of MFs affecting the plant, and germination parameters have been the subject of interest. Lately, many researchers have stated the positive effects of MFs on germination %, seedling growth, growth of meristem cells, and chlorophyll contents (Qados and Hozayn, 2010; Hozayn et al., 2014). Chickpea (Cicer arietinum) seeds were treated with MFs for 1–4 h in steps of 50 mT intensity from 0 to 250 mT and increased germination speed, seedling length, and dry weight as compared to control (Vashisth and Nagarajan, 2008). Static MFs having intensities 4 or 7 mT with 0, 2, 6, and 10 atm osmotic pressure created with sucrose or salt were applied to bean or wheat seeds. The MFs treatment improved the germination ratios, without having any influence of increased osmotic pressure. The greatest germination and growth rates observed in both wheat and bean plants were exposed to 7 mT MF as compared to the untreated seeds. In wheat seedlings, the root and shoot length was 7.63 ± 0.08 and 9.62 ± 0.07%, respectively. In bean seedlings, the root and shoot length was 5.46 ± 0.09 and 7.65 ± 0.08%, respectively (Cakmak et al., 2010). Application of non-uniform MFs having intensities of 60, 120, and 180 mT for different durations of 5, 10, and 15 min, respectively, resulted in significant improvement in pea germination. The high germination leads to increased emergence index, and vigor index by 86 and 205%, respectively (Jamil and Ahmad, 2012). Similarly, in another experiment treatment of corn seed with the pulsed electric magnetic field (EMFs) for different time durations of 0, 15, 30, and 45 min enhanced germination %, vigor, chlorophyll content, leaf area, fresh and dry weight, and yields (Bilalis et al., 2012).

After overnight soaking wheat grains were treated for consecutively 4 days and 5 h/day with a 30 mT static magnetic field (SMF) and a 10 kHz EMF. Results showed an increased germination speed and seedling growth compared to the control group (Payez et al., 2013). An MF applied to dormant seeds of barley, corn (Zea mays), wheat, and beans significantly enhanced the rate of their seedling growth. Exposure of mung bean (Vigna radiata) seeds to static MFs having an intensity of 87 to 226 mT for a duration of 100 min, resulted in a direct enhancement in germination % with increasing MFs intensity. At an intensity level of 0.194 T, the maximum germination of ~80% was observed as compared to the control (Mahajan and Pandey, 2014). Calculated mean values of germination time, germination rate, germination rate coefficient, magnetic constant, transition time, and water uptake, showed the positive effect of static MF in improving germination (Mahajan and Pandey, 2014). A summary of the findings of the effects of magnetic fields on seedling performance of the different plant species has been provided in Table 5.

TABLE 5

Table 5. Effect of magnetic fields waves on seedlings germination parameters.

Plasma seed treatment

In the agriculture sector, applications of plasma treatment are also gaining attention to influence germination and plant growth (Hayashi et al., 2011; Klämpfl et al., 2012). In various studies, scientists showed improved germination and growth pattern through the application of plasmas with various vapors and gases such as aniline, cyclohexane, and helium, respectively (Jiayun et al., 2014). In this regard, different types of plasma techniques have been used such as atmospheric plasma, microwave plasma, and magnetized plasma (Zhou et al., 2011). Figure 5 represents the schematic setup of the plasma seeds treatment. The influence of different gases used in plasma treatment is generally investigated. Different studies discovered that active oxygen species such as O, $O_{2}^{-}$ , O₃, and OH in water vapor plasma influenced the redox reaction which in results affects the plant development by controlling thiol groups' redox status (Henselová et al., 2012). In substitution for scarification and stratification, non-thermal plasma techniques were used as a seed priming method (Dhayal et al., 2006; Mahendran et al., 2017). Plasma application has several benefits including less seed destruction and being environment friendly having no chemical utilization (Volin et al., 2000; Dhayal et al., 2006; Bourke et al., 2018). Besides improving seed quality and plant growth, plasma can sterilize seeds and also cause variations of enzymatic reactions (Sera et al., 2010; Henselová et al., 2012).

FIGURE 5

Figure 5. Schematic diagram of the plasma seeds treatment.

Plasma can affect seed germination by suspending or enhancing the process. The new significant plasma-related studies include the use of microwave discharges (Sera et al., 2010) and low-density radio frequency discharges (Bormashenko et al., 2012; Filatova et al., 2013). Plasma treatment induced the development of thin (0.5–2 μm) hydrophobic and hydrophilic layers in the seed, which become very helpful in different cultivation environments including climate conditions, temperature, humidity, lighting, nutrition, and water volume. Thus, in wet and cold soil, the hydrophobic layers interrupt the water absorption which results overcome the chilling injury and improving the seed viability (Volin et al., 2000; Kavak and Eser, 2009). In another study, the use of the plasma method caused increased hydrophilicity which as result stimulated the water uptake and germination process (Bormashenko et al., 2012).

The examination of several seed germination studies showed the effect of different plasma treatments applied on various seeds including wheat, maize, radish, oat, safflower, and blue lupine (Lynikiene et al., 2006; Sera et al., 2010). Different examples from the literature of the plasma treatment on different seeds have been provided in Table 6. Safflower (Carthamus tinctorium L. semen) seeds were treated with argon-containing low-pressure capacitively-coupled RF plasma at a pressure of 1.6 and 16 Pa for a time duration of 30 and 130 min, respectively. Treatment of 1.6 Pa, for 30 min resulted in a 30% increase in germination rate while a 50% increase was attained at 16 Pa, for 130 min (Dhayal et al., 2006). The authors claimed that the plasma treatments have caused biochemical modification on the seeds as compared to removing germination inhibitors (Kim, 2019; Guo et al., 2020). Filatova et al. (2013) used air plasma 5.28 MHz at a pressure of 0.3–0.7 Torr to treat blue lupine (Lupinus angustifolius), soy, honey clover, and Galega (Galega virginiana) seeds and investigated a 10–20% increase in seed germination and crop viability.

TABLE 6

Table 6. Effect of plasma treatments on germination parameters of different seeds.

Henselová et al. (2012) used a low temperature diffuse coplanar surface barrier discharge air plasma at atmospheric pressure for 60 and 120 s and investigated the growth, anatomy, and biochemical changes that occurred in maize seeds (Zea mays L.). After 60 s plasma treatment seedlings showed an increase in root length (21%), root fresh weight (10%), and root dry weight (14%). The authors also detected significant changes in dehydrogenase, superoxide dismutase, catalase, and guaiacol-peroxidase. Bormashenko et al. (2012) treated the grains of lentil (Lens culinaris), beans (Phaseolus vulgaris), and wheat (Triticum spp.) with non-equilibrium plasma. Filatova et al. (2013) and Filatova et al. (2014) treated seeds wheat (Triticum aestivum L.), narrow-leaf lupine (Lupinus angustifolius), and corn (Zea Mays L.) for 10 min with capacitively-coupled low-pressure (40–80 Pa) RF air discharge plasma by using frequency 5.28 MHz and specific power 0.34–0.65 Wcm³. Similarly, in another study treatment of soybean seeds with cold plasma, treatment having helium with 0, 60, 80, 100, and 120 W for 15 s showed positive effects on seed germination and seedling growth and water uptake was also greater (Ling et al., 2014).

Gamma irradiations seed invigoration

In agriculture sciences, gamma radiation has several applications including food microbiological safety, storability subjects, slow fruit ripening, and vegetable sprouting, along with stimulation of seed germination (Araújo et al., 2016). Among IR, gamma (γ) radiation has high energy and is produced from Cobalt-60. Gamma radiation can penetrate and interact with biological materials (Islam, 2017). Units of Gray (Gy), are used to express the level of absorbed IRs, while 1 Gy dose is equal to 1 Joule radiation energy absorbed per kilogram. Another unit called Sievert unit (Sv) is also used to express the level of absorbed IRs but in the case of interaction biological material, like 1 Gy, 1 Sv is equal to 1 Joule radiation energy absorbed per kilogram of biological material. Another important factor to consider while using the IR technique is the dose rate defined as the rate of energy deposition (Gyh⁻¹) (Moussa, 2006). Gamma radiations can improve product quality, grain yield, and salinity tolerance (Kiong et al., 2008; Majeed et al., 2018). The biological effects of IRs depend on their chemical reactions with biological molecules and water for producing free radicals which control the activity of biomolecules (Araújo et al., 2016). Figure 6 gives the schematic outline of the gamma irradiation setup. This leads to the activation of an antioxidant system that prepares the defensive mechanism of plants against stresses (Wi et al., 2007; Ashraf, 2009).

FIGURE 6

Figure 6. Schematic setup of the gamma irradiation treatment.

Gamma radiations do not damage the deoxyribonucleic acid (DNA) and structural integrity of seeds and thus can activate various biochemical reactions in the seed (Bhosale and More, 2013). These radiations can affect the different components of seeds like cell membranes, proteins, and nucleic acids. γ-rays can be used as a seed priming technique to boost the germination process but its effect depends on several factors like radiation dose, intensity, and exposure time (Kovacs and Keresztes, 2002; Majeed et al., 2017). The biological or molecular mechanisms involved in the effects of radiation are not properly understood and several ideas have been given.

The application of γ-rays in seeds produces reactive oxygen species (ROS) as the result of water radiolysis. These species react as regulators which can amplify the stress and activate antioxidant responses, thus in this mechanism, ROS has an important role as a signaling molecule [gibberellins signaling pathway and oxidation of negative regulators of germination like abscisic acid] (Borzouei et al., 2010; Esnault et al., 2010). Therefore, γ-rays treated plant seeds easily overwhelmed the fluctuations in daily stress conditions, including light intensity, temperature, and water loss (Gicquel et al., 2012; Qi et al., 2015). Low dose γ-irradiations in seeds induce positive effects on enzymatic reactions and also affect nucleic acids and proteins synthesis which consequently enhances metabolic activities in the seed leading to breaking the seed dormancy and boosting germination speed and plant development (Abdel-Hady et al., 2008). The impact of γ-irradiation in seed technology as a seed invigorating technique has been an impactful way to improve germination. Arabidopsis thaliana seeds were treated with γ-ray's 50 Gy dose which in result showed positive results on all the tested growth parameters such as germination index, seedling growth, root length, and fresh weight (Qi et al., 2015).

Similarly, in another study, Oryza sativa L. cv-2233 and Phaseolus mungo L. dried seeds were treated with γ-rays ranging between 50 and 350 Gy. Oryza sativa, showed a stimulating response at 50 Gy giving approximately plant height 19+1.4%; panicle length 27+2.1%; seed number per panicle 64+2.8%, tiller number 17+1.7% while Phaseolus mungo showed at 200 Gy giving approximately plant height 51+1.4%; pod length 49+4%; seed number per pod 56+2.8% (Maity et al., 2005). Similarly, the impact of gamma irradiation on maize (Zea mays, hybrid Turda Star) seeds was studied, and a radiation sensitivity test was performed to compare germination capacity, plant growth, and photosynthetic pigment contents between treated and untreated seeds. Again, the stimulatory effects of γ-ray were seen at low doses (2–30 Gy) (Marcu et al., 2013). Different examples from literature about gamma irradiation affecting germination characters of different seedlings have been provided in Table 7.

TABLE 7

Table 7. Effect of gamma irradiation on germination percentage of different seeds.

Sound waves based stimulation of seeds

Audible sound within a frequency of 20 Hz to 20 kHz can be heard by human beings. Among different environmental factors like moisture, light, wind, and temperature affecting plant growth, comparatively limited data is available about the effect of audible sounds on plant growth (Hassanien et al., 2014). Acoustic biology has become progressively more popular. Recently different plants have been treated with sound waves to check their effect at various physiological growth stages. The use of sound waves as an invigoration method could reduce the requirement for chemical fertilizers by opening the stomata, and also enhance disease resistance in plants by strengthening the immune system (Junfang, 2012; Carlson, 2013; Jung et al., 2018). Naturally, plants can generate low-frequency sound waves 50–120 Hz and can also absorb or resonate specific frequencies of external sound waves (Frongia et al., 2020). Plants emit also ultrasonic vibrations of 20–100 kHz, measured by connecting a sensor directly to the stem of the plant (Hassanien et al., 2014). Plants release sound emissions from different organs and at different growth stages or in response to different situations. Through the use of small highly sensitive sound receivers, it has been shown that plants emit sound from the xylem and faint ultrasound in case of stress (Jung et al., 2018; Khait et al., 2018). Sound waves can induce various changes such as cell cycle changes, the vibration of plant leaves, and the acceleration of cellular protoplasmic movement (Godbole, 2013). The experimental diagram of sound wave treatment of seed is given in Figure 7.

FIGURE 7

Figure 7. Experimental setup of sound wave treatment for seeds.

It has been stated that sound waves activate stress-induced genes and also increase their transcription level (Xiujuan et al., 2003). Sound waves treatment enhances the uptake of dews and sprays fertilizers by stimulating the leaf stomata. Moreover, the process of photosynthesis is also influenced by the conversion of sound energy into chemical energy (Meng et al., 2012). According to previous studies, musical sounds can directly influence the biological system and thus could significantly affect the seeds sprouting (Creath and Schwartz, 2004).

Several studies have been undertaken to understand the influence of sound and music on plants and plant growth (Table 8). Rideau wheat seeds treated with the sound frequency of 5 kHz and pressure level of 92 dB resulted in stimulated growth along with increased dry weight and number of roots (Weinberger and Measures, 1979). In an experiment, paddy rice seeds were treated with a sound frequency of 0.4–4 kHz and a pressure level of 106–111 dB. The biological effect of sound waves resulted in a significant increase in germination index, stem height, fresh weight, root system activity, and the permeability of the cell membrane (Bochu et al., 2003). According to some studies, music or sound containing hard-core vibrations can cause harmful effects on plant growth. While classical music has gentle vibrations that increase plant growth similarly violin music also induced positive effects significantly (Aladjadjiyan and Kakanakova, 2009). Plants can use acoustic signals from the surrounding environment and spread them rapidly (Gagliano, 2012). A sound wave can transfer energy, to initiate the cytoplasmic streaming and influence the membrane materials thus resulting in variations in biological function and increased metabolic reactions. In another study, at 0.2 and 0.3 kHz, sound frequency young root tips of Zea mays showed a clear bend toward a sound source (Gagliano, 2012).

TABLE 8

Table 8. The application of sound waves to different crops affects their phenotype.

Laser irradiation seed treatment

As in the agriculture field, irradiation is known as a new branch of seed invigoration technique. Laser radiations have features such as coherence, high density, monochromatic, and polarization, and all these properties make laser irradiations applicable in agriculture (Hasan et al., 2020). Various parameters of laser radiation can affect the physiological process in seeds, these parameters include the type of laser radiation, intensity, wavelength, intensity, and exposure time (Govindaraj et al., 2017). The synergistic effect of different mechanisms interacting with laser light results in a range of noticeable effects in the agriculture field. In several studies, laser irradiation showed a positive effect on germination and disease prevention in different crops such as rice, maize, wheat, peas, radishes, and corn (Aladjadjiyan and Kakanakova, 2009; Hernandez et al., 2010). The schematic diagram of the laser irradiation treatment is given in Figure 8.

FIGURE 8

Figure 8. Schematic diagram of laser irradiation treatments.

Laser light treatment in seeds induces a series of reactions like accelerated maturity, improved disease resistance, improved energy potential, alpha-amylase action, and free radical's concentration (Klimek-Kopyra et al., 2021). In irradiated seeds, all these reactions lead to reduced seed dormancy, increased rate and percentage of germination, an improved profile of chlorophyll and carotenoid content, higher seed vigor, and a positive effect on the process of respiration and photosynthesis (Wang et al., 2019).

The authors also reported that in addition to serving as a pre-sowing seed treatment, laser irradiations can also affect the quality and quantity of production. Table 9 presents a list of several studies about the effect of laser stimulation on plants that can be found in the literature. In an experiment, He-Ne laser application as a pre-sowing treatment on four spring barley cultivars showed an increment in the germination capacity (Szajsner and Drozd, 2003). Treatment of wheat grains with semiconductor laser influenced their germination and development (Hernandez et al., 2008) while irradiation of tissue culture significantly caused changes in a lipid matrix structure (Salyaev et al., 2007).

TABLE 9

Table 9. Effect of laser irradiation stimulation on plants.

The application of light from a laser diode of 650 nm and a power of 27.4 mW increased the germination of photosensitized wheat seeds (Aguilar et al., 2008). Soybean seedlings treated with 532 nm laser improved the photosynthesis proficiency and enhanced the isoflavone content (Tian et al., 2010). Irradiation treatment of A. farnesiana seeds for 9 min with He-Ne laser light at 1.70 W cm⁻² affected the germination indices (Soliman and Harith, 2009). Several other studies used He Ne laser and approved promising effects on germination, in winter wheat genotypes, morphological characters were studied (Szajsner, 2009), in maize hybrids seeds activity of amylolytic enzymes was observed (Podlesny and Stochmal, 2005), and developmental phased of white lupine and fava bean plants were observed (Podlesny and Podlesna, 2004). Similarly, in different experiments, pre-sowing treatment of seeds with irradiation using specific application parameters showed significantly enhanced the production of fava bean seeds (Podlesny, 2007), alfalfa (Dziwulska et al., 2006), wheat (Szajsner, 2009), maize (Szajsner et al., 2007b), and barley (Szajsner et al., 2007a).

Conclusion and future prospects

High vigor seeds represent improved establishment and productivity of crops. Therefore, sustainable crop production requires the use of low-cost and environment-friendly techniques of seed enhancement. Several pre-sowing treatment attempts have been made to improve the yield. Physical methods are an innovation in the research area of seed invigoration to improve crop yield. These physical techniques are the substitute for chemical-based techniques in the development of new biotech-based solutions. These techniques are environmentally friendly and can be used on a high throughput scale. Although, plants respond to the physical treatment but still on a commercial scale it has not been fully exploited. Enough facilities are present to conduct physical treatment of seeds but still, there is a lack of information regarding pre-germinative metabolic reactions occurring in seeds. This information gap is hindering the successful application of these techniques, as seen for chemical treatments. There is also needed to explain all biochemical reactions affecting these processing technologies which result influence the growth and development of plants. These processing techniques have a challenge which is that not all techniques may result in improved germination of seed. The invigoration methods can make the seeds vulnerable to stress conditions if an unsuitable technique is applied to seeds. Therefore, it's important to determine all working conditions and protocols specific to plant seeds. Because the efficiency of each technique is directly linked to different factors including plant species, cultivars, environmental conditions, type of technique, processing treatment dose, exposure timings, etc. There is also needed to expand the number of tested species with each technique to identify factors best appropriate for each physical treatment. Further, for each treatment to modulate the seed response, the study of environmental parameters and their impact could not be ignored.

Author contributions

SH and AR designed the study and wrote the manuscript's first draft. MK and MF conducted sample selection and data management. Mahwish, TT, and AK managed the literature searches and analyses. X-AZ and SH edited the manuscript and supervised the work. AA and AL contributed in writing-review and editing the final manuscript. All authors contributed to and have approved the final manuscript.

Funding

This work was supported by National Natural Science Foundation of China (32150410363 and 32172348), and 111 Project (B17018); Guangdong Provincial Key Laboratory of Food Intelligent Manufacturing (2022B1212010015) and Science and Technology Project Xinjiang Uygur Autonomous Regions (2022A02002-4); and Department of Nutritional Sciences, Faculty of Science and Technology, Government College Women University, Faisalabad-38000, Pakistan.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Abbreviations

MWs, Microwave radiations; US, Ultrasonic waves; IR, Infrared; WLAN, Wireless router; MFs, Magnetic fields; DNA, Deoxyribonucleic acid; ROS, Reactive oxygen species; RF, Radio-frequency; SMF, Static magnetic field; EMF, Electric magnetic field.

References

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REVIEW article

Recent developments in physical invigoration techniques to develop sprouts of edible seeds as functional foods

Introduction

Physical seed invigoration techniques

Ultrasound seed processing

Microwave seed treatment

Magnetic fields for seed processing

Plasma seed treatment

Gamma irradiations seed invigoration

Sound waves based stimulation of seeds

Laser irradiation seed treatment

Conclusion and future prospects

Author contributions

Funding

Conflict of interest

Publisher's note

Abbreviations

References

This article is part of the Research Topic

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