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Impact of Nanomaterials on Plant Economic Yield and Next Generation
C H A P T E R
7 Impact of Nanomaterials on Plant Economic Yield and Next Generation Tariq Shah⁎, Jinsong Xu⁎, Xiling Zou⁎, Yong Cheng⁎, Xuekun Zhang⁎, Quaid Hussain⁎, Rafaqat Ali Gill† ⁎
Key Laboratory of Biology and Genetic Improvement of Oil Crops, Oil Crops Research Institute, Chinese Academy of Agricultural Sciences, Wuhan, China †Department of Genomics, Oil Crops Research Institute, Chinese Academy of Agricultural Sciences, Wuhan, China O U T L I N E 1 Introduction
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2 Nanotechnology
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Unraveling Potential Application of Nanotechnology in Crop Production 3.1 Nano-Fertilizers 3.2 Nano-Pesticides 3.3 Pesticides Degradation 3.4 Pesticides Residues Detection 3.5 Nano-Herbicides
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3.6 3.7 3.8 3.9
Plant Pathogens Detection Water Retention Improving Seed Germination Genetic Material Delivery
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Safety and Ethical Issues
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Future Outlook
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6 Conclusion
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References
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1 INTRODUCTION Agriculture presently is facing a number of challenges, such as stagnation in crop yields, low nutrient use efficiency, multinutrient deficiencies, the onset of pests and disease, water availability, shrinkable arable land, climate change, and shortage of labor. Moreover, demand for more food is increasing day by day due to the ever-increasing global population. This rapidly increasing global population needs sufficient food supply to feed millions of new mouths. For global food security, crop production must be increased several-fold sustainably
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with reduced inputs. Hence, there is an urgent need for proper management to utilize optimum plant nutrients and pesticides for enhancing crop production so as to cope with this alarming situation. For this, one of the frontier technologies, such as nanotechnology, can be explored to detect precisely and supply accurate quantities of plant nutrients and pesticides to increase crop production and conservation of inputs. Nanotechnology is the science in which materials are manipulated at nanoscale displaying different properties from bulk materials due to their size and surface area. This is a fast-growing field of science and technology. Nanotechnology and its derived applications are very important in the area of agriculture to address a number of issues pertaining to sustainable agricultural inputs aiming for enhancing crop productivity. Nanotechnology has several applications in various stages of crop production, processing, storing, packaging, and transporting of agricultural products. This chapter briefly reviews and describes some of the applications of nanotechnology in crop production practices to enhance crop productivity.
2 NANOTECHNOLOGY The term “nanotechnology” was first defined in 1974 by Norio Taniguchi of the Tokyo Science University as the study of manipulating matter on an atomic and molecular scale (Agrawal and Rathore, 2014; Taniguchi et al., 1974). The definition of nanotechnology is based on the prefix “nano” which is from the Greek word meaning “dwarf.” A nanometer is one billionth of a meter. The word nanotechnology is generally used when referring to materials with the size of 1–100 nm and deals with the alteration of materials at nanoscale and creation of nano-sized materials generally less than 100 nm in at least one dimension. These materials display different properties from bulk materials due to their size, including physical strength, chemical reactivity, electrical conductance, magnetism, and optical effects. Nanotechnology involves the manipulation or self-assembly of individual atoms or molecules or molecular clusters into structures to create materials and devices with new or vastly different properties. Nanoparticles are synthesized from organic or inorganic nanomaterials (NMs) using a number of chemical and physical methods (Table 1). The techniques for synthesis of nanoparticles usually involve either a bottom-up or a top-down method (Fig. 1). In the top-down approach, the size of particles is reduced by different chemical and physical procedures, such as milling, homogenization using high pressure, and sonication, while in the bottom-up method, the nanostructure building blocks of the nanoparticles are formed first and subsequently assembled to produce the final nanoparticles. Metal oxide nanoparticles such as ZnO, TiO2, MgO, and AgO are some examples of inorganic materials. Metallic nanomaterials are very interesting materials with unique electronic and electro-catalytic properties depending on their size and morphology. They can be utilized for nanostructured materials with specific forms, such as quantum dots (QDs). Other examples of inorganic materials are clay nanoparticles having a structure of stacked platelets. Organic materials, such as carbon nanotubes (CNTs), polymers, and lipids, are also materials with various applications in nanotechnology. Quantum dots are inorganic nanocrystals used for development of optical biosensors which are widely used to detect organic compounds and biomolecules, such as proteins, enzymes, and amino acids. Similarly, surface plasmon resonance (SPR) is a robust tool that can measure the binding kinetics of two molecules without the help of any fluorescent tag.
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TABLE 1 Nanomaterials Examples and Their Application Nanomaterials
Application
INORGANIC METAL NANOPARTICLES AgO, TiO2, ZnO, CeO2; Fe2O3
Delivery of biomolecules (proteins, peptides, nucleic acids), biosensors, diagnostic techniques, pesticide degradation
FePd, Fe-Ni; Silica; CdTe, CdSe CLAY Montmorillonite layered double hydroxides
Delivery of pesticides, fertilizers, plant growth-promoting factors
ORGANIC CARBON NANOTUBES Nanofibers
Biocatalysts, sensing
LIPIDS Liposomes
Delivery of DNA, and pesticides, essential oils
Lippopolyplexes Solid lipid nanoparticles POLYMERIC Natural
Biocompatible, biodegradable, nontoxic for delivery of DNA/ RNA
Cellulose, starch, gelatin, albumin Chitin, chitosan SYNTHETIC Dendrimers
Delivery of pesticides and DNA/RNA
Polyethylene oxide Polyethylene glycol Polylactides
Bottom-up method
Arrangement of smaller components in to more complex (layer-by-layer self assembly)
Top-down method Creation of smaller objects using larger objects
FIG. 1 Two approaches of nanoparticles synthesis.
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Thus, this technique can be adopted for biomolecular interaction analysis taking less time to detect binding events. Dendrimers are synthetic, organic macromolecules with highly branched and defined three-dimensional structures which provide a high degree of surface functionality and versatility. Similarly, aptamers are single-stranded nucleic acids which fit for the target in all the way forming three-dimensional structures with strict bonding of target specific binding with high affinity. This kind of nanosensor gives more specific and effective detection of plant diseases. Carbon nanotubes are hollow cylindrical tubes with one (single-walled CNTs), two (double-walled CNTs), or several (multiwalled CNTs) concentric graphite layers capped by a fullerenic hemisphere. Easy electron transfer reactions and high surface area by virtue of unique metallic, structural, and electronic characteristics of CNTs make them an important category of nanomaterials. Lipid nanoparticles used as delivery systems are composed of aqueous dispersion of dry powder as well as solid lipids, such as triglycerides, waxes, steroids, fatty acids, and emulsifiers and are prepared by a homogenization process using high pressure. Polymeric nanoparticles prepared from natural and synthetic polymers by wet synthetic methods are commonly used because surface modification can be carried out easily in subsequent steps for stability. Nanoparticles prepared from natural sources and biopolymers have some merits of biocompatibility and biodegradability. A biosensor is a device that integrates a biological recognition element with a physical or chemical transducer to detect a biological product. Broadly, a biosensor comprises three components: the biological recognition element, the transducer, and the signal processing electronics (Fig. 2). Different categories of nanomaterials, such as nanotubes, quantum dots, or other biological nanomaterials, have been used in biosensor technology to produce nano-biosensors (Prasad et al., 2016). These nanomaterials can contribute to either the bio- recognition element of a biosensor or the transducer or both. Nanoparticle-based biosensors are particularly attractive because they can be easily synthesized in bulk using standard chemical methods. Biosensors may be categorized according to the mechanism of biological selectivity (bioreceptor) or mode of physiochemical signal transduction (transducer). A bioreceptor is a molecular species which exploits a biochemical mechanism of recognition and is accountable for binding the concerned analyte to the sensor for measurement. The bioreceptor can broadly be categorized into various distinct classes such as antibodyantigen bioreceptor, enzymatic bioreceptor, nucleic acid bioreceptor, cellular bioreceptor, biometric bioreceptor, and bacteriophage bioreceptor. The transducer plays a crucial part in the detection as well as the identification process of a biosensor. The transduction methods, such as optical, electrochemical, and mass-based, are the most favored and universal methods. Several types of biosensors are being developed for different applications in enhancing agricultural crop productivity.
FIG. 2 A typical illustration of a biosensor.
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3 UNRAVELING POTENTIAL APPLICATION OF NANOTECHNOLOGY IN CROP PRODUCTION As traditional approaches for crop productivity reach their limits, agriculture has to adopt novel approaches based on nanotechnology to meet the demand of an ever-growing world population. Precision farming has been a long-derived aim to maximize output while minimizing inputs through monitoring environmental variables and applying targeted action. One application of agricultural nanotechnology addresses low use efficiency of inputs. For example, controlled release mechanisms through nanoscale carriers avoid temporal overdose and reduce the amount of agricultural chemicals used and consequently minimize inputs and waste. The potential applications of nanotechnology in agricultural research include slow and control-released chemical fertilizers, micronutrients, and biofertilizers for high efficiency and efficacy, delivery of chemical pesticides encapsulated in nanomaterials for slow and controlled release, stabilization of biopesticides with nanomaterials, agricultural diagnostics for pest and disease detection, water retention, and nano-genetic manipulation of agricultural crops (Prasad et al., 2014, 2017; Bhattacharyya et al., 2016) (Fig. 3).
3.1 Nano-Fertilizers Much of the fertilizers are not available to plants because of lost runoff and leaching causing pollution. Nanotechnology has provided the feasibility of exploiting nanomaterials as fertilizer carriers for controlled release of nutrients. These smart fertilizers increase nutrient use efficiency and reduce costs of environmental protection. Nanomaterials, having potential contribution in slow release of fertilizers, can be used for encapsulation of fertilizers which is done by various ways such as encapsulation of nutrient inside nano-porous materials and coating of the fertilizer with thin polymer film. Fertilizers can be delivered as emulsion or particles of nanoscale dimension. Fertilizers with sulfur nano-coatings are used for slow release of nutrients. Kaolin and polymeric biocompatible nanoparticles are some nanomaterials with potential applications in slow and controlled release of fertilizers (Wilson et al., 2008). Biodegradable, polymeric chitosan nanomaterials are used for the preparation of controlled release of the NPK fertilizer (Corradini et al., 2010).
FIG. 3 Nanotechnology application in enhancing crop production.
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Smart nano-fertilizers can be converted into intelligent smart nano-fertilizers by combining nanodevices so as to synchronize the release of fertilizers with their uptake by crops. Dr. Carlos Montreal of Agriculture and Agri-Food Canada is one of several research scientists developing such intelligent smart fertilizers that release nitrogen only when the plant needs it and in the amount the plant requires (Monreal et al., 2016). The plants communicate with their surrounding environment by producing various kinds of chemical signals. Many chemical compounds that are associated with nitrogen uptake have been identified. These compounds can be used to synchronize the release of fertilizer with nitrogen uptake by the crop. Nano-biosensor binding to these compounds can be developed so as to control the release of fertilizers. Nano-sized biosensors made up of specific chemical composition with polymer coating have been developed by this team. This system allows the fertilizers to be released into the soil when the plant requires it. The research team is trying to make intelligent fertilizers with biodegradable three-dimensional polymer coating less than 100 nm thick. Biofertilizers are formulations with beneficial microorganisms, such as plant growth- promoting rhizobacteria (PGPR), Rhizobium, Azotobacter, Azospirillum, and blue-green algae, fungal mycorrhizae, and phosphate-solubilizing bacteria (PSB), for example, Pseudomonas sp. and Bacillus sp. (Wu et al., 2005; Prasad et al., 2015). There are some constraints in their usage due to lack of suitable carrier materials, short shelf life, and problems in transportation and storage. Formulations resistant to desiccation can be developed by using polymeric nanoparticle coatings in biofertilizer preparation. Similarly, micronutrients can be trapped in nanomaterials for their slow release to promote optimum plant growth and development (Peteu et al., 2010).
3.2 Nano-Pesticides Nanomaterial-based formulations have potential applications for efficient delivery of pesticides. The process of absorption, attachment, encapsulation, or entrapment of the active ingredient into the nano-matrix occurs in nano-pesticide formulations. These formulations release the active ingredient slowly and in a controlled way. The advantages of nanomaterial- based formulations are improvement in efficiency and efficacy and lower toxicity. Nanopesticides are prepared either by very small particles of pesticidal active ingredients or some other nanostructured molecules with pesticidal properties. Nano-pesticide-based formulations have better dispersion and wettability quality as compared to conventional formulations (Bergeson, 2010a). Nanomaterials used for formulating nano-pesticides have desired properties such as biodegradability, solubility, permeability, and thermal stability (Bouwmeester et al., 2009). Nano-pesticides have large specific surface area enhancing affinity to the target. Nano-emulsions, nano-encapsulates, nano-containers, and nano-cages are some of the nano-pesticide delivery methods which have been widely discussed and reviewed recently (Bergeson, 2010b). The ability to delay or control delivery of pesticides to the target organisms is achieved by these nano-pesticide delivery methods (Singh et al., 2015). Spraying of pesticides involves large droplets associated with splash loss, so droplet size is diminished by using nano-pesticides which contribute to effective spraying and, thus, decrease splash losses. In order to keep the residue level below the critical limits as permissible by the regulatory criteria in foodstuffs, the nano-formulation should be degraded fast in the soil but slowly in plants. The sodium dodecyl sulfate (SDS) can play a role to enhance the photodegradation of the NPs in soil. Pesticide (imidacloprid) nano-formulations were p repared using
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SDS-modified silver/titanium dioxide (Ag/TiO2) along with chitosan and alginate by microencapsulation technique to increase the photodegradation by Guan et al. (2010). Keeping in view the formulation stability, which is an important aspect at the nano-level, polymer stabilizers such as polyvinylpyrrolidone, polyvinyl alcohol, and poly(acrylic acid)-b-poly(butyl acrylate) were used by Liu et al. (2008a) for formulating a stable nano-pesticide. Plants also provide a source of biomolecules which are not persistent in water and soil but biologically efficient (Isman, 2000). These phytochemicals, such as essential oils and secondary metabolites, face problems of cost-effectiveness and stability. Incorporation of essential oil obtained from Artemisia arborescens L. into solid lipid nanoparticles diminished the fast evaporation of essential oil (Lai et al., 2006). Similarly, amorphous nano-silica obtained from various sources, such as phytoplankton, volcanic soil is used as a useful biopesticide against stored grain, fungal organisms, and worms (Liu et al., 2008a,b). There are some biological control agents, such as fungi, bacteria, and viruses, which are used against insect pests. Bacterial and viral formulations prepared from these organisms are usually susceptible to desiccation, ultraviolet light inactivation, or even heat. Nano-formulations may provide new ways to enhance the stability of these biological agents. Mycopesticides (fungal biocontrol agents) are promising biological pesticides as there is no need of ingestion; instead, they act by contact, are very specific, and can be easily mass-produced (Barik et al., 2008). Microbial products such as enzymes, antibiotics, inhibitors, and toxins are promising as biopesticides against plant pathogens and pests (Bhattacharyya et al., 2016). Microbial-based formulations need stabilization and a directed delivery mechanism toward identified targets. Chitosan and clay, being biocompatible nanomaterials, can be used as stabilizing and delivery agents and, hence, have potential applications in the development of biopesticide formulations.
3.3 Pesticides Degradation Pesticide-contaminated soil and water are generally treated using phytoremediation, photochemical processes, and oxidation processes. Biorecalcitrant pollutants can be degraded using nanoparticles (Joo and Cheng, 2006). It has been shown that pesticides such as chlorpyrifos and atrazine can be degraded using nano-sized zerovalent iron. Similarly, some pesticide residues can also be photo-catalytically decomposed using TiO2 doped with Fe2O3 either incorporated into the pesticide formulations or sprayed directly on crops (Sasson et al., 2007).
3.4 Pesticides Residues Detection Pesticide residues are characterized by a high persistence in the environment and toxicity to both wildlife and humans. Pesticide residues in soil affect the soil microbial biodiversity. Some pesticides, particularly organochlorine, suppress symbiotic nitrogen fixation resulting in reduced crop production. Nitrification bacteria are very sensitive to pesticides and herbicides, and sulfonylurea herbicide has been found to inhibit this process (Gigliotti and Allievi, 2001). Some fungicides, such as chlorothalonil and dinitrophenyl, affect nitrification and denitrification microbial processes (Kinney et al., 2005; Lang and Cai, 2009). Some pesticides, such as benomyl and dimethoate, negatively affect symbiotic mycorrhizal fungi (Menendez et al., 1999; Chiocchio et al., 2000). The use of fertilizers, pesticides, herbicides, and other chemicals contribute soil and water pollution. Pesticides can get into water by runoff from treated areas, leaching through the soil and
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through drift during pesticide spraying. More recent studies also reported the presence of pesticides in surface water and groundwater close to agricultural land all over the world (Cerejeira et al., 2003; Konstantinou et al., 2006; Gilliom, 2007; Woudneh et al., 2009; Añasco et al., 2010). Soil organisms such as nematodes, mites, earthworms, spiders, and insects enhance soil aggregation and porosity, hence increasing infiltration and reducing runoff. It was found that the mixture of insecticides and fungicides at different concentrations caused a neurotoxic effect in earthworms and was physiologically damaging due to their high toxicity (Schreck et al., 2008). For organic farming certification, inspection of pesticide residue in soil and water is also necessary. Pesticide residue detection using nanosensors has higher sensitivity, low detection limits, fast response, and superselectivity (Liu et al., 2008b). Nanomaterial-based nanosensors can be used to detect such pesticide residues in soil and water instead of traditional gas or liquid chromatography-mass spectroscopy techniques. Although traditional techniques are accurate and reliable, these techniques involve time-consuming steps. Enzyme-based biosensors for organochlorine, carbamate, and organophosphate residue detection have been reviewed in detail by Van Dyk and Pletschke (2011).
3.5 Nano-Herbicides Weeds are unwanted plants alongside the desired plant crops and herbicides are used to kill the weeds. Conventional herbicides, when sprayed, have a chance of killing food crops too, and there might be a huge loss in the crop yield. Nano-herbicide destroys the entire weed from its roots but does not affect the desired food crops. As nanoparticles are target specific, they can be used to kill the weed and destroy it to get a better yield. Herbicides such as atrazine and triazine could be encapsulated to get efficient release to the plants in the same way as nano-fertilizers and nano-pesticides.
3.6 Plant Pathogens Detection Nanoparticles can be used as an identification tool for detection of bacterial, viral, and fungal plant pathogens in agriculture (Boonham et al., 2008; Prasad, 2014). Nano gold-based immune-sensors were used by Singh et al. (2010) to detect Karnal bunt disease in wheat using surface plasmon resonance. Plants respond to different stress conditions through physiological changes. Induction of systemic defense is one such response regulated by plant hormones such as salicylic acid, jasmonic acid, and methyl jasmonate. This indirect stimulus was used by Wang et al. (2010) to develop a sensitive electrochemical sensor, using modified gold electrodes with copper nanoparticles, to monitor the level of salicylic acid in oil seeds to detect the fungi (Sclerotinia sclerotiorum). They successfully measured the salicylic acid accurately using this type of sensor. Hence, research could be aimed at detecting pathogens and their by-products using nano-biosensor technology.
3.7 Water Retention Crop production needs a large amount of fresh water, so research should focus to improve water usage conservation. Some precise water delivery systems are water absorption efficiency of plants, water distribution near roots, in situ water holding capacity, e ncapsulated
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water released on demand, and interaction with field intelligence through distributed nano- sensor systems (Diallo et al., 2009). Nano-porous materials are capable of storing water and releasing it slowly, depending on drought level controlled by wireless nanosensors. This permits less water use and decreased losses into the environment. Nano-zeolites are crystalline aluminum silicates that allow the exchange of ions and reversible dehydration. They can improve the water retention of sandy soils and improve the porosity in clay soils. Hence, zeolites could be used for nano-organozeolite fertilizers and their use could reduce surface runoff (Prasad et al., 2014).
3.8 Improving Seed Germination Seed is the most important input determining crop productivity. There are many studies on the effects of nanomaterials on seed germination and growth to promote its use for crop production. Zheng et al. (2005) reported the effect of nano-TiO2 on the growth of spinach seeds. It was reported that nano-TiO2-treated seeds produced plants with 73% more dry weight compared to the control over a germination period of 30 days. Khodakovskaya et al. (2009) reported the use of CNTs for improving the germination of tomato seeds through better permeation of moisture. The carbon nanotubes serve as new pores for water permeation by penetration of the seed coat, and these CNTs act as a passage to channel the water into the seeds. Shah and Belozerova (2009) studied the effect of different metal nanoparticles, such as Si, Pd, Au, and Cu, on lettuce seed germination indicating positive influence at different concentration range. This facilitated germination could be exploited in a rain-fed agricultural system.
3.9 Genetic Material Delivery Nanotechnology is likely to play an important role in the development of genetically modified crops. The development of the biotic and abiotic variety of crop plants involves the delivery of genetic material, either DNA or RNA, resulting in the alteration of gene expression (Price and Gatehouse, 2008). There are many obstacles for gene delivery to transform plants (Ghormade et al., 2011). Microinjection, microprojectile bombardment, viral gene delivery, and Agrobacterium-mediated transformation are some techniques applied to transform plants. These methods are either applied only for dicotyledonous plant transformation or have low efficiency. For efficient gene transformation, nanoparticles such as gold nanoparticle-embedded carbon matrices as a carrier were used successfully for the delivery of genetic material (Vijayakumar et al., 2010).
4 SAFETY AND ETHICAL ISSUES The application of nanotechnology in agriculture is crucial because it directly affects human health. For the delivery of pesticides and fertilizers to plants, nanomaterials are used to decrease the dosage for controlled slow delivery. But risk assessment for using nanomaterials has still not been studied and defined properly. Hence, some concerns related to the use of nanomaterials in agriculture include phytotoxicity and potential residue carry-over
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in foodstuffs which have been reviewed in detail by Bouwmeester et al. (2009). There is a need to evaluate the toxico-kinetics and toxico-dynamics of nanomaterials used in crop production. Nanomaterials might have toxic effects in agricultural crop production (Nel et al., 2006). Nanomaterials with good wettability and dispersion and with biodegradable, biocompatible, nontoxic, stable, and well-understood toxico-kinetics and toxico-dynamics would be desirable and ideal for their efficient and effective applications in agriculture (Khot et al., 2012).
5 FUTURE OUTLOOK Nanotechnology could be developed for real-time monitoring of crop growth and field conditions including temperature, moisture level, soil fertility, crop nutrient status, insects, plant diseases, and weeds. Smart precision farming will make use of global satellite positioning systems and remote sensing devices and computers to measure environmental conditions enabling us to know whether crops are growing at maximum efficiency. Networks of wireless nanosensors positioned across crop fields will provide necessary data. This data will facilitate the best agronomic intelligence processes with the goal to minimize resource inputs and maximize outputs. It is also well known that humidity, light, temperature, soil conditions, fertilization, insects, and plant diseases all affect the release of volatile organic compounds which could be detected by electronic noses. In the future, electronic noses will detect crop diseases, identify insect infestation, and monitor food quality and could also be used in the food industry to assess the freshness spoilage of fruits and vegetables during the processing and packaging process. Similarly, smart dust technology will be used for monitoring various parameters such as temperature, humidity, insect, and disease infestation in the field. Smart nano-biosensors will be scattered like dust across the farms, working like the eyes, ears, and noses of the farming world. These tiny wireless smart nano-biosensors are capable of communicating the information they sense and will be programmed to respond various parameters such as nutrient deficiency, insect attack and disease infestation, and variation in humidity and temperature for real-time monitoring of the crop growth and field conditions (Ingale and Chaudhari, 2013).
6 CONCLUSION Nanotechnology is becoming a promising technology with the ability to create massive changes in agricultural crop productivity such as pesticide, biopesticide, and fertilizer delivery systems. Nanoscale devices can be used to make agricultural systems “smart.” Smart sensors and smart delivery systems will help in enhancing crop productivity in agriculture by providing accurate information. This can help in maintaining farms and fields with precise control and report timely needs of crops, thus helping farmers to make better decisions. Smart nanosensors and real-time monitoring systems will have a big impact on smart precision methodologies. Networks of wireless nanosensors positioned across cultivated fields will provide data leading to the best agronomic intelligence processes and practices with the goal to reduce resource inputs and enhance crop productivity.
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7 Impact of Nanomaterials on Plant Economic Yield and Next Generation Tariq Shah⁎, Jinsong Xu⁎, Xiling Zou⁎, Yong Cheng⁎, Xuekun Zhang⁎, Quaid Hussain⁎, Rafaqat Ali Gill† ⁎
Key Laboratory of Biology and Genetic Improvement of Oil Crops, Oil Crops Research Institute, Chinese Academy of Agricultural Sciences, Wuhan, China †Department of Genomics, Oil Crops Research Institute, Chinese Academy of Agricultural Sciences, Wuhan, China O U T L I N E 1 Introduction
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1 INTRODUCTION Agriculture presently is facing a number of challenges, such as stagnation in crop yields, low nutrient use efficiency, multinutrient deficiencies, the onset of pests and disease, water availability, shrinkable arable land, climate change, and shortage of labor. Moreover, demand for more food is increasing day by day due to the ever-increasing global population. This rapidly increasing global population needs sufficient food supply to feed millions of new mouths. For global food security, crop production must be increased several-fold sustainably
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with reduced inputs. Hence, there is an urgent need for proper management to utilize optimum plant nutrients and pesticides for enhancing crop production so as to cope with this alarming situation. For this, one of the frontier technologies, such as nanotechnology, can be explored to detect precisely and supply accurate quantities of plant nutrients and pesticides to increase crop production and conservation of inputs. Nanotechnology is the science in which materials are manipulated at nanoscale displaying different properties from bulk materials due to their size and surface area. This is a fast-growing field of science and technology. Nanotechnology and its derived applications are very important in the area of agriculture to address a number of issues pertaining to sustainable agricultural inputs aiming for enhancing crop productivity. Nanotechnology has several applications in various stages of crop production, processing, storing, packaging, and transporting of agricultural products. This chapter briefly reviews and describes some of the applications of nanotechnology in crop production practices to enhance crop productivity.
2 NANOTECHNOLOGY The term “nanotechnology” was first defined in 1974 by Norio Taniguchi of the Tokyo Science University as the study of manipulating matter on an atomic and molecular scale (Agrawal and Rathore, 2014; Taniguchi et al., 1974). The definition of nanotechnology is based on the prefix “nano” which is from the Greek word meaning “dwarf.” A nanometer is one billionth of a meter. The word nanotechnology is generally used when referring to materials with the size of 1–100 nm and deals with the alteration of materials at nanoscale and creation of nano-sized materials generally less than 100 nm in at least one dimension. These materials display different properties from bulk materials due to their size, including physical strength, chemical reactivity, electrical conductance, magnetism, and optical effects. Nanotechnology involves the manipulation or self-assembly of individual atoms or molecules or molecular clusters into structures to create materials and devices with new or vastly different properties. Nanoparticles are synthesized from organic or inorganic nanomaterials (NMs) using a number of chemical and physical methods (Table 1). The techniques for synthesis of nanoparticles usually involve either a bottom-up or a top-down method (Fig. 1). In the top-down approach, the size of particles is reduced by different chemical and physical procedures, such as milling, homogenization using high pressure, and sonication, while in the bottom-up method, the nanostructure building blocks of the nanoparticles are formed first and subsequently assembled to produce the final nanoparticles. Metal oxide nanoparticles such as ZnO, TiO2, MgO, and AgO are some examples of inorganic materials. Metallic nanomaterials are very interesting materials with unique electronic and electro-catalytic properties depending on their size and morphology. They can be utilized for nanostructured materials with specific forms, such as quantum dots (QDs). Other examples of inorganic materials are clay nanoparticles having a structure of stacked platelets. Organic materials, such as carbon nanotubes (CNTs), polymers, and lipids, are also materials with various applications in nanotechnology. Quantum dots are inorganic nanocrystals used for development of optical biosensors which are widely used to detect organic compounds and biomolecules, such as proteins, enzymes, and amino acids. Similarly, surface plasmon resonance (SPR) is a robust tool that can measure the binding kinetics of two molecules without the help of any fluorescent tag.
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TABLE 1 Nanomaterials Examples and Their Application Nanomaterials
Application
INORGANIC METAL NANOPARTICLES AgO, TiO2, ZnO, CeO2; Fe2O3
Delivery of biomolecules (proteins, peptides, nucleic acids), biosensors, diagnostic techniques, pesticide degradation
FePd, Fe-Ni; Silica; CdTe, CdSe CLAY Montmorillonite layered double hydroxides
Delivery of pesticides, fertilizers, plant growth-promoting factors
ORGANIC CARBON NANOTUBES Nanofibers
Biocatalysts, sensing
LIPIDS Liposomes
Delivery of DNA, and pesticides, essential oils
Lippopolyplexes Solid lipid nanoparticles POLYMERIC Natural
Biocompatible, biodegradable, nontoxic for delivery of DNA/ RNA
Cellulose, starch, gelatin, albumin Chitin, chitosan SYNTHETIC Dendrimers
Delivery of pesticides and DNA/RNA
Polyethylene oxide Polyethylene glycol Polylactides
Bottom-up method
Arrangement of smaller components in to more complex (layer-by-layer self assembly)
Top-down method Creation of smaller objects using larger objects
FIG. 1 Two approaches of nanoparticles synthesis.
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Thus, this technique can be adopted for biomolecular interaction analysis taking less time to detect binding events. Dendrimers are synthetic, organic macromolecules with highly branched and defined three-dimensional structures which provide a high degree of surface functionality and versatility. Similarly, aptamers are single-stranded nucleic acids which fit for the target in all the way forming three-dimensional structures with strict bonding of target specific binding with high affinity. This kind of nanosensor gives more specific and effective detection of plant diseases. Carbon nanotubes are hollow cylindrical tubes with one (single-walled CNTs), two (double-walled CNTs), or several (multiwalled CNTs) concentric graphite layers capped by a fullerenic hemisphere. Easy electron transfer reactions and high surface area by virtue of unique metallic, structural, and electronic characteristics of CNTs make them an important category of nanomaterials. Lipid nanoparticles used as delivery systems are composed of aqueous dispersion of dry powder as well as solid lipids, such as triglycerides, waxes, steroids, fatty acids, and emulsifiers and are prepared by a homogenization process using high pressure. Polymeric nanoparticles prepared from natural and synthetic polymers by wet synthetic methods are commonly used because surface modification can be carried out easily in subsequent steps for stability. Nanoparticles prepared from natural sources and biopolymers have some merits of biocompatibility and biodegradability. A biosensor is a device that integrates a biological recognition element with a physical or chemical transducer to detect a biological product. Broadly, a biosensor comprises three components: the biological recognition element, the transducer, and the signal processing electronics (Fig. 2). Different categories of nanomaterials, such as nanotubes, quantum dots, or other biological nanomaterials, have been used in biosensor technology to produce nano-biosensors (Prasad et al., 2016). These nanomaterials can contribute to either the bio- recognition element of a biosensor or the transducer or both. Nanoparticle-based biosensors are particularly attractive because they can be easily synthesized in bulk using standard chemical methods. Biosensors may be categorized according to the mechanism of biological selectivity (bioreceptor) or mode of physiochemical signal transduction (transducer). A bioreceptor is a molecular species which exploits a biochemical mechanism of recognition and is accountable for binding the concerned analyte to the sensor for measurement. The bioreceptor can broadly be categorized into various distinct classes such as antibodyantigen bioreceptor, enzymatic bioreceptor, nucleic acid bioreceptor, cellular bioreceptor, biometric bioreceptor, and bacteriophage bioreceptor. The transducer plays a crucial part in the detection as well as the identification process of a biosensor. The transduction methods, such as optical, electrochemical, and mass-based, are the most favored and universal methods. Several types of biosensors are being developed for different applications in enhancing agricultural crop productivity.
FIG. 2 A typical illustration of a biosensor.
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3 UNRAVELING POTENTIAL APPLICATION OF NANOTECHNOLOGY IN CROP PRODUCTION As traditional approaches for crop productivity reach their limits, agriculture has to adopt novel approaches based on nanotechnology to meet the demand of an ever-growing world population. Precision farming has been a long-derived aim to maximize output while minimizing inputs through monitoring environmental variables and applying targeted action. One application of agricultural nanotechnology addresses low use efficiency of inputs. For example, controlled release mechanisms through nanoscale carriers avoid temporal overdose and reduce the amount of agricultural chemicals used and consequently minimize inputs and waste. The potential applications of nanotechnology in agricultural research include slow and control-released chemical fertilizers, micronutrients, and biofertilizers for high efficiency and efficacy, delivery of chemical pesticides encapsulated in nanomaterials for slow and controlled release, stabilization of biopesticides with nanomaterials, agricultural diagnostics for pest and disease detection, water retention, and nano-genetic manipulation of agricultural crops (Prasad et al., 2014, 2017; Bhattacharyya et al., 2016) (Fig. 3).
3.1 Nano-Fertilizers Much of the fertilizers are not available to plants because of lost runoff and leaching causing pollution. Nanotechnology has provided the feasibility of exploiting nanomaterials as fertilizer carriers for controlled release of nutrients. These smart fertilizers increase nutrient use efficiency and reduce costs of environmental protection. Nanomaterials, having potential contribution in slow release of fertilizers, can be used for encapsulation of fertilizers which is done by various ways such as encapsulation of nutrient inside nano-porous materials and coating of the fertilizer with thin polymer film. Fertilizers can be delivered as emulsion or particles of nanoscale dimension. Fertilizers with sulfur nano-coatings are used for slow release of nutrients. Kaolin and polymeric biocompatible nanoparticles are some nanomaterials with potential applications in slow and controlled release of fertilizers (Wilson et al., 2008). Biodegradable, polymeric chitosan nanomaterials are used for the preparation of controlled release of the NPK fertilizer (Corradini et al., 2010).
FIG. 3 Nanotechnology application in enhancing crop production.
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Smart nano-fertilizers can be converted into intelligent smart nano-fertilizers by combining nanodevices so as to synchronize the release of fertilizers with their uptake by crops. Dr. Carlos Montreal of Agriculture and Agri-Food Canada is one of several research scientists developing such intelligent smart fertilizers that release nitrogen only when the plant needs it and in the amount the plant requires (Monreal et al., 2016). The plants communicate with their surrounding environment by producing various kinds of chemical signals. Many chemical compounds that are associated with nitrogen uptake have been identified. These compounds can be used to synchronize the release of fertilizer with nitrogen uptake by the crop. Nano-biosensor binding to these compounds can be developed so as to control the release of fertilizers. Nano-sized biosensors made up of specific chemical composition with polymer coating have been developed by this team. This system allows the fertilizers to be released into the soil when the plant requires it. The research team is trying to make intelligent fertilizers with biodegradable three-dimensional polymer coating less than 100 nm thick. Biofertilizers are formulations with beneficial microorganisms, such as plant growth- promoting rhizobacteria (PGPR), Rhizobium, Azotobacter, Azospirillum, and blue-green algae, fungal mycorrhizae, and phosphate-solubilizing bacteria (PSB), for example, Pseudomonas sp. and Bacillus sp. (Wu et al., 2005; Prasad et al., 2015). There are some constraints in their usage due to lack of suitable carrier materials, short shelf life, and problems in transportation and storage. Formulations resistant to desiccation can be developed by using polymeric nanoparticle coatings in biofertilizer preparation. Similarly, micronutrients can be trapped in nanomaterials for their slow release to promote optimum plant growth and development (Peteu et al., 2010).
3.2 Nano-Pesticides Nanomaterial-based formulations have potential applications for efficient delivery of pesticides. The process of absorption, attachment, encapsulation, or entrapment of the active ingredient into the nano-matrix occurs in nano-pesticide formulations. These formulations release the active ingredient slowly and in a controlled way. The advantages of nanomaterial- based formulations are improvement in efficiency and efficacy and lower toxicity. Nanopesticides are prepared either by very small particles of pesticidal active ingredients or some other nanostructured molecules with pesticidal properties. Nano-pesticide-based formulations have better dispersion and wettability quality as compared to conventional formulations (Bergeson, 2010a). Nanomaterials used for formulating nano-pesticides have desired properties such as biodegradability, solubility, permeability, and thermal stability (Bouwmeester et al., 2009). Nano-pesticides have large specific surface area enhancing affinity to the target. Nano-emulsions, nano-encapsulates, nano-containers, and nano-cages are some of the nano-pesticide delivery methods which have been widely discussed and reviewed recently (Bergeson, 2010b). The ability to delay or control delivery of pesticides to the target organisms is achieved by these nano-pesticide delivery methods (Singh et al., 2015). Spraying of pesticides involves large droplets associated with splash loss, so droplet size is diminished by using nano-pesticides which contribute to effective spraying and, thus, decrease splash losses. In order to keep the residue level below the critical limits as permissible by the regulatory criteria in foodstuffs, the nano-formulation should be degraded fast in the soil but slowly in plants. The sodium dodecyl sulfate (SDS) can play a role to enhance the photodegradation of the NPs in soil. Pesticide (imidacloprid) nano-formulations were p repared using
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SDS-modified silver/titanium dioxide (Ag/TiO2) along with chitosan and alginate by microencapsulation technique to increase the photodegradation by Guan et al. (2010). Keeping in view the formulation stability, which is an important aspect at the nano-level, polymer stabilizers such as polyvinylpyrrolidone, polyvinyl alcohol, and poly(acrylic acid)-b-poly(butyl acrylate) were used by Liu et al. (2008a) for formulating a stable nano-pesticide. Plants also provide a source of biomolecules which are not persistent in water and soil but biologically efficient (Isman, 2000). These phytochemicals, such as essential oils and secondary metabolites, face problems of cost-effectiveness and stability. Incorporation of essential oil obtained from Artemisia arborescens L. into solid lipid nanoparticles diminished the fast evaporation of essential oil (Lai et al., 2006). Similarly, amorphous nano-silica obtained from various sources, such as phytoplankton, volcanic soil is used as a useful biopesticide against stored grain, fungal organisms, and worms (Liu et al., 2008a,b). There are some biological control agents, such as fungi, bacteria, and viruses, which are used against insect pests. Bacterial and viral formulations prepared from these organisms are usually susceptible to desiccation, ultraviolet light inactivation, or even heat. Nano-formulations may provide new ways to enhance the stability of these biological agents. Mycopesticides (fungal biocontrol agents) are promising biological pesticides as there is no need of ingestion; instead, they act by contact, are very specific, and can be easily mass-produced (Barik et al., 2008). Microbial products such as enzymes, antibiotics, inhibitors, and toxins are promising as biopesticides against plant pathogens and pests (Bhattacharyya et al., 2016). Microbial-based formulations need stabilization and a directed delivery mechanism toward identified targets. Chitosan and clay, being biocompatible nanomaterials, can be used as stabilizing and delivery agents and, hence, have potential applications in the development of biopesticide formulations.
3.3 Pesticides Degradation Pesticide-contaminated soil and water are generally treated using phytoremediation, photochemical processes, and oxidation processes. Biorecalcitrant pollutants can be degraded using nanoparticles (Joo and Cheng, 2006). It has been shown that pesticides such as chlorpyrifos and atrazine can be degraded using nano-sized zerovalent iron. Similarly, some pesticide residues can also be photo-catalytically decomposed using TiO2 doped with Fe2O3 either incorporated into the pesticide formulations or sprayed directly on crops (Sasson et al., 2007).
3.4 Pesticides Residues Detection Pesticide residues are characterized by a high persistence in the environment and toxicity to both wildlife and humans. Pesticide residues in soil affect the soil microbial biodiversity. Some pesticides, particularly organochlorine, suppress symbiotic nitrogen fixation resulting in reduced crop production. Nitrification bacteria are very sensitive to pesticides and herbicides, and sulfonylurea herbicide has been found to inhibit this process (Gigliotti and Allievi, 2001). Some fungicides, such as chlorothalonil and dinitrophenyl, affect nitrification and denitrification microbial processes (Kinney et al., 2005; Lang and Cai, 2009). Some pesticides, such as benomyl and dimethoate, negatively affect symbiotic mycorrhizal fungi (Menendez et al., 1999; Chiocchio et al., 2000). The use of fertilizers, pesticides, herbicides, and other chemicals contribute soil and water pollution. Pesticides can get into water by runoff from treated areas, leaching through the soil and
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through drift during pesticide spraying. More recent studies also reported the presence of pesticides in surface water and groundwater close to agricultural land all over the world (Cerejeira et al., 2003; Konstantinou et al., 2006; Gilliom, 2007; Woudneh et al., 2009; Añasco et al., 2010). Soil organisms such as nematodes, mites, earthworms, spiders, and insects enhance soil aggregation and porosity, hence increasing infiltration and reducing runoff. It was found that the mixture of insecticides and fungicides at different concentrations caused a neurotoxic effect in earthworms and was physiologically damaging due to their high toxicity (Schreck et al., 2008). For organic farming certification, inspection of pesticide residue in soil and water is also necessary. Pesticide residue detection using nanosensors has higher sensitivity, low detection limits, fast response, and superselectivity (Liu et al., 2008b). Nanomaterial-based nanosensors can be used to detect such pesticide residues in soil and water instead of traditional gas or liquid chromatography-mass spectroscopy techniques. Although traditional techniques are accurate and reliable, these techniques involve time-consuming steps. Enzyme-based biosensors for organochlorine, carbamate, and organophosphate residue detection have been reviewed in detail by Van Dyk and Pletschke (2011).
3.5 Nano-Herbicides Weeds are unwanted plants alongside the desired plant crops and herbicides are used to kill the weeds. Conventional herbicides, when sprayed, have a chance of killing food crops too, and there might be a huge loss in the crop yield. Nano-herbicide destroys the entire weed from its roots but does not affect the desired food crops. As nanoparticles are target specific, they can be used to kill the weed and destroy it to get a better yield. Herbicides such as atrazine and triazine could be encapsulated to get efficient release to the plants in the same way as nano-fertilizers and nano-pesticides.
3.6 Plant Pathogens Detection Nanoparticles can be used as an identification tool for detection of bacterial, viral, and fungal plant pathogens in agriculture (Boonham et al., 2008; Prasad, 2014). Nano gold-based immune-sensors were used by Singh et al. (2010) to detect Karnal bunt disease in wheat using surface plasmon resonance. Plants respond to different stress conditions through physiological changes. Induction of systemic defense is one such response regulated by plant hormones such as salicylic acid, jasmonic acid, and methyl jasmonate. This indirect stimulus was used by Wang et al. (2010) to develop a sensitive electrochemical sensor, using modified gold electrodes with copper nanoparticles, to monitor the level of salicylic acid in oil seeds to detect the fungi (Sclerotinia sclerotiorum). They successfully measured the salicylic acid accurately using this type of sensor. Hence, research could be aimed at detecting pathogens and their by-products using nano-biosensor technology.
3.7 Water Retention Crop production needs a large amount of fresh water, so research should focus to improve water usage conservation. Some precise water delivery systems are water absorption efficiency of plants, water distribution near roots, in situ water holding capacity, e ncapsulated
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water released on demand, and interaction with field intelligence through distributed nano- sensor systems (Diallo et al., 2009). Nano-porous materials are capable of storing water and releasing it slowly, depending on drought level controlled by wireless nanosensors. This permits less water use and decreased losses into the environment. Nano-zeolites are crystalline aluminum silicates that allow the exchange of ions and reversible dehydration. They can improve the water retention of sandy soils and improve the porosity in clay soils. Hence, zeolites could be used for nano-organozeolite fertilizers and their use could reduce surface runoff (Prasad et al., 2014).
3.8 Improving Seed Germination Seed is the most important input determining crop productivity. There are many studies on the effects of nanomaterials on seed germination and growth to promote its use for crop production. Zheng et al. (2005) reported the effect of nano-TiO2 on the growth of spinach seeds. It was reported that nano-TiO2-treated seeds produced plants with 73% more dry weight compared to the control over a germination period of 30 days. Khodakovskaya et al. (2009) reported the use of CNTs for improving the germination of tomato seeds through better permeation of moisture. The carbon nanotubes serve as new pores for water permeation by penetration of the seed coat, and these CNTs act as a passage to channel the water into the seeds. Shah and Belozerova (2009) studied the effect of different metal nanoparticles, such as Si, Pd, Au, and Cu, on lettuce seed germination indicating positive influence at different concentration range. This facilitated germination could be exploited in a rain-fed agricultural system.
3.9 Genetic Material Delivery Nanotechnology is likely to play an important role in the development of genetically modified crops. The development of the biotic and abiotic variety of crop plants involves the delivery of genetic material, either DNA or RNA, resulting in the alteration of gene expression (Price and Gatehouse, 2008). There are many obstacles for gene delivery to transform plants (Ghormade et al., 2011). Microinjection, microprojectile bombardment, viral gene delivery, and Agrobacterium-mediated transformation are some techniques applied to transform plants. These methods are either applied only for dicotyledonous plant transformation or have low efficiency. For efficient gene transformation, nanoparticles such as gold nanoparticle-embedded carbon matrices as a carrier were used successfully for the delivery of genetic material (Vijayakumar et al., 2010).
4 SAFETY AND ETHICAL ISSUES The application of nanotechnology in agriculture is crucial because it directly affects human health. For the delivery of pesticides and fertilizers to plants, nanomaterials are used to decrease the dosage for controlled slow delivery. But risk assessment for using nanomaterials has still not been studied and defined properly. Hence, some concerns related to the use of nanomaterials in agriculture include phytotoxicity and potential residue carry-over
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in foodstuffs which have been reviewed in detail by Bouwmeester et al. (2009). There is a need to evaluate the toxico-kinetics and toxico-dynamics of nanomaterials used in crop production. Nanomaterials might have toxic effects in agricultural crop production (Nel et al., 2006). Nanomaterials with good wettability and dispersion and with biodegradable, biocompatible, nontoxic, stable, and well-understood toxico-kinetics and toxico-dynamics would be desirable and ideal for their efficient and effective applications in agriculture (Khot et al., 2012).
5 FUTURE OUTLOOK Nanotechnology could be developed for real-time monitoring of crop growth and field conditions including temperature, moisture level, soil fertility, crop nutrient status, insects, plant diseases, and weeds. Smart precision farming will make use of global satellite positioning systems and remote sensing devices and computers to measure environmental conditions enabling us to know whether crops are growing at maximum efficiency. Networks of wireless nanosensors positioned across crop fields will provide necessary data. This data will facilitate the best agronomic intelligence processes with the goal to minimize resource inputs and maximize outputs. It is also well known that humidity, light, temperature, soil conditions, fertilization, insects, and plant diseases all affect the release of volatile organic compounds which could be detected by electronic noses. In the future, electronic noses will detect crop diseases, identify insect infestation, and monitor food quality and could also be used in the food industry to assess the freshness spoilage of fruits and vegetables during the processing and packaging process. Similarly, smart dust technology will be used for monitoring various parameters such as temperature, humidity, insect, and disease infestation in the field. Smart nano-biosensors will be scattered like dust across the farms, working like the eyes, ears, and noses of the farming world. These tiny wireless smart nano-biosensors are capable of communicating the information they sense and will be programmed to respond various parameters such as nutrient deficiency, insect attack and disease infestation, and variation in humidity and temperature for real-time monitoring of the crop growth and field conditions (Ingale and Chaudhari, 2013).
6 CONCLUSION Nanotechnology is becoming a promising technology with the ability to create massive changes in agricultural crop productivity such as pesticide, biopesticide, and fertilizer delivery systems. Nanoscale devices can be used to make agricultural systems “smart.” Smart sensors and smart delivery systems will help in enhancing crop productivity in agriculture by providing accurate information. This can help in maintaining farms and fields with precise control and report timely needs of crops, thus helping farmers to make better decisions. Smart nanosensors and real-time monitoring systems will have a big impact on smart precision methodologies. Networks of wireless nanosensors positioned across cultivated fields will provide data leading to the best agronomic intelligence processes and practices with the goal to reduce resource inputs and enhance crop productivity.
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