Introduction

Agriculture is the backbone of almost every country and every person either directly or indirectly relies on agricultural products for livelihood (Fao FAO 2013). In India, nearly 70% of the population are reliant on agriculture for economy and food (Swaminathan and Bhavani 2013). However, agriculture is not that profitable business in recent times, and it requires novel techniques to feed an increasing worldwide population, which can keep it productive, competitive, and profitable in the future (Chen and Yada 2011). In India, agricultural growth is decreased to 2% in 1995–2005 from 3.6% in 1985–1995 and annual food grain production per capita is also decreased to 179 kg in 2014–2017 from 207 kg in 1991–1995 (Ministry of Agriculture, Government of India, 2017–2018). Thus, there is a requirement of distinct technique, different from traditional techniques, such as use of hybrid varieties, synthetic fertilizers, and pesticides to achieve targeted agricultural growth in the future. In the past, several techniques were used to increase the yield of crops and to minimize limitations, such as stagnation in yield, multi-nutrient deficiency, climate change, shrinking of available land and water availability, urbanization, sustainable use of resources, environmental issues, including runoff of nutrients, accumulation of pesticides and fertilizers resistance to genetically modified organism (GMO) crops, and shortage of labors are associated with agriculture (Shah and Wu 2019). Hence, it is necessary to accomplish sustainable growth in agriculture at the rate of 4–5% to encounter food security challenges (Diaz-Ambrona and Maletta 2014) and agriculture requires novel improved technology for sustainable growth (Chen and Yada 2011; Mitchell 2001; Robinson and Salejova-Zadrazilova 2010), that should be eco-friendly and increase per unit production by utilizing minimum natural resources.

Nanotechnology is a novel emerging option for a revolution in agriculture (Opara 2004) and it has potential to change the fate of agricultural industry and its allied fields (Batsmanova et al. 2013; Scott and Chen 2013). Nanotechnology is a branch of science which deals with the synthesis and application of nanoparticles, that are 1–100 nm in size (Roco 2003). These nanoparticles are currently used and are under extensive research in various sectors, such as defence and security, medicine, textiles, biotechnology, electronics, cosmetics and paints, optical engineering, energy, and communications (Biswas and Wu 2005); however, its application in agriculture is still limited (Tile et al. 2016) even research work-related nanotechnology started around 50 years ago (Mukhopadhyay 2014). Further, nanoparticles possess typical physical, chemical, and biological properties due to enhanced specific surface area, high surface energy, and quantum confinement (Nel et al. 2006; Lin and Xing 2008). These nanosized particles can easily penetrate into the cell wall of living organisms, due to its small size, which differentiate them from their bulk counterparts (Zhu et al. 2006). Moreover, nanoparticles have also gained great consideration in agriculture sector, due to its highly reactive surface-to-volume ratio property (Lyons et al. 2018).

Applications of distinct nanoparticles are increasing with time in agriculture sector in several ways (Shang et al. 2019a); however, super-dispersive metal powders (SDMP) also called metal nanoparticles are being used more for the seed treatment of different crops, due to their low toxicity, compared to their salts and chelates. Further, only a few concentrations of SDMP are required to stimulate physiological and biological progressions in plants (Pavlov 2000; Folmanis and Kovalenko 1999). SDMP serve as free active electrons used for stimulating metabolic processes of cells (Folmanis and Kovalenko 1999). Churilov et al. (2000) reported that seed treatment with SDMP may increase protein content up to 40%, depending upon the concentration of nanoparticles (Churilov et al. 2000). Conversely, Goswami et al. (2019) demonstrated that the nanoparticles might have positive or negative effects on seed quality parameters (Goswami and Mathur 2019). The distinct type, size, shape, and dose of nanoparticles have peculiar direct or indirect effect on the seed quality parameters, such as germination percentage, seedling length, seedling dry weight, and seed vigor indices (Monica and Cremonini 2009). However, most of the current publications regarding nanoparticles and crops are centered around the effects of nanoparticles on seed germination and other quality parameters (Stampoulis et al. 2009), as rapid germination and establishment of seedling are the most crucial factors, which affects the crop production (ur Rehman et al. 2015; Ibrahim 2016; Liu et al. 2012). Moreover, Opoku et al. (1996) stated that seeds with better germination cause vigorous growth of plant and robust root system (Opoku et al. 1996). Thus, the aim of this article is to provide an overview of three different SDMP (zinc, silver, and titanium) in various agriculture crops and its potential application to increase their yield. In addition, the drawback of conventional seed growth enhancers, impact of metal nanoparticles toward seeds, and mechanism of nanoparticles to increase seed germination were also discussed.

Seed quality parameters

Seed is considered as the initial determinant of the future plant development. Thus, quality of seed is crucial in crop performance, whereas the effect of other inputs, such as fertilizer dose and irrigation in crop performance also depend on the seed quality (Wimalasekera 2015). Quality of a seed is defined as the genetic purity of a plant material with high germination rate, proper moisture content, and infestation of disease, due to insects or pests. Good quality seed alone can increase yield of plant up to 40%, which varies depending on the crops. Further, the quality of a seed is determined by its physical, physiological, genetic, and health (Thompson 1987). Physical quality of any seed can be increased via specific processes, which include cleaning of seed, removal of damaged and diseased seeds, as well as separation of other crop seeds. Similarly, genetic quality of a seed depends on the production method and adopted crop seed chain method. Likewise, physiological quality of a seed is determined by germination percentage as well as vigor of seed, and it can be achieved via distinct seed treatments (Villa et al. 2019). It is worthy to note that seeds of high quality are always free from any type of disease and pest. Further, yield of any crop is dependent on the planting value of seed, which is determined by distinct seed quality parameters, such as germination percentage, seed vigor, and seed health as shown in Fig. 1. Moreover, high germination percentage is essential to maintain plant population with recommended seeding rate of crop and vigor, which ensures performance of seed in the field or laboratory as well as storage, even in adverse conditions. In addition, highly vigorous seed, which denotes degree of seed aliveness, always perform better in any condition over low-quality seed (Hampton 2002).

Fig. 1
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Parameters affecting the quality of seeds

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Conventional seed growth enhancers

The enhancement of seed quality is usually performed by exclusive methods, after the harvesting of seeds and before next agriculture cycle to improve germination, stress tolerance, and to minimize dormancy, seed-borne diseases, and facilitate better storage. Seed enhancement techniques, such as pre-sowing, pre-storage, and mid-storage treatments, are the conventional approaches, that are used widely in the cultivation of agriculture crops (Sharma et al. 2015). Pre-sowing treatments are performed to break dormancy, improve germination, and for precision sowing of seeds via distinct approaches, such as scarification, stratification, seed pelleting, seed priming, seed coating, and seed protection treatments for the removal of harmful microorganisms (Ahmed and Kumar 2020). Even though, these seed treatment methods are used for a long time to enhance their growth, there are several limitations, which affects the seed dormancy and long-term stability (Broersma and Luckmann 1967). Thus, there is a need for a potential alternative seed growth enhancement technique to overcome the challenges posed by conventional methods.

Nanotechnology in improving seed quality

Nanoparticles are recently used as a potential seed growth enhancer, due to their ability to provide effective seed dormancy and possess capability to serve as a fertilizer, pesticide, and nutrient for the seed growth. There are several experiments that have revealed the positive effects of nanoparticles in agriculture and environmental applications, especially to improve seed quality parameters via increasing water uptake and enzymatic activity of seeds and by supporting their defence mechanisms against pathogens and environmental threats. This may be due to the degradation of nanoparticles into ions in the soil, which would have been absorbed by the plants or seeds as nutrients and improve their positive effects. In addition, high concentration of these nanoparticle-mediated nutrient application in the soil, especially engineered synthetic nanoparticles, can lead to negative effects toward the plants, such as reduced root and shoot length, seedling vigor index, and induce stress as well as inhibit beneficial microbes in the environment in certain cases (Lewis et al. 2019; Hussain et al. 2017), depending on the soil type and nutrient requirement by specific plant (Lin and Xing 2007). However, the mechanism in which nanoparticle-induced water uptake in plants or seeds is still unknown and is a topic of interest among recent researchers to be beneficial in agriculture (Shang et al. 2019a). Table 1 is the summary of nanotechnology-based agricultural approaches to protect and enhance the seed growth, compared to conventional approaches.

Table 1 Comparison of conventional and nanotechnology-mediated agricultural methods to enhance seed quality and growth
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In recent times, conventional methods in agriculture, due to various limitations, are replaced with nanotechnology-based approaches as mentioned in Table 1. Conventional methods, which includes distinct techniques, such as excessive use of chemical fertilizers, insecticides, and pesticides, that are responsible to reduce insect, pest, and disease attack in agricultural crops (Sharma and Singhvi 2017), but leads to nutrient imbalance in soil (Savci 2012a); soil pollution (Rahman and Debnath 2015); production of greenhouse gases, such as nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2) (Savci 2012b); economic loss due to low efficiency (Miao et al. 2015); and environmental pollution (Anju et al. 2010). Further, excessive use of chemical fertilizers is also responsible for the loss of beneficial soil organisms, such as nitrifying bacteria (Savci 2012b). Hence, nanoparticle-based fertilizers are introduced as an alternative for chemical fertilizers, as they are required in less quantity to exhibit enhanced effectiveness, compared to chemical fertilizers (Lateef et al. 2016; Wang et al. 2015a) and maintain soil health (Peteu et al. 2010). Furthermore, Du et al. (2019) reported that use of zinc oxide nanoparticles in wheat crop can provide high grain yield, compared to conventional fertilizers with zinc sulfate (Du et al. 2019). Moreover, there are various experiments that have revealed the application of nanoparticles in the agricultural soil to increase the uptake of nutrients among seeds and plants, which eventually elevates the nutrition status in the plant yield (Aziz et al. 2016; Dwivedi et al. 2016; Sabir et al. 2014; Mastronardi et al. 2015).

Potential of nano-seed priming over conventional seed priming

Seed priming technique has been used in several crops for better germination, high yield, quality seed production, and stress management, since ancient times (Chen and Arora 2013). In general, seed priming is performed with distinct agents, such as water, inorganic salts including zinc sulfate, magnesium chloride, manganese sulfate, sodium chloride, and sodium sulfate for the production of high seedling vigor via uniform germination process within the seed, without radical emergence (Ibrahim 2016). However, conventional seed priming agents are required in large quantities, where most of them will be discarded as waste and pollute soil as well as water. Further, conventional seed growth enhancers are not as stable, compared to nanosized particles. Due to non-stability and loss of large quantities as waste, there is a need for repeated application of these seed growth enhancers (Tsuji 2001). Furthermore, it is reported that conventional herbicides have phytotoxic effect on several crops that could be controlled by nano-herbicide (Pérez-de-Luque and Rubiales 2009). Similar to nano-herbicides, nanoinsecticides and nanopesticides are also required in small quantities and could replace highly toxic chemical insecticides and pesticides, that can lead to soil pollution (Kalia and Gosal 2011). Thus, nanoparticles are introduced as a novel and efficient seed priming and growth enhancer agents in various recent researches.

Recently, conventional priming agents are being replaced by nanoparticles, especially by metal-based nanosized particles, such as silver nanoparticles (AgNPs), titanium dioxide (TiO2) nanoparticles, zinc nanoparticles (ZnNPs), iron nanoparticles (FeNPs), copper nanoparticles (CuNPs), and carbon-based nanoparticles, due to their enhanced effect in seed germination. Moreover, less concentration of nanoparticles is required to improve seedling vigor of various crops, compared to conventional seed priming agents (Mahakham et al. 2017a). Acharya et al. (2020) reported that seed priming of watermelon seeds with silver nanoparticles has enhanced seed germination, yield, and quality of the fruit (Acharya et al. 2020). Further, Maroufi et al. (2011) also reported that seed priming with nanoparticles has more positive effects on seed germination and seedling vigor in green gram, compared to hydropriming of seeds (Maroufi et al. 2011). Furthermore, Pawar et al. (2019) demonstrated that nano-seed priming with iron oxide nanoparticles exhibited positive effects on seed growth of chickpea (Cicer arietinum L.), compared to hydropriming of seeds (Pawar et al. n.d.).

Types of nanoparticles in agriculture

Various types of nanoparticles, such as nanosized metal and metal oxide, carbon-based, polymer, and nanocomposite particles, are currently employed in several agricultural applications as shown in Table 2.

Table 2 Application of nanoparticles in agriculture and plant growth
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Metal and metal oxide NPs

Various metals are utilized as a precursor for the preparation of metal-based nanoparticles via distinct approaches, such as chemical, biosynthesis, photochemical, and electrochemical methods. These metal-based nanoparticles are being used in agriculture for different purposes as listed in Table 3. Certain metal-based nanoparticles are used for seed treatment and preparation of nanoinsecticide as well as nanopesticides. Mahakham et al. (2016) reported that gold nanoparticles can be useful as a priming agent for the seed treatment of aged maize seeds, exhibit positive effects on the germination of seeds (Mahakham et al. 2016), and also increase seed quality parameters. However, these gold nanoparticles are reported to have negative effect on the root and shoot length of rice (Ndeh et al. 2017). Increment in germination percentage and root length was also observed by Adhikari et al. (2013) in rice seeds, when they were treated with silica (SiO2) and molybdenum (Mo) nanoparticles. However, they also demonstrated that a dose of above 50 ppm of Mo nanoparticles had negative effects on the root length or seedlings of rice (Adhikari et al. 2013). Further, Adhikari et al. (2012) reported that seed treatment with copper (Cu) oxide nanoparticles did not possess any significance on the germination of soybean and chickpea, while the root length was decreased via seed treatment with nanoparticles (Adhikari et al. 2012). However, the germination percentage of soybean seeds was increased by the treatment of seeds with iron, cobalt, and copper nanocrystalline powder (Ngo et al. 2014). Furthermore, Alam et al. (2015) reported that iron nanoparticles (2 ppm) for the seed treatment of wheat can increase their germination percentage, root length, and shoot length, whereas concentration above 2 ppm has been demonstrated to have negative effects on these parameters (Alam et al. 2015).

Table 3 Treatment of metal-based nanoparticles in plants for agricultural applications
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Carbon-based NPs

Carbon is a key element in compounds, such as carbohydrates, proteins, and lipids of plants. Plants utilize carbon in the form of carbon dioxide (CO2) to synthesize food and oxygen (O2) by converting sunlight via photosynthesis process. Numerous types of carbon-based nanoparticles, such as single-walled carbon nanotube (SWCNT), double-walled carbon nanotube (DWCNT), multi-walled carbon nanotubes (MWCNT), and fullerenes were used in several agricultural applications to identify their efficacy in seed germination and plant growth. Similar to other nanoparticles, carbon-based nanoparticles also have both advantages and limitations on seed germination and seedling vigor (Vithanage et al. 2017; Mukherjee et al. 2016; Zaytseva and Neumann 2016). However, it has been concluded by various studies that most positive effects on seed quality parameters was obtained by use of SWCNT, MWCNT, carbon nanotubes, fullerenes, and graphene, while using them in low concentrations (Tripathi et al. 2011; Lahiani et al. 2015; Villagarcia et al. 2012; Khodakovskaya et al. 2011; Khodakovskaya et al. 2012). Milewska-Hendel et al. (2016) reported that the use of SWCNT can increase the germination of tobacco, maize, soybean, switchgrass, tomato, rice, and barley. However, the SWCNT exhibited positive effect toward rice root growth and negative effect toward Medicago sativa, when quantum dots (QDs) were used (Milewska-Hendel et al. 2016). Further, use of MWCNTs has helped in the uptake of water by the seeds of tomato resulting in swift germination (Aslani et al. 2014). Moreover, Hossain et al. (2015) identified that MWCNT possess ability to activate stress-related gene in tomato resulting in better germination of seeds (Hossain et al. 2015).

Polymer NP

Polymer nanoparticles are synthesized by mixing nano-organic material in inorganic solution or metal nanoparticles in organic mixture. These polymer nanoparticles are prepared via three exclusive methods, such as solution casting, melt blending, and in-situ polymerization. Nanosphere and nanocapsules are the most common example of polymer-based nanoparticles (Rao and Geckeler 2011). Clemente et al. (2014) reported that triazine class of herbicide is more toxic to the environment and prepared herbicide nanocapsules with less toxicity for environmental applications (Clemente et al. 2014). Likewise, Kumar et al. (2015) stated that Acetamprid-loaded alginate-chitosan nanocapsules possess enhanced pesticidal efficiency with reduced side effects among plants (Kumar et al. 2015). Moreover, Sun et al. (2012) developed an extremely sensitive acetylcholinesterase (AChE) inhibition-based amperometric biosensor with the help of hollow gold nanospheres that can detect pesticide residue in fruits and vegetables (Sun et al. 2012). Similarly, Ge et al. (2011) also developed a biosensor with the help of silica nanospheres to detect deltamethrin in fruits and vegetables (Ge et al. 2011).

Nanocomposites

In general, nanocomposites are formed by blending a nanomaterial called filler with various bulk materials (Frimpong et al. 2007). Nanocomposites can be organic or inorganic in nature based on their main component and filler (Shi et al. 2013). If any metal is used as main component, then it is considered as inorganic nanocomposite, whereas organic nanocomposites are made up of carbon-based nanoparticles (Camargo and Smith 2009). Nanocomposites are fabricated by synthesis approaches, such as sol-gel, photo reduction, hydrothermal, and ball milling method (Khan et al. 2015). Besides, these nanocomposites are used in the water purification applications for inhibiting harmful microorganism in contaminated water and utilize them for irrigation in agriculture (Takafuji et al. 2004). Moreover, nanocomposites are extensively used in food packing applications, due to their barrier properties against oxygen and carbon dioxide to safeguard several agricultural products and improve their self-life (Shankar and Rhim 2016b). Thus, the novelty of these nanoparticles and their exclusive properties are widely used in various agricultural applications.

Application of nanotechnology in agriculture

Nanotechnology has been introduced in the agricultural applications, such as fertilizers, precision farming, environmental remediation, plant breed development, nanoinsecticides, and nanopesticides as shown in Fig. 2.

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Applications of nanotechnology in plant growth and crop protection (Shang et al. 2019b)

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Advantages of nano-fertilizers over chemical fertilizers

It can be noted that one-third of the total crop yield is depended on the use of fertilizers, and hence, proper use of fertilizers is highly essential to minimize the cost of production and to reduce environmental pollution, that are caused due to excessive use of chemical fertilizers. Large quantity of chemical fertilizers is used for decades, especially during green revolution, to increase crop yield (S B et al. 2016). Further, excessive use of chemical fertilizers has been reported to degrade total cultivable area, as soil pollution is predominant due to excessive leaching of chemicals through run-off, evaporation, drift, hydrolysis by soil moisture, and microbial and photolytic degradation. However, nanoparticles that are applied as fertilizer to enhance nutrients in soil are demonstrated to migrate slowly (Yang et al. 2017). Around 40–70% of nitrogen, 80–90% of phosphorus, and 50–90% of potassium as chemical fertilizers is lost in the environment before reaching the target site and leads to soil pollution as well as economic loss (Trenkel 1997; Ombódi and Saigusa 2000). Chemical fertilizers are also responsible for improper nutrient balance of soil, increased disease and insect-pest resistance, reduced soil microflora, and weakened nitrogen fixation (Tilman et al. 2002). Thus, there is a need to increase nitrogen (nutrient) usage efficiency (NUE) of fertilizers by introducing eco-friendly fertilizers, that do not disturb soil properties (Miransari 2011). Several researchers reported that the introduction of nano-fertilizer is the most suitable alternative option for conventional fertilizers, as they can exhibit three times more NUE, due to their higher surface area, high solubility (Sasson et al. 2007), and controlled release of nutrients over a long period of time (Tsuji 2001), compared to chemical fertilizers. Fertilizers in nanometer regime or fertilizer encapsulated inside a nanomaterial (DeRosa et al. 2010), that can increase the nutrient uptake (Tarafdar et al. 2012a) of plants and reduce environmental degradation (Chinnamuthu and Boopathi 2009), is called nano-fertilizer. Rai et al. (2012) reported that nano-fertilizers are available in three forms, such as fertilizers encapsulated in a nanomaterial, coated with thin polymer film, and delivered as a particle or emulsion of nanoscale dimensions (Rai and Ingle 2012). Similarly, Kannan (2011) reported that nano-fertilizers possess ability to release nutrient for up to 50 days, while urea as a fertilizer can release nutrients for up to 12 days (Kannan 2011). Likewise, nano form of sulfur has been used as a better fertilizer, compared to their bulk counterpart (Wilson et al. 2008). Further, Wanyika et al. (2012) demonstrated that the urea encapsulated with silica nanoparticles have longer (around five times) release time than conventional fertilizers (Wanyika et al. 2012). Furthermore, Tarafdar et al. (2012a) and Tarafdar et al. (2012b) reported that foliar application of nanophosphors at 640 mg/ha has led to an increment in the yield of cluster bean and pearl millet under arid conditions (Tarafdar et al. 2012b; Tarafdar et al. 2012c). Moreover, Jinghua (2004) also stated an increment in the uptake and utilization of nutrient by grain crops due to the addition of nanocomposites as a potential fertilizer (Guo 2004).

Precision farming application

Production of crops, that are maximized by utilizing exact dose of different inputs, such as water and fertilizers, are decided according to the need of plants at specific location to minimize waste generation and energy demand is known as precision farming approach. The incorporation of less toxic nanoparticles, compared to their bulk counterparts, in precision farming approach is considered as an eco-friendly practice (Batley et al. 2013). Blackmore (1994) stated that the crop production is dependent on the inputs, that are provided to crops, according to their location (Blackmore 1994). Recently, wireless nanosensor network (WNSN) is used to identify exact nutrient requirement of plants in the field condition. Nano-nodes, nano-micro interface, nano-router, and nano-gateway are the distinct components of WNSN, that are used to obtain data about soil temperature, soil moisture, nutrient status of crop and soil, as well as weed infestation in the field (Joseph and Morrison 2006). In addition, nanosensors are also helpful in deciding the exact time of sowing and harvesting of crop, according to the availability of soil moisture and soil temperature (Jeevanandam et al. 2020).

Environmental remediation

In recent times, the environment has been polluted severely, due to excessive use of fertilizers and other practices via traditional crop production methods, where different pollutants are being deposited in the soil and ground water that can disturb the growth of plants. These pollutants are identified to be degraded, dissolved, or converted into less toxic compounds with the aid of nanoparticles (Dhewa 2015). Nanoscale zerovalent iron particles are used recently for environmental remediation as it produces very less amount of toxic hazardous effects during the degradation of pollutant in the contaminated sites (Nadagouda and Varma 2009). Further, toxic heavy metals in ground water, such as lead (Pb2+), chromium (Cr3+), cadmium (Cd2+), and zinc (Zn2+) can be removed by using carbon nanotubes (Li et al. 2003; Rao et al. 2007). Furthermore, bimetallic (iron and palladium) nanoparticles are also used to remove toxic trichloroethane (TCE) (Elliott and Zhang 2001). Thus, it is worthy to note that the nanoparticles are highly beneficial for the removal of environmental contaminants, such as heavy metals, dyes, chlorinated organic compounds, organophosphorus compounds, volatile organic compounds, and halogenated herbicides, due to their high reactivity, high surface area-to-volume ratio, longer stability, and target specific delivery property (Guerra et al. 2018).

Nanotechnology in the development of plant breeds

Plant breeders are using different techniques, such as mutations via X-rays, gamma rays, and beta rays to modify genetic make-up of the plants to develop a new variety from the existing varieties. Currently, nanoparticles have gained the attention of scientists to amend genetic make-up of the crops (Prasanna and Hossain 2007) and to make desirable genetic alterations without altering other functions of the cells. It has been predicted that the success of this technique will lead to the development of photo-insensitive crop varieties that can be produced throughout the year. Further, use of nanotechnology can halt the debate on the usage of genetically modified crop (GMO) and take it to the next level as atomically modified crop (AMO) (S B et al. 2016). In certain studies, scientists have altered the purple color of leaves and stem of rice variety “Khao Kam” to green color via nanotechnology. Further, nanoparticles are also used to develop insect and pest resistant varieties. Furthermore, silica nanoparticles were used for transferring DNA into tobacco plant, whereas gold nanoparticles were used for the capping of targeted gene to avoid gene leaching at non-targeted site of plant (Torney et al. 2007). Notably, nanoparticles are also utilized for the development of distinct plant varieties, instead of mutation causing X-rays, gamma rays, and beta rays, due to their high cell penetration ability and RNA binding properties (Zhang et al. 2010). Further, it has been identified that nanoparticle-coated genes have more target specific delivery without any undesirable side effects, compared to other conventional gene delivery approaches (Galbraith 2007).

Nanoinsecticides and nanopesticides

Infestation of insects and diseases in crop production is increased, since the time of green revolution, due to higher usage of chemical fertilizers and thus, utilization of insecticides and pesticides were also increased. Globally, around 2 million tonnes of chemical-based toxic pesticides are used in agriculture (De et al. 2014), which is harmful for beneficial soil microbes and increases resistance toward insects and pests (Tilman et al. 2002). Higher usage of insecticides and pesticides is not an eco-friendly practice as it can destroy the soil biodiversity. Thus, replacement of insecticides and pesticides by eco-friendly materials is the need of the hour (Ragaei and Sabry 2014) and nanomaterials are considered as a potential alternative to minimize limitations of traditional insecticide and pesticide (Sasson et al. 2007). Further, smaller size of nanoparticles is beneficial in making them highly active, compared to conventional insecticide or pesticide (Madhuri et al. 2010). Furthermore, application of nanomaterials is eco-friendly and less toxic to human health (Mousavi and Rezaei 2011). Several metallic nanoparticles, such as silver, gold, and iron are widely used for insect and pest control (Al-Samarrai 2012). Debnath et al. (2011) reported that less nanomaterial concentration is required for plant protection, due to their higher reactivity, compared to bulk material (Debnath et al. 2011). Similarly, nanomaterials, that are applied as foliar spray, are reported to be more effective than absorption of nanomaterial by roots (Lauterwasser 2005). Likewise, Seo et al. (2011) and Pinto et al. (2013) reported that the use of thiamine di-lauryl sulfate (TDS) nanoparticles and nanocomposites have significant inhibitory effect on Colletotrichum gloeosporiodies and Aspergillus niger, respectively (Seo et al. 2011; Pinto et al. 2013). Moreover, Ing et al. (2012) stated that the copper-chitosan nanocomposites possess ability to control Fusarium solani with enhanced biodegradability and less toxicity (Yien et al. 2012). Besides, quantum dots are extensively used to monitor plant pathogens and advantageous microbes (Lévesque 2001). In addition, antimicrobial property of silver nanoparticles can further reduce the use of pesticides (Bhagat et al. 2019) and Barik et al. (2008) reported that nanosilica is used to control several insect and pests in agricultural field (Barik et al. 2008). Thus, it can be noted that several nanoparticles are widely used in agricultural applications and can be beneficial in seed priming applications to improve seed quality parameters. Moreover, insecticidal properties of nano-engineered materials, such as nanostructured alumina (NSA), has been reported to be due to the surface charge of nanomaterials, that are generated during the synthesis, and their strong sorptive action (Stadler et al. 2018). Meanwhile, insects also generate electrostatic charges on their surface via actions such as walking, flying, or rubbing with any surfaces (Edwards 1962; Stadler et al. 2010; Stadler et al. 2017). Thus, the electrostatic force of attraction between the nanomaterials and the insect leads to attachment as well as internalization of nanomaterials. Later, the nanomaterial degrades into its ionic state, lead to damage of cuticular wax, which is responsible for the water balance in the insect body, and inhibits the growth of insects via dehydration. For instance, NSA is recognized as a strong insecticide against storage pests, including Rhyzopertha dominica and Sitophilus oryzae (Stadler et al. 2010).

Metal nanoparticles on seed quality parameters

Plant growth is highly dependent on several factors; however, uptake and utilization of distinct nutrients is one of the most crucial factors as they are essential for proper functioning of plants. Nanoparticles have influence on several mechanisms in plants as they can enter cells easily through cell wall due to their small size (Moore 2006). Pore size of the cell wall is also enlarged by the application of nanoparticles, which ensures their internalization into the cells (Nair et al. 2010). Uzu et al. (2010) reported that nanoparticles can internalize into the leaves via stomata, when nanoparticles are sprayed on the leaf surface (Uzu et al. 2010). Further, treatment of nanoparticles has led to the internalization into seeds or cell organelles (Khodakovskaya et al. 2009; Etxeberria et al. 2006). Furthermore, Zhu et al. (2008) also identified the presence of iron oxide nanoparticles in pumpkin roots after treating them for few days (Zhu et al. 2008). Moreover, Dimkpa et al. (2015) reported that nanoparticles can internalize into the cell wall, even when the pore size of the cell is less than nanoparticle size (Dimkpa et al. 2015), as TiO2 nanoparticles of up to 140 nm in size were identified to be present in the wheat seeds (Larue et al. 2012). Similarly, Geisler-Lee et al. (2013) reported the presence of silver nanoparticles in the root cap and other parts of the roots, after nanoparticle treatment for few days (Geisler-Lee et al. 2012). There are numerous evidences to emphasize that nanoparticles are transportable within plants via plasmodesmata or other channels (Lin et al. 2009). Servin et al. (2013) demonstrated that the soil treated with TiO2 nanoparticles can allow them to internalize into cucumber leaves and fruits, and these nanoparticles possess ability to increase the activity of catalase enzyme that can protect the plant from reactive oxygen species (ROS) (Servin et al. 2013). ROS production within the cells or plants is very harmful as it can lead to damages in DNA and membrane, lipid peroxidation, protein oxidation, and ultimately cause cell death. The production and toxic effect of ROS can be reduced by various enzymes, namely superoxide dismutase (SOD) (Scandalios 1993), ascorbate peroxidase (APX) (Rico et al. 2015), glutathione peroxidase (GPX), and catalase (CAT) (Garg and Manchanda 2009).

Super-dispersive metal powders to improve seed quality

Metal oxide nanoparticles are reported to have ability to increase the defence mechanism of plants by increasing the production level of SOD, APX, and CAT. Increased SOD level can be obtained by exposing plants toward metal-based nanoparticles. Faisal et al. (2013), Ma et al. (2016), and Rajeshwari et al. (2015) demonstrated that nanosized NiO, CeO2, and Al2O3 particles possess ability to elevate SOD level in tomato, rice, and onion plants, respectively (Faisal et al. 2013; Ma et al. 2016; Rajeshwari et al. 2015). Similarly, the production of CAT can be induced by metallic nanoparticles, such as Fe3O4 NPs, Co3O4 NPs, and CeO2 NPs, whereas Au NPs, MnO2 NPs, Fe3O4 NPs, CuO NPs, Co3O4 NPs, and CeO2 NPs possess capability to increase the production of GPX and Pt NPs as well as CeO2 NPs can increase the production SOD in specific plants (Wei and Wang 2013). Further, silica nanoparticles were also identified to increase the salinity tolerance in tomato (Haghighi and Pessarakli 2013), drought tolerance in wheat (Ahmed et al. 2016), stimulate antioxidant enzyme system, and results in better seedling growth (Bao-Shan et al. 2004; Neethirajan et al. 2009). Furthermore, application of ZnO nanoparticles have positive effect toward the growth of several plants as zinc is an essential element for plant growth and main component of certain enzymes and plants (Cakmak 2000a). Besides, zinc also serves as a precursor of indole acetic acid (IAA) or auxin production, which is known as the growth inducer (Buchanan et al. 2015). Also, Senthilkumar (2011) stated that ZnO nanorods can improve biochemical and physiological properties in black gram seeds (Senthilkumar 2011), whereas application of ZnO nanoparticles are proven to increase the lycopene content in tomato for up to 113% (Raliya et al. 2015) and phosphorus uptake in mung bean up to 10.8%, compared to bulk ZnO (Raliya et al. 2016). Moreover, TiO2 nanoparticles possess photocatalytic property, which is beneficial to convert light energy into chemical or electrical energy and can be used to enhance the photosynthesis process of the plants (Chen and Mao 2007). Likewise, Zheng et al. (2005) reported that TiO2 nanoarticles are proven to increase the activity of Rubisco activase enzyme in spinach that helps in the better seedling growth. Different experiments were performed to analyse the response of TiO2 nanoparticles toward seed germination, plant growth, plant protection, and also to identify pesticide wastage in the soil (Zheng et al. 2005). Antibacterial and antifungal properties of silver nanoparticles are widely used in biomedical applications, due to their biocompatibility and bioavailability (Singh et al. 2008), which elevates its application in the agriculture to control phytopathogens without affecting environment (Panáček et al. 2009). Seed treatment with AgNPs is demonstrated to protect seed and seedlings from seed-borne and soil-borne bacterial and fungal diseases. Further, application of silver nanoparticles are reported to reduce diseases, such as powdery mildew (Park et al. 2006) and spot blotch in wheat (Mishra et al. 2014), to address fungal infections in plants (Min et al. 2009) and to control conidial germination in fungal species of Raffaelea genus (Kim et al. 2009).

Effect of zinc nanoparticles on seed quality parameters

Zinc is an integral part of plants, which act as a co-factor for various enzymes. Further, it is also engaged in synthesis and functioning of indole-acetic acid (IAA), protein, chlorophyll, carbohydrate, cytochromes, and leaf cuticle (Marschner and Marschner 1996; Buchanan et al. 2000) and are utilized for the detoxification of reactive oxygen species (Cakmak 2000b). Deficiency of zinc cause shortening of internodes that consequences in reduced plant growth and reduced number of plant leaves and ultimately yield is reduced. Nanosized zinc particles can have an adverse effect on the seed quality parameters, as they can associate with the integral part of plants, similar to micro and macro-sized particles of zinc. Generally, zinc nanoparticles are used in the form of metal oxide, which is an inorganic compound, that is nearly insoluble in water and appears as a white powder so seed treatment with zinc oxide nanoparticles did not leach out in presence of soil water but normal zinc salt are leached out in presence of water present in soil and decline the efficiency of seed treatment with zinc in comparison to seed treatment with zinc oxide nanoparticles. These zinc oxide nanoparticles (ZnONPs) are extensively used in various coatings and personal care products, due to their ultraviolet radiation absorption and transparency properties (Luna-delRisco et al. 2011; Poynton et al. 2011). Furthermore, United States Food and Drug Administration (USFDA) recognized nanosized ZnO as generally recognized as safe (GRAS) compound (Duvall 2012). Moreover, ZnONPs possess enhanced antimicrobial properties against Staphylococcus aureus and Escherichia coli, which can be attributed to their reactive oxygen species production ability (Emami-Karvani and Chehrazi 2011; Senthilkumar and Sivakumar 2014). In addition, Molina et al. (2006) also reported beneficial effects of ZnONPs on Pseudomonas putida bacteria, which is a strong plant root colonizer (Molina et al. 2006).

ZnONPs has positive as well as the negative effect on seed quality parameters, in the context of agricultural crops. A possible increment in the germination percentage, seedling length, and seedling dry weight was observed, when the seeds of chili, rice, and onion were treated with ZnONPs (Afrayeem and Chaurasia 2017; Panda 2017; Raskar and Laware 2014a). Likewise, Latef et al. (2017) reported that seed priming with different concentrations (20 ppm, 40 ppm, and 60 ppm) of ZnONPs possess ability to increase the root length, shoot length, fresh and dry weight of the root, as well as shoot of lupine (Lupinus termis) and the seeds were primed with 60 ppm concentration to achieve maximum growth. The study also emphasized that zinc oxide nanoparticles can suppress the salinity stress and elevate the growth of lupine (Latef et al. 2017). Similarly, Singh et al. (2013) investigated the effect of nano and bulk zinc oxide particles on seed quality parameters of cabbage, cauliflower, and tomato and identified that the nanosized zinc oxide had positive effect on germination percentage and seedling length in all three crops; however, the bulk form had inhibitory effects on the seed (Singh et al. 2013). Further, Anandaraj and Natarajan (2017) reported that seed treatment of aged onion seeds with zinc nanoparticles possesses ability to increase the germination percentage, shoot length, root length, and vigor indices over control (non-treated seeds) (Anandaraj and Natarajan 2017). Similar findings of increased seed quality parameters in aged seeds were reported by Shyla and Natarajan (2016) in groundnut (Shyla and Natarajan 2016). Furthermore, Gokak and Taranath (2015) found that seed treatment with zinc nanoparticles had no significant effect of germination percentage as well as root and shoot elongation of Macrotyloma uniflorum (Lam.) Verdc; however, a delay in germination was reported after treating the seeds with nanoparticles (Gokak and Taranath 2015). Conversely, Lin and Xing (2007) reported that nano-Zn and nano-ZnO have an inhibitory effect on root elongation of radish, rape, ryegrass, lettuce, corn, and cucumber. The study also added that the nano-Zn and nano-ZnO have an inhibitory effect on the seed germination of ryegrass and corn at 2000 ppm, respectively, and the seed germination of radish, rape, lettuce, and cucumber was not affected, significantly (Lin and Xing 2007). Moreover, Xiang et al. (2015) investigated the effect of ZnONPs toward the seed germination of Chinese cabbage and revealed that the germination was not significantly affected, whereas the root and shoot elongation was inhibited by the ZnONPs (Xiang et al. 2015). Similarly, no effect was reported on germination and inhibitory effect toward the root and shoot elongation was identified in rice, due to higher doses of nanoparticles (Boonyanitipong et al. 2011).

Lopez-Moreno et al. (2010) reported that ZnONPs had no significant effect on seed germination of soybean; however, the root length was increased at lower concentration (500 ppm) and the higher concentration (4000 ppm and 5000 ppm) had an inhibitory effect on the root length of soybean (López-Moreno et al. 2010). Further, Raskar and Laware (2014) found that treatment of seeds with a lower concentration of ZnONPs can positively affect the germination percentage of onion; however, higher dose of nanoparticles can inhibit the germination percentage by reducing mitotic division and causing abnormalities in chromosomes (Raskar and Laware 2014b). Similar findings of increased germination with a lower concentration and decreased germination with higher dosage was also reported in wheat (Ramesh et al. 2014) and Allium cepa (Ghodake et al. 2011). Furthermore, Mahajan et al. (2011) reported that seedling growth and biomass of mung (Vigna radiata) and gram (Cicer arietinum) was increased due to seed treatment with 20 ppm of ZnONPs; however, higher doses are proven to cause negative effect on seedling growth and biomass (Mahajan et al. 2011). This might be attributed to the fact that higher dose of nanoparticles exhibit toxicity due to accumulation of nanoparticles in seedlings (Mahajan et al. 2011). Likewise, Dhoke et al. (2013) demonstrated that foliar spray of ZnONPs can increase the root length, shoot length, and biomass of mung (Dhoke et al. 2013). Similarly, Zafar et al. (2016) also revealed that the lower concentration (1–20 ppm) of zinc oxide nanoparticles can increase the germination percentage and seedling growth in Brassica nigra; however, higher concentration (500–1500 ppm) had an inhibitory effect on these parameters (Zafar et al. 2016). Moreover, De la Rosa et al. (2013) stated that higher dose (1600 ppm) of ZnONPs nanoparticles had a significant effect on germination of cucumber seeds; however, it reduces the germination percentage by 40% and 20% in alfalfa (Medicago sativa) and tomato (Solanum lycopersicum), respectively. Further, lower dose (250 ppm) also reduced the germination percentage in tomato (de la Rosa et al. 2013). In addition, Prasad et al. (2012) emphasized that the nano form of zinc has a significant effect on germination, root and shoot growth, as well as the dry weight of peanut seeds; however, bulk form of zinc had non-significant effect on these parameters. They also reported that lower dose (1000 ppm) of nanoparticles had a significant effect, whereas higher doses (2000 ppm) had an inhibitory effect on these parameters (Prasad et al. 2012). Correspondingly, Singh et al. (2016) also described that the treatment of seed with zinc oxide nanoparticles had a positive effect on seed germination, radicle length, plumule length, and seed vigor indexes, whereas the bulk form (zinc sulfate) had inhibitory effects on these parameters (Singh et al. 2016).

Effect of silver nanoparticles on seed quality parameters

Silver nanoparticles are one of the most common metal nanoparticles, that are widely used in personal care items, water purification, construction materials, food packaging industry, and various medical related instruments (Borase et al. 2014). Further, it has been reported that silver nanoparticles have a biocidal effect on bacteria (Wakshlak et al. 2015; Prasad 2014). Application of silver nanoparticles is increased in agriculture sector due to its antimicrobial property that is used for plant protection (Morones et al. 2005). Furthermore, Chen and Schluesener (2008) stated that AgNPs are the most studied and used nanoparticles in the agriculture sector for higher yield and sustainability of agricultural crops (Chen and Schluesener 2008). Likewise, the silver nanoparticles are also proven to possess both positive and negative effect on seed quality parameters of different crops, depending on the method and duration of treatment, size, shape, concentration, experimental conditions, and synthesis approach (Mirzajani et al. 2013; Tripathi et al. 2017; Tripathi et al. 2015; Da Costa and Sharma 2016).

Savithramma et al. (2012) stated that seed treatment with AgNPs in Boswellia ovalifoliolata has doubled the seedling height and speed of germination, while the seed emergence was also increased by 28% over the control (Savithramma et al. 2012). Likewise, Jasim et al. (2017) demonstrated that silver nanoparticle-treated seedlings of fenugreek had more root and shoot growth, compared to the control (Jasim et al. 2017). Similarly, Hojjat and Hojjat (2015) also reported that the seed treatment of fenugreek with AgNPs can increase the germination percentage, germination rate, seedling length, and weight (Hojjat and Hojjat 2015). Further, Almutairi (2016) investigated the effects of tomato seed treatment with AgNPs under salinity stress and revealed that the nanoparticles mitigated salt stress during germination and has led to higher germination rate, seedling length, seedling fresh, and dry weight (Almutairi 2016). Similar findings were also observed in Thymus vulgaris L. and Thymus daenensis Celak under salinity stress (Ghavam 2018). Furthermore, Jassem et al. (2018) reported that seed quality parameters were significantly increased due to nano-priming of wheat seeds with AgNPs (Jassem et al. 2018); however, the contrary results of nano-priming were also observed in recent times (Yasmeen et al. 2015). Moreover, Rezvani et al. (2012) stated that silver nanoparticles possess ability to encourage root growth by obstructing ethylene signals in Crocus sativus (Rezvani et al. 2012). In another study by Sharma et al. (2012), the seeds of Brassica juncea were raised in MS (Murashige and Skoog) medium, that are supplemented with silver nanoparticles at 0, 25, 50, 100, 200, and 400 ppm of concentration and identified that the lower concentrations (25 and 50 ppm) had no significant effect on germination; however, root length was increased by 167 and 277% and shoot length by 15 and 25%, respectively (Sharma et al. 2012a). Besides, higher dose of nanoparticles had an inhibitory effect on germination and root length. The similar finding of increased biomass due to supplementation of MS medium with silver nanoparticles was also reported in Arabidopsis thaliana (Kaveh et al. 2013). Further, Pokhrel and Dubey (2013) revealed that citrate-coated silver (Citrate–nAg) nanoparticles and zinc oxide nanoparticles (nZnO) can increase the root length in maize and cabbage. In both maize and cabbage, measurement of germination level has revealed that the lower nanoparticles can lead to toxicity (Pokhrel and Dubey 2013). Likewise, another report revealed an increment in the germination of wheat seeds, root length, and shoot length, due to AgNPs treatment (Sabir et al. 2011).

Mahakham et al. (2017) revealed that the silver nanoparticle treatment of aged seeds can increase the germination percentage and seed vigor of rice (Mahakham et al. 2017b). Further, Almutairi and Alharbi (2015) observed that silver nanoparticles had a significant effect on germination and root length of watermelon as well as zucchini; however, germination and root length of the corn was affected positively and negatively due to AgNPs, respectively (Almutairi and Alharbi 2015). According to Yasur and Rani (2013), AgNPs had no significant effect on seed germination as well as root and shoot length of castor bean (Ricinus communis L) plant (Yasur and Rani 2013). Furthermore, Karimi et al. (2012) also reported that AgNP-coating of wheat seed had no significant effect on seed germination (Karimi et al. 2012). Similarly, Dimkpa et al. (2013) demonstrated that different concentrations (0, 0.5, 1.5, 2.5, 3.5, and 5 mg/kg) of silver nanoparticles and silver had a negative effect on the root growth of wheat (Dimkpa et al. 2013). Likewise, Gubbins et al. (2011) revealed that the plant growth as well as fresh and dry weight of Lemna minor L was decreased, when the fronds of Lemna minor L were exposed to different concentration (0, 5, 10, 20, 40, 80, and 160 μg/L) of AgNPs for 7 and 14 days. The study further reported that higher concentration and more exposure time had led to an inhibitory effect on the seed growth (Gubbins et al. 2011). Moreover, small sized (2 nm) AgNPs and bulk form of silver are proven to have reduced effect on seed germination, root length, and weight in cucumber and lettuce (Barrena et al. 2009). Additionally, El-Temsah and Joner (2012) reported that minimum sized nanoparticles (Agcoll) at 10 ppm of concentration can reduce the germination of rye grass seeds; however, they are proven to have a positive impact on the germination of flax and barley. The intermediate (Ag5nm) and a large-sized silver nanoparticles (Ag20nm) at 10, 20, and 100 ppm are further proven to decrease the seed germination percentage of barley and do not have any effect on the germination of flax and rye grass. Also, the shoot length was more sensitive to silver nanoparticles and every concentration of distinct nanoparticle sizes is proven to decrease the shoot length of all the three crop seedlings (El-Temsah and Joner 2012).

Amooaghaie et al. (2015) revealed that both nano and bulk form of silver had an inhibitory effect on germination of Brassica nigra seeds and nanoparticles were more toxic than the bulk form of silver (Amooaghaie et al. 2015). Likewise, Thuesombat et al. (2014) reported that different sized (20, 30–60, 70–120, and 150 nm) silver particles of distinct concentration (0.1, 1, 10, 100, and 1000 ppm) had an inhibitory effect on germination percentage, root length, shoot length, root dry, and fresh weight of jasmine rice and they also added that small sized nanoparticles at lower concentration were less toxic to the seed quality parameters than large-sized nanoparticles at higher concentration (Thuesombat et al. 2014). Further, Stampoulis et al. (2009) designed an experiment to check out the effect of nano and bulk form of silver in zucchini (Cucurbita pepo) and identified that the seed germination was not significantly affected by both forms of silver; however, plant biomass was reduced for silver nanoparticle treated seeds, compared to control and bulk form, due to higher accumulation of nanoparticles, that has led to phytotoxicity (Stampoulis et al. 2009). Furthermore, Gruyer et al. (2013) demonstrated that the nano and bulk form of silver nitrate had different effects on root length of lettuce, barley, and radish. The study also revealed that the seeds of radish and barley can exhibit higher root elongation, when they were exposed to nano and bulk form of silver nitrate; however, the lettuce root growth was suppressed due to these materials (Gruyer et al. 2014). Similarly, Geisler-Lee et al. (2013) identified that the germination of Arabidopsis thaliana seeds was not significantly affected, when they were exposed to different sized (20, 40, and 80 nm) AgNPs and Ag+; however, the growth rate of plants was reduced. While the maximum reduction was identified in the seeds treated with smaller (20 nm) sized nanoparticles (Geisler-Lee et al. 2012). Similar findings of no effect on germination of wheat seeds treated AgNPs, that are applied with MS medium, were observed in a recent study; however, the number of seminal roots, root biomass, fresh weight, and dry weight was increased at lower concentrations of silver nanoparticles and higher concentration had an inhibitory effect on above-mentioned parameters (Razzaq et al. 2016).

Abdel-Azeem and Elsayed (2013) reported that different sized (65, 50, and 20 nm) AgNPs had no noteworthy effect on the seed germination of Vicia faba; however, the root length was decreased with a reduction in size of nanoparticles and with more exposure time to nanoparticle (Ahmed et al. 2013). Similarly, Qian et al. (2013) demonstrated that AgNPs and Ag+ ion had no adverse effect on germination; however, the root growth was decreased due to both nanoparticles and ions. This study also showed that the silver nanoparticles were found to be more toxic for root growth, compared to Ag ion (Qian et al. 2013). Likewise, Pandey et al. (2014) examined the influence of silver nanoparticles and microparticles on the germination percentage, root length, and shoot length on seeds of Brassica juncea L. and identified that the nano form of silver had a positive effect on every parameter, compared to bulk form, which has led to a negative effect on abovementioned parameters (Pandey et al. 2014). Besides, Krishnaraj et al. (2012) investigated the effect of nano and bulk form of silver nitrate on the seed germination of Bacopa monnieri (Linn.) and reported that nano form of silver did not significantly affect the germination percentage; however, higher concentration (above 100 ppm) of silver nitrate possesses ability to completely halt the seed germination (Krishnaraj et al. 2012). Furthermore, Yin and co-workers (2012) calculated the influences of different silver nanoparticles (polyvinyl pyrrolidone coated-AgNPs, gum Arabic coated-AgNPs, and AgNO3) on the germination of eleven different wetland plants (Eupatorium fistulosum, Lolium multiflorum, Carex vulpinoidea, Panicum virgatum, Scirpus cyperinus, Carex lurida, Carex scoparia, Carex crinita, Phytolacca americana, Lobelia cardinalis, Juncus effusus) and concluded that GA-AgNPs enhanced the germination rate of a species (E. fistulosum) and reduced germination rate of three species (Scirpus cyperinus, Juncus effusus and Phytolacca americana). However, PVP-AgNPs had no significant effect on the germination and the AgNO3 increased the germination rate of 5 species out of 11 (Yin et al. 2012). Moreover, Salama (2012) stated that seedling treatment with different concentration (20, 40, and 60 ppm) of AgNPs had a positive effect on the root and shoot length; however, higher concentration (80 and 100 ppm) had a negative effect on the root and shoot length of common bean and corn (Salama 2012). Recently, similar findings of enhanced seed quality parameter at lower concentrations of silver nanoparticle-treated seeds and decrement of the quality parameters at higher concentration were also reported in Cassia occidentalis plant (Mujeeb et al. 2018). Likewise, Oukarroum et al. (2013) exposed aquatic Lemna gibba plant to the nutrient medium, that are supplemented with AgNPs with a dose of 0, 0.01, 0.1, 1, and 10 ppm and identified the toxic effect of higher AgNPs concentration (1 and 10 ppm) on total number of fronds (Oukarroum et al. 2013). Additionally, Parveen and Rao (2014) reported that the treatment of Pennisetum glaucum seeds with different concentration (25 and 50 ppm) of nano-silver had a significant positive effect on the germination percentage; however, they have exhibited inhibitory effects toward its root length (Parveen and Rao 2015). Further, no significant effect was found in germination, due to nano and bulk form of silver treatment; however, root length was decreased due to silver treatment in radish (Wang et al. 2015b).

Effect of titanium nanoparticles on seed quality parameters

Titanium dioxide nanoparticles are known for its diverse use in common products, such as sunscreen to advanced products, including photovoltaic cells. The nanosized metal oxide (TiO2) of titanium is extensively under research to be used in agriculture. Various research related to titanium dioxide nanoparticles (nTiO2) in agriculture revealed that nTiO2 has both positive and negative or no effect on different vegetative growth, similar to other metal-based nanoparticles and yield parameters according to the dose of nanoparticles, type of treatment and crop, as well as the duration of the treatment (Biswas and Wu 2005; Jiang et al. 2008). Vijayalakshmi et al. (2018) treated naturally aged maize seeds with different (200, 400, 600, and 800 mg/kg) concentration of TiO2 nanoparticles and identified that the lower concentration (200 mg/kg) had a significant effect on germination, seedling length, seedling dry weight, and seed vigor over control treatment (Vijayalakshmi et al. 2018). Likewise, increased seed quality parameters, such as germination percentage, seedling length, seedling dry weight, and vigor index were observed in maize and fleawort (Plantago psyllium L.), respectively (Sani 2012; Farahani et al. 2012). Similarly, Wu et al. (2013) stated that nTiO2 could increase germination of aged lettuce seeds (Wu et al. 2013). Further, Ghorbanpour (2015) observed a significant increase in the root and shoot dry mass, when Salvia officinalis plant seedlings were exposed to 200 mg/L concentration of nTiO2; however, the dry mass of root and shoot was reduced at 1000 mg/L concentration (Ghorbanpour 2015).

Zheng et al. (2005) checked the effect of different concentration (0, 0.25%, 0.50%, 1.00%, 1.50%, 2.00%, 2.50%, 4.00%, and 6.00%) of bulk and nano-sized titanium dioxide on spinach seeds and identified that the germination percentage, germination index, seedling dry weight, and seed vigor was significantly increased and maximum germination as well as growth was found at 2.5% concentration of nano-TiO2 (Zheng et al. 2005). Further, an increment in the fresh weight of root and shoot after nanosized TiO2 treatment was also observed in spinach (Yang et al. 2007). Furthermore, Maroufi et al. (2011) revealed that nano-priming with different concentration (0, 0.1%, and 0.2%) of TiO2 had a significant effect on the seed germination, seedling dry weight, and seedling vigor of green gram seeds and 0.2% of concentration was the most encouraging for these parameters (Maroufi et al. 2011). Similarly, Linglan et al. (2008) stated that the fresh and dry weight of seedlings was increased by 58% and 69%, due to nano priming of spinach seed with titanium dioxide nanoparticles (Linglan et al. 2008). Likewise, Dehkourdi et al. (2014) demonstrated that different concentrations (3.5, 5.5, 7.5, and 9.5%) of nano-anatase had a significant positive effect on germination percentage, root length, shoot length, germination index, fresh weight of seedlings, and vigor index of pepper (Capsicum annum L.). The study also identified that an increment in the concentration favored the seed quality parameters and elevated properties at the nanoparticle dose of 7.5% (Dehkourdi et al. 2014). Besides, Lu et al. (2002) reported that nano-anatase could enhance germination and growth of soybean (Lu et al. 2002). Also, Dehkourdi and Mosavi (2013) evaluated and identified that parsley seed raised in MS medium, that are supplemented with different concentration (0, 10, 20, 30, and 40 mg/ml) of nano-anatase, showed better germination percentage, germination rate, root length, shoot length, and fresh weight over control and maximum improvement in these parameters was found at 30 mg/ml of concentration (Dehkourdi and Mosavi 2013). Moreover, Daghan (2018) raised maize seedlings in Hoagland nutrient solution medium, that are supplemented with different concentration (0, 0.5, 10, 20 mg/L) of titanium dioxide nanoparticles and observed that these nanoparticles have toxic effects on root and shoot length. Maximum shoot and root dry mass was identified in the treatment of control samples and minimum mass was identified at a 20 mg/L concentration of nTiO2 (Dağhan 2018).

Recently, reduced the germination percentage and root growth was observed in maize, when the seeds were treated with nano and bulk form of titanium dioxide (Karunakaran et al. 2016). Similar findings were also observed in Vicia narbonensis L. and Zea mays L (Castiglione et al. 2011). Likewise, Samadi et al. (2014) evaluated the effects of dissimilar bulk and nano form of titanium dioxide nanoparticle concentration (0, 100, 200, and 300 mg/L) on the seed germination, root length, and shoot length of Mentha piperita and identified their negative effect on all the parameters, except 100 mg/L concentration of nTiO2 showed a positive effect in increasing the root length (Samadi et al. 2014). Similarly, Mushtaq (2011) also identified the negative effect of nTiO2 on the germination percentage, germination index, and root length of cucumber (Cucumis sativus) up to 5000 μg/ml concentration (Mushtaq 2011). Conversely, Wu et al. (2012) demonstrated that titanium dioxide nanoparticles had toxic effects on the germination index of lettuce, radish, and cucumber (Wu et al. 2012). Further, Mahmoodzadeh et al. (2013) observed that the seed treatment with higher concentration (2000 mg/L) of nTiO2 had more significant effect on the seed germination and growth of canola (Brassica napus) over other concentrations (10, 100, 1000, 1200, 1500, 1700 mg/L) and the control. Highest and lowest germination rate was identified at 1200 mg/L and 1500 mg/L, respectively. Furthermore, lower concentrations (0, 10, 20, 30, 40 μg/ml) of nTiO2 had a significant effect on the germination and seedling length; however, higher concentration (50 μg/ml) had an inhibitory effect on the seed quality parameters of onion (Laware and Raskar 2014). Moreover, Song et al. (2012) revealed that the growth of Lemna minor was increased, while treating them with nano-anatase of concentration up to 200 mg/L; however, the growth of plants was reduced after proving more than 200 mg/L of concentration (Song et al. 2012). Besides, Feizi et al. (2013) demonstrated that different concentrations (0, 5, 20, 40, 60, and 80 mg/L) of bulk and nano titanium dioxide had no significant effect on root length, shoot length, and seedling dry weight of Salvia officinalis L.; however, the germination percentage was significantly increased, when 60 mg/L of bulk and nano TiO2 was used for the treatment (Feizi et al. 2013). In addition, several literatures also emphasized that exposure to titanium dioxide nanoparticles had no significant effect on the germination and root length of peas, rice, and tomato, respectively (Boonyanitipong et al. 2011; Fan et al. 2014; Song et al. 2013).

Toxicity of super-dispersive nanoparticles on seed quality parameters

There is a requirement of critical research in the field of nanotechnology, especially in the agriculture sector to identify the positive and negative effects of nanoparticles on seed quality parameters. In certain cases, ZnONPs has negative effect on seed germination and seedling growth, which can be attributed to the aggregation of nanoparticles that can block cell wall pores, decline water uptake, as well as gas exchange and results in poor plant growth. Lin and Xing (2008) confirmed by electron microscopy images that the application of ZnONPs can damage the epidermal, endodermal, vascular, and cortical cells, which can lead to oxidative stress in ryegrass plants resulting in the death of plants (Lin and Xing 2008). Higher ROS generation induced by ZnONPs application is also reported in the literature (Xia et al. 2006; Ryter et al. 2006). Further, Panda et al. (2003) stated that the application of ZnONPs can negatively affect the antioxidant system of wheat (Panda et al. 2003). Furthermore, decreased chlorophyll content in cucumber was observed after the application of ZnONPs (Aarti et al. 2006). Similarly, Kumari et al. (2009) identified that the application of silver nanoparticles can disturb cell division and can cause cell disintegration to be responsible for improper zucchini plant growth (Kumari et al. 2009). Likewise, Oukarroum et al. (2013) demonstrated that seed treatment with AgNPs for 1 week in Lemna gibba increased the ROS production and decreased the growth of plants (Oukarroum et al. 2013). During higher production of ROS, H2O2 will be converted into highly active OH− ion that can interact with biological molecules and cause damage to lipids, proteins and DNA (Foyer et al. 1997), where OH− ion cannot be detoxified by any enzymatic action. Besides, lipid damage due to ROS production cause different stresses in plant growth. Final product of lipid peroxidation is malondialdehyde (MDA) that can lead to cell membrane damages (Sharma et al. 2012b). Moreover, Mirzajani et al. (2014) and Vannini et al. (2014) reported that exposure to AgNPs can cause protein damage in rice and wheat seedlings, respectively (Mirzajani et al. 2014; Vannini et al. 2014). Recently, Hossain et al. (2016) also emphasized that both silver and ZnONPs can cause protein damage (Hossain et al. 2016). Similar to other nanoparticles, TiO2 nanoparticles also can cause oxidative stress in Allium cepa and Nicotiana tabacum, that can lead to chromosome damage (Ghosh et al. 2010). It is worthy to note that the DNA or chromosome damage in the cell of any plant can lead to misfunction or total inactivation of proteins, which can inhibit the plant growth (Imlay and Linn 1988). Thus, it is evident that the dose of nanoparticle to be used for seed priming application is highly crucial, as addition of dose above threshold level will lead to toxicity in plants and the environment.

Conclusion

Nanotechnology is an emerging science that has gained significant interest among scientists to be included in several agricultural applications and could be used more in the future for sustainable agriculture. Even though there is no toxic effect of a specific nanoparticle toward a particular crop, it can be concluded from this review that the toxicity of the nanoparticles toward the environment as well as crops depends on the type, chemical composition, size, density, method of preparation, and application of nanoparticles. Further, the type and age of the crop as well as the duration of treatment of a specific nanoparticle toward a particular crop can also play a crucial role in their toxicity. Thus, it can be concluded that nanoparticles possess both positive and negative effects on plants, depending to their properties. Hence, the future of nanotechnology in agriculture sector is still uncertain. Moreover, there is a necessity for more research in this sector to standardize the effect of nanoparticles on crops. Furthermore, there is a lack of research on the effect of nanoparticles in the environment and the human body as its effects can be irreversible. Therefore, there is a requirement to identify the internalization mechanism of nanoparticles within the plants and its ability to transmit from plants to human via food chain, which can be beneficial in utilizing nanoparticles for large-scale agriculture production without any hazardous effect toward humans and the environment.