Nanotechnology opens a large scope of novel applications in the fields of plant nutrition needed to meet the future demands of the growing population because nanoparticles (NPs) have unique physicochemical properties, i.e. high surface area, high reactivity, tunable pore size, and particle morphology. Management of optimum nutrients for sustainable crop production is a priority-based area of research in agriculture. In this regard, nanonutrition has proved to be the most interesting area of research and concerns with the provision of nano-sized nutrients for sustainable crop production. Using this technology, we can increase the efficiency of micro- as well as macronutrients of plants. In the literature, various NPs and nanomaterials (NMs) have been successfully used for better nutrition of crop plants compared to the conventional fertilizers. This review summarizes these NPs and NMs into macro-, micro-, and nanocarrier-based fertilizers and plant-growth-enhancing NPs with unclear mechanisms, describing their role in improving growth and yield of crops, concentration/rate of application, particle size, mechanism of action if known, toxic effects if any, and research gaps in the present research. Moreover, future research directions for achieving sustainable agriculture are also discussed in the appropriate section and at the end in the concluding remarks section.
The world of agriculture is facing many challenges, such as changing climate due to the greenhouse effect and global warming; urbanization due to life pattern changes; non-judicious use of resources like petroleum, natural gas, high-quality rock phosphate, etc., that are non-renewable; and environmental issues like run off, eutrophication related with the application of more chemical fertilizers than required. These problems get more intensified by the world population, which is increasing at an alarming rate and is expected to reach 9.6 billion by the year 2050 . An increasing demand for global food production has been observed during the last two decades due to a change in diet pattern and an increasing demand for bioenergy crops. An increase by 70% in global grain production is required to feed this increasing world population . Moreover, to fulfill the increasing demand for bioenergy, there will be an additional demand for agricultural production from the already limited arable land of the world. Of course, it will create new opportunities for the generation of energy and electricity from the biofuels and agricultural waste products; however, workable economics and encouraging policy is still pending. The above-mentioned scenario will be critical for the countries, especially the developing ones where agriculture is the backbone of their national economy and faces many challenges like the lack of new arable land and reduction of cultivable land due to competing demands for economic development activities, commodity dependence, poverty, and malnutrition.
Advancement in the field of science and technology could be a potential solution for value addition in their current production systems . A significant increase in agricultural production could be possible through utilization of current knowledge in the field of nanotechnology for efficient nutrient system, good plant protection practices, efficient photocapturing system in plants, precision agriculture, and many others .
In case of plant nutrition, high application rate of fertilizers can boost up the production of agricultural crops around the world, but it may cause serious threats to the environment in the form of eutrophication and contaminating freshwater sources, thus severely affecting people using that freshwater sources for drinking and also the aquatic life due to algal growth . Moreover, the concentration of nitrates in groundwater has increased to a toxic level due to the intensive use of fertilizers . So, the need of the hour is to develop an efficient plant nutrient system with minimum damage to the environment and for global sustainable development.
For sustainable agriculture, application of nanotechnology has been regarded as an innovative and promising technology to feed the ever-increasing population of the world. It has not only revolutionized agriculture with innovative nutrients in the form of nanofertilizers (NFs) but has also helped in the plant protection field through the development of nanopesticides, efficient water management system, and also increasing the efficiency of plant in utilizing the sun’s energy . In case of conventional fertilizers, low use efficiency (20–50%) and cost-intensive increase in application rates have urged scientists around the world to develop and promote the use of NFs . Many scientists worldwide have focused on this innovative field and have developed such NPs and NMs that could serve as nutrients for the plants to enhance germination rates, growth, yield, and many physiological parameters .
Recently, scientists around the world are focusing on the potential role of NPs in biotechnology, as these have the ability to transport DNA and other chemicals into the plant cells. This breakthrough has opened a new window for gene manipulation and their expression in the specific cells of the plants . In this context, a success has been achieved for plants’ augmented ability to harvest more light energy by delivering carbon nanotubes (CNTs) into the chloroplasts. Moreover, these tubes could also serve as an artificial antenna for capturing wavelengths of light such as ultraviolet, green, and near infrared, which are not in their normal range .
In this regard, the major objective of this review is to collate and analyze the most recent nanotechnological developments/breakthrough in the field of plant nutrition to increase growth and yield of crop plants with minimum destruction to the environment, mechanisms of action, factors affecting their efficacy, and future research gaps, which need to be elucidated for their successful implementation in sustainable agriculture. The following sections clearly explain the role of nanotechnology in different aspects of agriculture.
NMs are defined as an ingredient containing particles with at least one dimension that approximately measures 1–100 nm (United States Environmental Protection Agency). Accordingly, the NFs are the NMs that could serve as macro- or micronutrient(s) for the crop plants (macro- or micronutrient fertilizers) or help as carriers of the conventional chemical fertilizers – nanocarriers for efficient utilization of the nutrient. There are some NMs that are not included in the list of macro- or micronutrients for plants; however, these have shown an improvement in various growth processes of plants, so these have been discussed in the later sections of the review as “others.” NFs have been proved to be comparatively effective over the conventional chemical fertilizers due to their novel mechanisms of actions, increased use efficiency, reduced nutrient loss, and minimum deterioration of the environment.
Moreover, these have the ability to enter into the cells directly as these materials are small sized, which reduces/bypasses the energy-intensive mechanisms of their uptake/delivery into the cell, as clear from Figure 4 [13, 14]. Similar to the conventional fertilizers, NFs are dissolved in the soil solution and the plants can directly take them up. However, their solubility might be more than that of related bulk solids found in the rhizosphere due to their small size. These are more efficient compared to the ordinary fertilizers, as these reduce N loss due to leaching, emissions, and long-term incorporation by soil microorganisms. Moreover, controlled-release NFs may also improve fertilizer use efficiency (FUE) and soil deterioration by decreasing the toxic effects associated with overapplication of traditional chemical fertilizers . There are also reports about the use of nanoencapsulated slow-release fertilizers. Recently, biodegradable, polymeric chitosan NPs (~78 nm) have been used for controlled release of NPK fertilizer sources such as urea, calcium phosphate, and potassium chloride . Other NMs like kaolin and polymeric biocompatible NPs could also be utilized for this purpose . The details about NFs are given in the following sections.
2.1 Macronutrient NFs
These are chemically composed of one or more nano-sized macronutrients that are required by crop plants in large quantities. These include N, P, K, Ca, Mg, and S. The requirement of macronutrients by the crop plants is increasing with the increase in the demand for more food for the ever-increasing population of the world. The macronutrient demand is expected to increase to 263 Mt by 2050 . In order to reduce this, macronutrient NFs have provided the solution in the form of their increased use efficiency compared to conventional chemical fertilizers with a use efficiency of not more than 20%. NFs comprising macronutrients have been developed by scientists and technicians around the world, and these have shown a tremendously increased efficiency in increasing the growth and productivity of crops. So, these have not only increased the efficiency but also reduced the cost, and hence were found to be an economical alternative to the existing conventional chemical fertilizers. Their detailed description is given in the following sections.
2.1.1 Nitrogen (N)-NPs
N is the most important nutrient involved in many processes of crop plants. Various strategies have been employed to improve its use efficiency. Nitrogenous NFs have been reported by various scientists around the world [18–21]. For example, slow release of N was observed when urea (ammonium) was coated on zeolite chips . Similarly, urea-modified hydroxyapatite NPs were encapsulated under pressure into the cavities of soft wood of Gliricidia sepium, and were tested for slow and sustainable release of N into the soil. Interestingly, N supply through this strategy was found optimum up to 60 days compared to conventional nitrogenous fertilizers, which gave more N supply to the plants in the beginning and very low at the later stage up to 30 days .
2.1.2 Phosphorus (P)-NPs
Being an essential component of many metabolites and having a key role in many metabolic processes of plants, P is supplied to the crop plants through so-called chemical fertilizers, of which only up to 20% is taken up by crop plants and the rest is fixed in the soil and/or accumulates in water bodies through run off, causing eutrophication. Nanotechnology has played a key role in increasing the phosphorus use efficiency (PUE) of crop plants and decreasing environmental threats through eutrophication. In this regard, hydroxyl apatite (Ca5(PO4)3OH) NPs were synthesized using a one-step wet chemical method and compared with conventional chemical phosphatic fertilizers for their role in increasing PUE, and ultimately plant growth and yield . Soybean (Glycine max) was used as test crop under greenhouse conditions. A significant increase in growth rate (33%) and seed yield (20%) compared to the conventional chemical phosphatic fertilizers was observed due to the supply of Ca and P simultaneously. Moreover, the product had weaker interaction with the soil components compared to conventional chemical phosphatic fertilizers. The product showed no phytotoxicity effect on the germination rate of lettuce (Lactuca sativa). Similarly, in another approach, P-NPs were biosynthesized using Aspergillus tubingensis TFR-5 from tri-calcium phosphate (Ca3P2O8) . However, the biosynthesized P-NPs were not tested/reported to have efficacy in improving growth and yield parameters of crops, and need to be elucidated in future studies.
2.1.3 Potassium (K)-NPs
Still, there are no reports available in the literature about the use of K-NPs. However, carrier-based K-NPs have been developed and tested under controlled conditions (Table 1).
|Nutrient provided||Crop and experimental conditions||Size and rate of application||Growth enhancement||References|
|N||Lolium multiflorum, controlled conditions, sandy loam soil||Clinoptilolite NH4, fertilized with 0, 60, 120, and 180 kg N ha-1||Enhanced yield and NUE possibly due to salt to ion ability to retain and slowly break free NH4+ ions|||
|No crop involved; N release from urea-modified hydroxyapatite NPs||Showed subsequent slow release even on day 60 compared to commercial fertilizer, which released heavily early followed by release of low and non-uniform quantities until around day 30|||
|Zea mays, soil, 120 days, water irrigation, lysimeteric study||20 and 60 g NH4-N zeolite kg-1, 150 kg N ha-1 commercial fertilizer||N uptake rate (1–1.1), Zea mays yield (1–1.04), N leaching (0.78–0.94)|||
|Zea mays, loamy sand, water irrigation, 45 days, greenhouse test||23 g N kg-1 zeolite; 112, 224, or 336 kg N ha-1; NH4-N soaking in 1 m (NH4)2SO4 for 10 days, changing solution every 2–3 days||N leaching reduced and N-use efficiency improved|||
|P||Glycine max, 5 months greenhouse test, nutrient solution||Apatite, Ca5(PO4)3OH, 16 nm, 21.8 mg l-1 as P, soluble Ca(H2PO4)2, 21.8 mg l-1 as P||Growth and yield was more in case of apatite, Ca5(PO4)3OH compared to soluble Ca(H2PO4)2|||
|K||Chrysanthemums, potting medium, 100-days greenhouse test||K zeolite, 3 g l-1 as K, nutrient solution||Yield was increased and K leaching was reduced|||
|Triticum aestivum, soil, 25 days greenhouse test||K synthesized by kaolinite, KOH, and KCl at 100°C for 6 h, 2.8–89 mg kg-1 as K, nutrient solution||Aboveground biomass and leaf K content improved significantly|||
|Ca||Arachis hypogaea, 80 days greenhouse, sand medium||CaCO3, 20–80 nm, 160 mg l-1 as Ca, Ca(NO3)2, 200 mg l-1 as Ca, nutrient solution||Growth, yield, and quality parameters significantly improved; however, the highest yield was achieved at a combination of 1 g l-1 humic acid and Ca NPs|||
|Mg||Vigna unguiculata subsp. unguiculata, foliar application, field experiment||Mg-NPs, 500 mg l-1 as Mg||1000-Seed weight and leaf and stem Mg improved compared to regular Mg salts; the highest yield was achieved at a combination of 500 mg l-1 Fe and Mg-NPs|||
2.1.4 Calcium (Ca)-NPs
Ca participates in many metabolic processes of plants like cell elongation, strengthens cell wall structure via the formation of calcium pectate, improves stomatal functions, induces heat shock proteins, and protects against various fungal and bacterial diseases. Ca-NPs have also been formulated and tested for their role in increasing the crop growth and productivity. CaCO3 NPs (20–80 nm, 160 mg l-1 as Ca) in Hoagland solution were tested as a source of Ca for peanut, grown in sand for 80 days , and were compared with control (without Ca) and with soluble source of Ca as Ca(NO3)2 (200 mg l-1). A significant improvement in fresh biomass compared to the control was observed; however, this enhancement was similar on a dry weight basis in comparison to the soluble source of Ca [Ca(NO3)2]. The results were not able to justify why Ca(NO3)2 was compared as a Ca control, as it provides N besides Ca. Ca uptake by seedling stem and roots was enhanced compared to the control, which makes it justifiable that Ca-NPs enhanced Ca uptake and its transport from root to shoot due to their high surface area for being scavenged by the root surface of plant in the rhizosphere. Moreover, when there was combined application of Ca-NPs and humic acid (1 g l-1), maximum increase in seedling dry weight, i.e. 30% and 14% compared to the control and treated with Ca(NO3)2, respectively, was observed.
2.1.5 Magnesium (Mg)-NPs
Mg has a key role in photosynthesis as it is an essential component of chlorophyll, the light-absorbing green pigment found in plants. It also helps in the synthesis of amino acids and cell proteins, uptake and migration of P, and causes resistance against biotic and abiotic stress in plants. The effect of combined foliar application of Mg-NPs and Fe-NPs (0.5 g l-1) on the photosynthetic efficiency of black-eyed pea (Vigna unguiculata) was investigated in a field experiment . The results clearly showed that combined application of Fe- and Mg-NPs significantly improved photosynthetic efficiency, which ultimately improved growth and yield parameters. Interestingly, their alone application caused a decrease in grain yield (8%). However, the authors observed an increase in the uptake of Mg in different plant tissues compared to the control and regular application of Mg, which suggests that Mg uptake increases with the application of Mg-NPs.
2.2 Micronutrient NFs
Micronutrients play an important role in many physiological functions of plants. These are required in a very small amount (≤100 ppm) but have a very critical role in various plant metabolic processes. These include chloride (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni). These are applied to the plants either as Hoagland solution  or as foliar depending on crop species, and also on the nutrient to be applied. These are also applied to the crop plants with composite fertilizers containing multiple macronutrients like NPK. Micronutrients present in these composites usually provide enough nutrients and cause little environmental risks. However, their availability is severely affected by small changes in pH, soil texture, and organic matter . So, it is most likely that under such circumstances, their optimum availability could be achieved through the application of NFs containing these micronutrients. A summary of the studies conducted regarding the investigation of the efficacy of each micronutrient-containing NPs is given below Table 2.
|Nutrient provided||Crop and experimental conditions||Size and rate of application||Comments||References|
|Fe||Vigna unguiculata subsp. unguiculata, foliar application, field study||Fe-NPs, 0.25 and 0.5 g l-1||More 1000-seed weight, leaf Fe and chlorophyll content compared to regular Fe salt|||
|Glycine max, greenhouse test 7 days, perlite medium, nutrient solution||Superparamagnetic iron oxide NPs, Fe3O4, 18.9–20.3 nm, 30, 45, and 60 mg l-1||Chlorophyll contents increased up to 45 mg l-1 but decreased at 60 mg l-1|||
|Cucurbita pepo cultivated in vitro||carbon-coated Fe-NPs||–|||
|Mn||Vigna radiata, 15 days in growth chamber, perlite medium, nutrient solution||Metallic Mn, 20 nm, 0.05, 0.1, 0.5, and 1.0 mg l-1, MnSO4||Metallic Mn increased growth and physiological parameters more compared to MnSO4; Mn-NPs did not show phytotoxicity|||
|Zn||Vigna radiata and Cicer arientinum, incubated 60 h in agar medium||ZnO, 20 nm, 1–2000 mg l-1||Growth and yield parameters in both improved|||
|Cucumis sativus, 53 days greenhouse pot study||ZnO, 10 nm, 400 and 800 mg kg-1 soil||Root dry mass, fruit starch, glutelin, and Zn contents significantly improved|||
|Brassica napus and Lolium perenne, 5 days germination||ZnO, 20 nm, 1–2000 mg l-1 applied to Brassica napus and metallic Zn, 35 nm, 1–2000 mg l-1 applied to Lolium perenne||Improved root elongation in both and Zn-NPs at levels higher than the optimum showed phytotoxicity|||
|Arachis hypogaea, germination and field trial during 2008–2010||nZnO, 25 nm, 1000 ppm, and chelated bulk zinc sulfate (ZnSO4), a field experiment with nZnO applied at 15 times lower dose compared to chelated ZnSO4||Promoted both seed germination and seedling vigor, early flowering, higher leaf chlorophyll content, pod yield per plant compared to chelated bulk ZnSO4; in field experiment, there was higher pod yield compared to chelated ZnSO4|||
|Glycine max||nZnO at 0, 0.5,and 1 g l-1||Increased germination over control; greater radicle length and fresh weight in stressed seedling|||
|Triticum aestivum L.||nZnO 20–50 nm||Significantly increased chlorophyll and protein content|||
|Pennisetum americanum||Biosynthesized Zn-NPs, 15–25 nm, sprayed at 16 l ha-1 after 2 weeks of germination at 10 ppm||Improved growth, physiological, biochemical, and yield parameters over control in 6-week-old plants|||
|Solanum lycopersicum L.||TiO2- and ZnO-NPs, 25±3.5 nm, 0–1000 ppm||Promoted growth and development|||
|Allium cepa||ZnO-NPs, at 0.0, 10, 20, 30 and 40 g ml-1||Low concentrations increased seed germination but decreased under higher ones|||
|Cyamopsis tetragonoloba L.||Biosynthesized nZnO, foliar sprayed at 10 ppm||Improved growth, yield, and quality parameters|||
|Musa acuminatain vitro cultures||nZn and nZnO||Somaclones accumulated more proline, chlorophyll, antioxidant enzymes activity, and developed more dry weight than the control|||
|Arabidopsis thaliana (mouse-ear cress)||nZnO at 400, 2000, and 4000 mg l-1||nZnO was most phytotoxic; inhibition of seed germination depended on particle size at equivalent concentrations|||
|Medicago sativa, Cucumis sativus, and Solanum lycopersicum||nZnO at 0–1600 mg l-1 and Zn2+ at 0–250 mg l-1||Germination in Cucumis sativus increased with nZnO at 1600 mg l-1 while it decreased in Medicago sativa and Solanum lycopersicum, observed highest Zn content in alfalfa with 1600 mg l-1 nZnO and 250 mg l-1 Zn2+|||
|Lolium perenne||nZnO and nZn2+||Reduced biomass, root tips shrank, and root epidermal and cortical cells highly vacuolated/collapsed; nZnO was observed in apoplast and protoplast of the root endodermis and stele|||
|Cu||Phaseolus radiatus and Triticum aestivum||nCu||The 2-day median effective concentrations for P. radiatus and T. aestivum exposed to nCu were 335 and 570 mg l-1, respectively|||
|Cucurbita pepo||Bulk and Cu-NPs and Ag-NPs were directly compared||Bulk and NP Cu were highly phytotoxic; humic acid (50 mg l-1) decreased the ion content of bulk Cu solution but increased Cu2+ of NP solutions|||
|Lactuca sativa, 15-day germination, soil||Metallic Cu, b 50 nm, 130 and 600 mg kg-1 as Cu||Increased shoot/root ratio, total N, and organic matter at 130 mg kg-1|||
|Edodea densa planch, 3-day incubation, water||70% CuO and 30% Cu2O, 30 nm, 0.025, 0.25, 0.5, 1, and 5 mg l-1 as Cu, CuSO4||Increased photosynthesis rate but leaf Cu at <0.5 mg l-1 CuSO4 inhibitory at all concentrations|||
|Mo||Cicer arietinum, rhizosphere soil examination||Mo, 100–250 nm, 8 mg l-1, others unknown||Improved nodule number/mass, activity of antioxidant enzymes and symbiotic bacteria|||
2.2.1 Iron (Fe)-NPs
In a greenhouse study under a hydroponic system, application of lower concentrations of Fe-NPs (30, 45, and 60 mg l-1) significantly improved the chlorophyll contents of the subapical leaves of soybean compared to the regular application of Fe-EDTA . The results suggested that Fe-NPs could serve as an efficient source of Fe compared to the regular Fe-EDTA applied at <45 mg l-1 as Fe, thereby reducing the chloratic symptoms caused by its deficiency in soybean. Moreover, the uptake efficiency of Fe-NPs in the plant body was enhanced, which ultimately increased the chlorophyll contents of soybean plants. In another experiment, growth and yield parameters of black-eyed peas were significantly improved when Fe-NPs were applied as foliar at 500 mg l-1 . Moreover, the application of Fe-NPs improved the effect of another fertilizer nutrient applied in the form of Mg-NPs. Previously, Hoagland and Arnon  found that most of the plants generally require 1–5 mg l-1 Fe in soil solution.
2.2.2 Manganese (Mn)-NPs
A hydroponic culture experiment was conducted to find out the comparative efficacy of Mn-NPs and commonly used Mn-salt, i.e. MnSO4, on the growth and yield parameters of mung bean . Both were applied at 0.05, 0.1, 0.5, and 1.0 mg l-1. The results showed that application of Mn-NPs at 0.05 mg l-1 significantly improved growth and yield parameters compared to the control with no Mn applied. At higher doses, Mn-NPs did not show toxicity to the bean plants, while MnSO4 applied at 1 mg l-1 showed toxic effects like necrotic leaves, brown roots, and gradual disappearance of the rootlet after 15 days of treatment. Moreover, greater oxygen evolution and photophosphorylation in Mn-NP-treated chloroplasts was noted compared to the control. Greater oxygen evolution was caused by enhanced splitting of water in the oxygen-evolving center located in the chloroplast. The authors concluded that Mn-NPs could serve as a potential modulator of photochemistry in the agriculture sector.
2.2.3 Zinc (Zn)-NPs
Many researchers around the world have focused on finding the effect of ZnO-NPs on the growth and productivity of crops. Most important, out of the all micronutrients, it is the most widely studied in plant science worldwide. For example, optimal concentration of ZnO-NPs significantly enhanced the growth and yield parameters of mung bean and chickpea . Authors found that the optimal concentration of ZnO-NPs to be applied depends on the nature of the crop. With the application of 20 mg l-1 ZnO-NPs to mung bean plants, an increase of 42%, 41%, 98%, and 76% in root length, root biomass, shoot length, and shoot biomass, respectively, was recorded. Moreover, the application of higher doses of ZnO-NPs caused a decrease in the growth rates of mung bean and chickpea. In another greenhouse experiment, the application of ZnO-NPs at the rate of 400 and 800 mg kg-1 caused a significant increase in the growth and yield parameters of cucumber (Cucumis sativus) . The results clearly showed an increase of 10% and 60% in plant root dry mass with the application of 400 and 800 mg kg-1, respectively, as compared to control (without ZnO NPs). However, the same rates caused a slight increase of 0.6% and 6% in the dry fruit weight, respectively, as compared to the control. Similarly, Lin and Xing  reported a significant increase in the root elongation of germinated seeds of radish (Raphanus sativus) and rape (Brassica napus) with the application of ZnO-NPs at 2 mg l-1, in comparison to control (deionized water). The authors also found a significant improvement in the growth parameters of ryegrass (Lolium perenne) with the application rate of 2 mg l-1 metallic Zn-NPs. Seed germination was improved with the application of lower concentrations of ZnO-NPs in peanut , soybean , wheat , pearl millet , tomato , and onion . In another experiment, a significant improvement in Cyamopsis tetragonoloba plant biomass, shoot and root growth, root area, chlorophyll and protein synthesis, rhizospheric microbial population, acid phosphatase, alkaline phosphatase, and phytase activity in cluster bean rhizosphere was recorded with the application of ZnO-NPs . Similarly, Helaly et al.  found that ZnO-NPs supplemented with MS-media promoted somatic embryogenesis, shooting, regeneration of plantlets, and also induced proline synthesis, activity of superoxide dismutase, catalase, and peroxidase, thereby improving tolerance to biotic stress. In contrast to these studies, many researchers have reported phytotoxicity of the application of Zn-NPs in various crop plants [31–33, 42, 49]. However, phytotoxicity was found to depend on the nature of crop plants. Overall, most of the crop plants usually require merely 0.05 mg l-1 soil solution. The researchers in these studies applied metallic Zn-NPs at a very high rate, ranging from 400 to 2000 mg l-1, which was the main reason for their toxic effects. Even the application of Zn-NPs at 10 mg l-1 to ryegrass proved harmful for normal growth . In another study, among cucumber, alfalfa, and tomato, the application of ZnO-NPs only enhanced seed germination of cucumber .
2.2.4 Copper (Cu)-NPs
Previously, it has been clearly found that the application rate of Cu-NPs at 0.02 mg Cu l-1 in Hoagland solution is optimum for normal growth and yield of crops. Scientists around the world have found toxic effects of the application of Cu-NPs, as they have applied them at higher rates than required [44, 45]. They found that Cu-NPs applied at the rate of 200–1000 mg l-1 caused toxic effects on seedling growth of mung bean, wheat, and yellow squash. Similarly, reduced biomass of zucchini by 90% compared to that of the control (without Cu) after the seedlings were incubated in Hoagland solution for 14 days was recorded with the application of metallic Cu-NPs at 1000 mg l-1. However, researchers like Shah and Belozerova  recorded a significant increase of 40% and 91% in 15-day lettuce seedling growth rate with the application of Cu-NPs at 130 and 600 mg kg-1, respectively. Similarly, a 35% increase in photosynthetic rate of waterweed was recorded in a 3-day incubation study using a low concentration of Cu-NPs applied at ≤0.25 mg l-1 .
2.2.5 Molybdenum (Mo)-NPs
Mo is essential for legumes as it is involved in biological nitrogen fixation (BNF), being the component part of nitrogenase enzyme. For normal metabolism, crop plants usually require ≈0.01 mg Mo l-1 soil solution. Taran et al.  conducted a pot experiment using different combinations of N-fixing bacteria and Mo-NPs (water, Mo-NPs, microbial inoculation with nitrogen-fixing bacteria, and a combination of the microbes and Mo-NPs). The control was treated with distilled water. Chickpea seeds were soaked in each of the treatments for 1–2 h. The results clearly showed that the combined application of microbes and Mo-NPs significantly improved the microbiological properties of the rhizosphere, including all groups of agronomically important microbes. The same combination significantly improved the root number, nodule number per plant, and nodule mass per plant compared to control.
2.3 Nanocarrier-based fertilizers
These are the fertilizers that contain NMs, which, when mounted with a plant nutrient, can increase the nutrient efficiency of the plants and/or reduce the detrimental effect of conventional fertilizers associated with their high rates of application. These nanocarriers, in themselves, do not contain any nutrient at all. The most commonly used nanocarrier-based fertilizers are discussed below.
2.3.1 Nutrient-augmented zeolites
Zeolites are complicated silicate minerals with nanostructures that are well renowned for their unique nanoporous properties with high specific surface area (≈1150.5 m2 g-1) and cation exchange capacity (10 times more than that of soil). These are highly selective toward plant macronutrients like K+ and NH4+ . These mostly do not occur at nanoscale but acquire nanostructure during the arrangement of Al and Si in the three-dimensional framework of SiO4 and AlO4 tetrahedra of zeolites, which creates channels and voids that are within nanoscales (0.3–10 nm diameter). The essential nutrients may be mounted onto the exchange sites of zeolites, which, on their application to the field, slowly release nutrients, thereby reducing their loss through leaching and/or run off. Similarly, as the nutrients get adsorbed onto the exchange sites of zeolite, the chances of their availability to the soil microbes involved in ammonification are also reduced, thereby decreasing the loss of nutrients through volatilization or through nitrification and denitrification (NH3, N2, or N2O).
Long ago, zeolites have been well renowned for being used as carrier for plant nutrients and have been intensively studied around the world . Zeolites have been mostly used for increasing nitrogen use efficiency (NUE), reducing the nutrient loss, and thereby reducing the detrimental effects on the environment. For example, a pot experiment was conducted to find out the comparative efficacy of naturally occurring zeolite (Clinoptilolite) rich in exchangeable K and with regularly used nutrient solution containing 234 mg K l-1 using chrysanthemum (Chrysanthemum morifolium) as a test plant . The results clearly showed that a single application of naturally occurring zeolite at 33 g l-1 was sufficient to produce a 3-month yield of chrysanthemum compared to those recorded with a nutrient solution containing 234 mg K l-1. The nutrient solution contained about 2.3 times more K than the potting medium fertilized with K-augmented zeolite.
Two greenhouse studies were conducted with sweet corn (Zea mays L.) to find out the effect of ammonium-loaded clinoptilolite (A-Cp) in reducing N-leaching compared to ammonium sulfate (AS). Pots containing sandy soil were fertilized with either AS or one of three size fractions of A-Cp at rates of 112, 224, or 336 kg N ha-1 in a first experiment, and 112 or 224 kg N ha-1 in a second experiment. Soil amended with AS leached 10–73% of the added N, depending on N rate, whereas <5% of added N leached from A-Cp-amended soil regardless of N rate and Cp particle size. Plants fertilized with A-Cp assimilated significantly more N than AS-fertilized plants. It was concluded that fertilization with A-Cp would minimize N leaching from sandy soils while sustaining normal corn growth . In another study, application of NH4-exchanged zeolite-rich tuff (an equivalent rate of 30 tons ha-1) significantly decreased nitrification rate by 4% in a silty clay loam and 11% in loamy sand soil . It was suggested that decrease in nitrification was due to the adsorption of NH4+ on the exchange sites of zeolite, which ultimately reduced NH4+ exposure to nitrifying bacteria.
In a lysimetric study, 22% reduction in leaching of NO3-N (13.8 kg ha-1) was recorded with the application of zeolite amendment (60 g kg-1) compared to the control (18 kg ha-1). There was 10.3% and 4.9% increase in dry weight of stoves and grain yield of maize . Moreover, the zeolite amendment caused a significant increase in rate of fertilizer uptake from 77% to 86%. Pot experiment under controlled condition was conducted to compare the effect of K-augmented zeolite with KCl applied at varying concentrations on different growth stages of wheat . The results clearly showed that K-augmented zeolite significantly improved the aboveground dry biomass by 64% compared to KCl applied at rate of 89 mg kg-1. Recently, Li et al.  investigated the effect of NH4- and K-loaded zeolite (NK-Z) on slow release of N and potassium (K). The results clearly showed a significant increase in total harvest weight of kale (Brassica alboglabra Bailey) compared to that with the application of KCl or KOH. Moreover, higher levels of N and K were detected in soil applied with NK-Z.
So, nutrient-augmented zeolites significantly improved the growth and yield of various test crops as discussed in the above reports. However, their use is constrained by their extra cost to purchase zeolite and augment it with nutrient of interest. In the future, there is dire need to find out an automated system installed in the chemical manufacturing unit that automatically ensures the mounting required nutrient during the prilling process of fertilizer manufacture.
2.3.2 Other nanocarriers
Several laboratory studies have demonstrated the capability of several NPs (silica, Fe oxides, C-coated Fe, and polymers) for efficient DNA transport and chemicals into the plant tissues/cell [30, 56, 57]. Moreover, their use might become expensive as plants have a natural ability to absorb soluble nutrients (e.g. N, P, and K) directly from soil solution so their use might become an extra effort, which makes nutrient application more complex and also expensive. However, there is a possibility to find out certain NMs/NPs that are economical and more efficient compared to previously available carriers like zeolite. So, there is need to find out such NMs comprehensively in future research.
3 Plant-growth-enhancing NPs with unclear mechanisms
Several studies have reported NPs that could enhance plant growth with an unknown mechanism as these are neither a source of macro- or micronutrients nor carry any nutrient with them. Their detailed description is given in the following sections Table 3.
|Crop and experimental conditions||Size and rate of application||Comments||References|
|TiO2-NPs that enhanced plant growth parameters through partially known mechanisms|
|Solanum lycopersicum L.||TiO2- and ZnO-NPs, 25±3.5 nm, 0–1000 ppm||Promoted growth and development|||
|Spinacia oleracea, greenhouse, 35 days, perlite medium||Anatase, 5 nm, 2.5 g l-1, seed soak and leaf spray||More biomass, N, chlorophyll, and protein content observed with anatase compared to bulk rutile|||
|Spinacia oleracea||Nano-anatase TiO2 (5 nm), bulk TiO2||Greater amount and the activity of Rubisco activase with nano-anatase TiO2 compared to bulk TiO2|||
|Spinacia oleracea||Nano-anatase TiO2||Promoted spectral responses, leading to the improvement of primary electron separation, electron transfer, and light energy conversion of D1/D2/Cyt b559 complex|||
|Lemna minuta, growth chamber, 7 days, nutrient solution||Anatase, 5–10 nm, 10, 50, 100, 200, 1000, and 2000 mg l-1, bulk TiO2||More root length, biomass, and chlorophyll content with anatase at <500 mg l-1 compared to bulk TiO2|||
|Glycine max||Hastened germination, growth, and prevented from going moldy; increased nitrate reductase of root and leaves; ultimately improved resistance to adversities|||
|Vigna radiata L.||Biosynthesized TiO2, 12–15 nm, foliar sprayed at 10 ppm||Promoted growth, physiological, biochemical and yield parameters|||
|Brassica napus||nTiO2 at 10, 100, 1000, 1200, 1500, 1700, and 2000 mg l-1||Showed larger radicle and plumule growth of seedling at 1200 and 1500 mg l-1 compared to other concentrations and control|||
|Spinacia oleracea||nTiO2 (rutile) and non-nTiO2||Increased germination, growth, yield, and physiological functions. The best results were found at 2.5% n-TiO2|||
|Spinacia oleracea under different illumination times (1, 5, 10, 20, 30, and 40 min)||nTiO2 (rutile) 0.25% solution||Increased rate of evolution oxygen of chloroplasts in different illumination times|||
|Spinacia oleracea||Nano-anatase TiO2||Increased activities of reactive oxygen species (ROS) scavenging enzymes|||
|Spinacia oleracea chloroplasts under UV-B radiation||Nano-anatase||Improved ROS scavenging system by increasing activities of various enzymes involved in defense system under stress|||
|Triticum aestivum||nTiO2 (1, 2, 10, 100, and 500 ppm), bulk (1, 2, 10, 100, and 500 ppm), and control (without TiO2)||nTiO2 improved germination and various growth parameters at 10 ppm compared to bulk and control|||
|Triticum aestivum||Anatase and rutile TiO2-NPs with diameters ranging from 14 to 655 nm||The smallest NPs during the first stages of development caused an increase of root elongation|||
|Triticum aestivum, field experiment||nTiO2 at 0.01%, 0.02%, and 0.03%, control (bulk TiO2)||nTiO2 at 0.02% increased almost all agronomic traits including gluten and starch content|||
|Spinacia oleracea||n-TiO2 (rutile) 0.25%||Accelerated Hill reaction and noncyclic photophosphorylation (nc-PSP) activity of chloroplasts, the chloroplast coupling and activities of Mg2+-ATPase and chloroplast coupling factor I (CF1)-ATPase on the thylakoid membranes|||
|Zea mays L. and Vicia narbonensis L.||nTiO2 at concentration range from 0.2% to 4.0%||Delayed germination progression in both materials; reduced mitotic index and increased aberration emergence|||
|Ulmus elongate, foliar application||Nano-anatase TiO2 at 0.1%, 0.2%, and 0.4% solution||Enhanced leaf absorbance indicating the synthesis of carbohydrate and lipid compounds|||
|CNTs that enhanced plant growth parameters through partially known mechanisms|
|Cell culture with extracted chloroplasts||SWNTs||Enhanced photosynthetic activity than that of controls and reduced concentrations of ROS inside extracted chloroplasts|||
|Lolium multiflorum, Brassica napus, Zea mays, 5 days incubation study||MWCNTs with diameter 10–20 nm, length 1–2 μm, 2 g l-1||Significantly improved root elongation in Brassica napus, ryegrass, and Zea mays|||
|Solanum lycopersicum, 2 months growth chamber, soil mixture, water||MWCNTs diameter ~10–25 nm, 50 and 200 mg l-1, activated carbon (AC)||Both increased plant height and fruit number per plant compared to control and enhanced water uptake and utilization efficiency|||
|Solanum lycopersicum||Well-dispersed MWCNTs and MWCNTs functionalized with stronger -ve groups||Increased plant growth and production of water channel protein compared to control or plants exposed to poorly dispersed and highly aggregated MWCNTs|||
|Zea mays||Factory-synthesized MWCNTs||Improved water absorption, plant biomass, and the concentrations of the essential Ca, Fe nutrients|||
|Tobacco cell culture||MWCNTs and AC at 5–500 μg ml-1||Both MWCNTs and AC enhanced growth, expression of aquaporin gene, and production of NtPIP1 protein; however, AC stimulated cell growth only at low concentrations over control. The expression of marker genes for cell division and cell wall extension was also upregulated in cells compared to control cells or those exposed to AC only|||
|Barley, Zea mays, Glycine max, agar medium, 10-day germination study||MWCNTs diameter ~15–40 nm, 50, 100, and 200 mg l-1||Shoot and root length in barley, Zea mays and Glycine max significantly enhanced|||
|Triticum aestivum, 7 days of exposure||OMWCNTs with a length 50–630 nm, at 10–160 lg ml-1||Improved growth, yield and dehydrogenase activity at 80 lg ml-1|||
|Triticum aestivum under light and dark conditions||Water-soluble carbon nanodots (wsCND)||Enhanced growth of root and shoot both in light and dark conditions|||
|Allium cepa and Cucumis sativus 2–3-day germination study in growth chamber||SWCNTs, diameter ~8 nm, length 0.1–5 μm, at 104, 315, 1750 mg l-1||Improved root elongation of 1-day-old Allium cepa and Cucumis sativus|||
|Solanum lycopersicum||CNTs (10–40 μg ml-1)||Improved germination and growth rates compared to control|||
|Brassica juncea and Phaseolus mungo||MWCNTs at 10, 20, and 40 μg ml-1||Germination and root growth in both was enhanced at 10 and 20 μg ml-1 while phytotoxicity was evidenced at 40 μg ml-1|||
|Hybrid Bt Gossypium spp., in vitro conditions||MWCNTs||Improved growth and yield parameters both under in vitro and field conditions at 60 and 100 μg ml-1, respectively|||
|Brassica juncea||Oxidized MWCNTs having a diameter of 30 nm||Improved germination and growth under low concentration compared to non-oxidized and higher conc.-treated seeds|||
|Lactuca sativa, hydroponic study for 15 days||MWNTs treated Hoagland’s media, carbon black (CB)||Deteriorated root tip and penetrated into plant cell wall, causing cell death on root and leaves compared to CB|||
|Spinacia oleracea, hydroponic culture||MWCNTs at 0–1000 mg l-1||Exhibited growth inhibition and cell death, suggesting a role of ROS in MWCNT-induced toxicity|||
|Onobrychis arenaria seedlings||Nano-Taunit containing MWCNTs at 100 and 1000 μg ml-1||Stimulated root and stem growth and enhanced the peroxidase activity|||
|Cicer arietinum||Water-soluble carbon nanotubes (wsCNTs) at 6.0 μg ml-1||Increased growth rate, water absorption, and retention; wsCNTs are non-toxic to plant cells|||
|AG-NPs nanoparticles that enhanced plant growth parameters through partially known mechanisms|
|Bacopa monnieri L. Wettst, hydroponically||Biosynthesized Ag-NPs||Enhanced peroxidase and catalase activity; simulated the stress conditions induced by the silver nitrate treatment|||
|Arabidopsis||Ag-NPs with decahedral (45±5 nm), triangular (47±7 nm), and spherical (8±2 nm) shapes||Increased ROS accumulation and root growth, activated gene expression involved in cell proliferation, metabolism, and hormone signaling pathways|||
|Corms Crocus sativus||nAg at 0, 40, 80, or 120 ppm||Increased root number, length, and leaves dry weight with 80 ppm under flooding stress|||
|Raphanus sativus and Lactuca sativa, Hordeum vulgare as a reference, greenhouse study||AgNP, 10 nm PVP, at 1, 2.5, 5 and 10 mg l-1||Root length was increased for Hordeum vulgare, but was dramatically inhibited for Lactuca sativa|||
|Eleven species of wetland plants in simple pure culture and field soils in greenhouse experiment||20-nm Polyvinylpyrrolidone-coated AgNPs (PVP-AgNPs), 6-nm gum arabic-coated AgNPs (GA-AgNPs), and AgNO3 at equivalent Ag conc. (1, 10, or 40 mg Ag l-1).||The plant growth response differed by taxa with Lolium multiflorum growing more rapidly under both AgNO3 and GA-AgNP exposures|||
|Boswellia ovalifoliolata||Biosynthesized AgNPs at 10–30 mg ml-1 in MS basal medium||Improved germination and growth parameters|||
|Phaseolus vulgaris L., Zea mays L.||AgNPs at 20, 40, 60, 80, and 100 ppm||Increasing concentration from 20 to 60 ppm increased growth and physiological parameters|||
|Brassica juncea, 7-day-old seedlings||AgNPs at 0, 25, 50, 100, 200, and 400 ppm||Improved growth and physiological parameters involved in ROS scavenging, found optimum growth at 50 ppm|||
|Au-NPs that enhanced plant growth parameters through partially known mechanisms|
|Lactuca sativa, short term||Silica, palladium, gold, and copper||Enhanced growth of Lactuca sativa seeds as measured through shoot/root ratios of the germinated plant (p<0.05)|||
|Cucumis sativus, Lactuca sativa, and Photobacterium phosphoreum||Au, 10 nm, germination, and anaerobic toxicity test||In all cases, low or zero toxicity was observed|||
|Brassica juncea, field conditions||AuNPs at 0, 10, 25, 50, and 100 ppm||Increased growth, yield, and quality parameters up to 25 ppm, improved the redox status of the treated plants|||
|Gloriosa superba||Spherical biosynthesized Au NPs 25 nm at 500 and 1000 μm||Exposure at 1000 μm concentration had the most significant effect on seed germination rate and vegetative growth|||
|Arabidopsis thaliana||Au-NPs, 24 nm, at 10 and 80 μg ml-1||Enhanced total seed yield, seed germination rate, vegetative growth and free radical-scavenging activity|||
|SiO2-NPs that enhanced plant growth parameters through partially known mechanisms|
|Glycine max||Unknown nanometer materials||Increased nitrate reductase activities, water use efficiency, total antioxidant capacities, and stimulated the antioxidant system|||
|Lycopersicum esculentum Mill.||nSiO2, 12 nm||Among the treatments, 8 g l-1 nSiO2 improved germination and growth parameters|||
|Zea mays||Si-NPs, 20–40 nm, at 5–20 kg ha-1, bulk silica at 15–20 kg ha-1||SNPs increased growth up to 15 kg ha-1; silica accumulation in leaves was high at 10 and 15 kg ha-1 concentrations of SNPs||[104, 105]|
|Larix olgensis soaked for 6 h||TMS (nSiO2) at 2000, 1000, 500, 250, 125, and 62 μl l-1||Promoted seedling growth and improved seedling quality, 500 μl l-1 produced the best result, compared to that of control|||
|Solanum lycopersicum||nSi at 1 and 2 mm and NaCl at 25 and 50 mm||1 mm nSi+25 mm NaCl improved germination, root length, and dry weight while higher conc. reduced germination|||
|Cucurbita pepo L. cv. white bush marrow||nSiO2||Improved germination, growth, and physiological parameters; induced ROS scavenging|||
|Ocimum basilicum under salinity stress in greenhouse condition||No Si, Si fertilizer, and nSi, salinity 1, 3, and 6 dS m-1||Leaf dry and fresh weight, chlorophyll and proline content increased, which was due to tolerance induction in the plant|||
|Indocalamus barbatus McClure, foliar spraying||nSiO2 at 0, 150, 300, and 450 mg l-1||Improved quality and nutrient contents, stimulated SOD and POD activities, and decreased MDA content at 300 mg l-1|||
|Indocalamus barbatus McClure, foliar spraying||nSiO2 at 150, 300, and 450 mg l-1||Improved physiological and photochemical parameters; the optimal concentration of 300 mg l-1 compared to control|||
|SBA, 15 nm (SBA15) and 23 nm (SBA23)||Adsorbed 4.7 and 15 mg PSII g SBA-1 with 15 nm (SBA15) and 23 nm (SBA23), respectively|||
3.1 Titanium dioxide (TiO2)-NPs
Traditionally, Ti is not included in the list of essential plant nutrients. Moreover, most of the soils contain sufficient amount of Ti ranging from 0.1% to 0.9% with an average of about 0.03 mg l-1 in soil solution . So, there is no need to apply an external source of Ti to crop plants. However, several researchers have reported about its positive effect on photosynthesis efficacy and enzyme activities like that of nitrogenase, thereby increasing N supply to the crop plants via BNF, and ultimately the plant growth [58–60]. For example, Yang et al.  noted a significant increase (twofold) in the fresh and dry weights of spinach (Spinacia oleracea) plants when 2.5 g l-1 Ti-NP solution was sprayed once a week for 35 days compared to that of the control (without Ti-NP application). Moreover, the same treatment caused an increase of 23%, 34%, and 13% in total N, chlorophyll, and protein contents over control, respectively. Gao et al.  noted similar results with the application of Ti-NP at 0.3 g l-1, about 10 times less concentration than that used by Yang et al. . In a 7-day growth chamber study, a significant increase in plant height and fresh weight of duckweed (Lemna minor) was noted with the application of Ti-NP at 0.5 g l-1 compared to bulk TiO2 solution . Application of Ti-NP >0.5 g l-1 showed an inhibitory effect on the plant. Combined application of TiO2 and SiO2 at lower concentrations resulted in a significant increase in the activity of nitrate reductase in the rhizosphere of soybean, which ultimately improved germination and growth . Yang et al.  and Mishra et al.  recorded improvement in plant fresh and dry weights, and suggested this to be due to the improvement in various enzyme activities (nitrate reductase, glutamate dehydrogenase, glutamine synthase, and glutamic-pyruvic transaminase) involved in N metabolism. Ti-NPs favor the conversion of inorganic N to organic N in the form of proteins and chlorophyll, which results in an overall increase in plant productivity. Seed germination, radicle and plumule growth of canola seedlings, tomato and mung bean [38, 63], spinach (Spinacia oleracea), and wheat (Triticul aestivum L.) were significantly improved with the application of Ti-NP [64–70]. Similarly, under water-deficit conditions, wheat growth and yield parameters were significantly improved with the application of Ti-NPs at 0.01%, 0.02%, and 0.03% .
Moreover, these are also effective photocatalysts and have been extensively used for the production of H2 as a fuel using solar energy through photocatalytic decomposition of organic compounds [115, 116]. Yang et al.  found that Ti-NPs noticeably promote aged seeds’ vigor and chlorophyll formation and stimulate ribulose 1,5-bisphosphate carboxylase (Rubisco) activity, thereby increasing photosynthesis and ultimately plant growth and development. It has also been found that Ti-NPs increase light absorbance, hasten transport and conversion of light energy, protect chloroplasts from aging, and prolong the photosynthetic time of chloroplasts. Hong et al. [66, 72] suggested that it might be due to protection of chloroplast from excessive light by augmenting the activity of antioxidant enzymes (catalase, peroxidase, superoxide dismutase). However, more comprehensive studies are required to elucidate how photocatalysis of these NPs coordinate with plant’s biochemical process, i.e. photosynthesis to improve its growth and productivity [7, 9]. These studies would also help elucidate the mechanisms/process behind why plants do not utilize Ti, sufficiently present in the soil, for their biochemical processes. Above all, there are also reports about the negative effects of Ti-NPs on seed germination applied at 4 g l-1 .
3.2 Carbon nanotubes
CNTs have acquired an important position due to their unique mechanical, electrical, thermal, and chemical properties. However, there has been scant information available on the efficacy of CNTs in plant nutrition and their relation with plant metabolism. Various studies have reported magically positive effects of multiwalled CNTs (MWCNTs) on the seed germination of various crops and ultimately plant growth [75–77]. The proposed mechanism behind this plant growth improvement involves the induction of water channel proteins due to the presence of surface charges of CNTs, thereby enhancing water and essential nutrient (Ca and Fe) uptake efficiency that could enhance seed germination and plant growth and development [77, 78, 117]. In another study, embedded single-walled CNTs (SWCNTs) in isolated chloroplast augmented photosynthetic activity (three times) compared to control and enhanced electron transport rates . Moreover, SWCNTs enabled plants to sense nitric oxide, a signaling molecule. The authors suggested that a nanobionics approach to engineered plants would enable new and advanced functional properties in photosynthetic organelles. For this purpose, extensive research is required to elucidate the impact of CNTs on ultimate products of photosynthesis such as sugars and glucose.
Seed germination of barley, soybean, and corn was significantly enhanced with the addition of MWCNTs in agar medium . The authors found that MWCNTs have the ability to penetrate into seed coats as nanotube agglomerates were detected through Raman spectroscopy and transmission electron microscopy. Moreover, they also found that MWCNTs regulated the expression of genes encoding several types of water channel proteins in soybean, corn, and barley seed coat. In another study, Khodakovskaya et al.  found an upregulation in marker genes for cell divisions (CycB), cell wall formation (NtLRX1), and water transport (aquaporin, NNtPIP1) in tobacco cell culture with the application of MWCNTs, hence resulting in better seed germination and ultimately plant growth. Similarly, MWCNTs improved water retention capacity and biomass, flowering, and fruit yield, and increased the medicinal properties of tomato . Wang et al.  applied oxidized MWCNTs and found a significant improvement in dehydrogenase activity and cell elongation of the root system. The presence of water-soluble CNTs inside wheat plants using scanning electron and fluorescence microscope was also confirmed, which resulted in improved root and shoot growth under light and dark conditions . Improvement in root growth of germinated seeds of ryegrass with the application of MWCNTs and in onion (Allium cepa) and cucumber seeds with the application of SWCNTs was recorded [33, 83] in comparison to control (0 CNTs). Many researchers have confirmed the role of CNTs in improving seed germination and ultimately the growth of various crops like hybrid Bt cotton, Brassica juncea, Phaseolus mungo, tomato (Lycopersicum esculentum Mill.), mustard (Brassica juncea), black gram (Phaseolus mungo), and rice (Oryza sativa L.), etc. [75, 76, 79, 83–86, 118].
However, some researchers have reported the inhibitory effect of MWCNTs at higher concentrations on the growth of various crop plants [77, 82, 87, 88, 119]. Similarly, Lin and Xing  in radish, lettuce, or cucumber seeds have reported no effect even at higher concentrations.
The exact mechanism behind this enhancement is still unknown. However, various researchers have suggested that improvement in plant growth and enzyme activities (peroxidase and dehydrogenase) through the application of MWCNTs might be due to their uptake and accumulation in roots followed by translocation to leaves, which ultimately induces gene expression [78–80, 89]. From these reports, it might be concluded that effect of SWCNTs and MWCNTs varies from plant to plant, within and across species, and depends on growth stage, concentration, and nature of CNTs. So, more field research is needed to confirm the positive effect of CNTs on yield of various crops. Studies to elucidate responsible mechanisms are also required to be conducted.
3.3 Silver (Ag)-NPs
The available data show that a few studies regarding the effect of Ag-NPs have been conducted. Their positive/neutral/negative effect was entirely dependent on the plant species exposed, their particle size, and the rate of application . Recently, exposure of Ag-NPs with three different morphologies to physiological and molecular response of Arabidopsis suggested that decahedral Ag-NPs showed the highest degree of root growth promotion (RGP); however, spherical Ag-NPs had no effect on RGP but triggered the highest levels of anthocyanin accumulation in Arabidopsis seedlings . The spherical and decahedral Ag-NPs resulted in the highest and lowest yield of Cu/Zn superoxide dismutase, respectively. Moreover, three different sizes and shapes of Ag-NPs regulated protein accumulations, gene expression involved in cellular events, activated the aminocyclopropane-1-carboxylic acid (ACC)-derived inhibition of root elongation in Arabidopsis seedlings, and reduced the expression of ACC synthase 7 and ACC oxidase 2, suggesting that Ag-NPs acted as inhibitors of ethylene perception and could interfere with ethylene biosynthesis. Similarly, Rezvani et al.  also reported that Ag-NPs induced root growth of Crocus sativus by blocking ethylene signaling. Recently, Gruyer et al.  reported root length increase in barley but was inhibited in lettuce. Also, Yin et al.  studied the effects of Ag-NPs on germination of 11 wetland plant species (Lolium multiflorum, Panicum virgatum, Carex lurida, C. scoparia, C. vulpinoidea, C. crinita, Eupatorium fistulosum, Phytolaca americana, Scirpuscyperinus, Lobelia cardinalis, and Juncus effusus) and found improved germination rate only in one species (E. fistulosum). In another hydroponic study, a significant improvement in germination rate of Bacopa monnieri was recorded with the application of biologically synthesized Ag-NPs . Moreover, the same treatments enhanced protein and carbohydrate synthesis, while total phenol contents and catalase and peroxidase activities were reduced. Similar results were reported in Boswellia ovalifoliolata , Brassica juncea, common bean, and corn [97, 98].
3.4 Gold (Au)-NPs
Similar is the case with Au-NPs that a few studies have been conducted regarding their effect on plant productivity. Various researchers around the world reported positive effects of Au-NPs application on seed germination of various crops like lettuce and cucumber , Brassica juncea , Boswellia ovalifoliolata , and Gloriosa superba . Moreover, Au-NPs application improved seed germination and antioxidant system in Arabidopsis thaliana and altered levels of expression of miRNAs that regulate various morphological, physiological, and metabolic processes in plants . On the other hand, Shah and Belozerova  reported a toxic effect of Au-NPs on the function of aquaporins, a group of proteins involved in the transportation of a wide range of molecules including water. So, a comprehensive study involving the growth and physiological aspects of plants needs to be conducted in the future to make the positive effects of Au-NPs on plant more authentic.
3.5 Silicon dioxide (SiO2)-NPs
Plant growth and development starts from the germination of seeds followed by root elongation and shoot emergence as the earliest signs of growth and development. Although silicon is not an essential nutrient for plant growth, it has played an important role in germination and growth through its effect on metabolism of various nutrients like carbon, N, and P [13, 120–122]. The reported data from various studies suggested that the effect of SiO2-NPs on seed germination was concentration dependent. Their lower concentrations improved the seed germination of tomato . Similarly, SiO2-NPs improved seed germination through better nutrient availability to maize [104, 105]. Bao-shan et al.  also reported improved seedling growth and quality of Changbai larch (Larixolgensis) through exogenous application of SiO2-NPs. Haghighi et al.  in tomato and Siddiqui et al.  in squash reported that SiO2-NPs enhanced seed germination and stimulated the antioxidant system under NaCl stress. Exogenous application of SiO2-NPs and nano-titanium dioxide (nano-TiO2) improved seed germination of soybean by increasing the activity of nitrate reductase  and also by enhancing seeds’ ability to absorb and utilize water and nutrients . Under salinity stress, SiO2-NPs improve leaf fresh and dry weights, chlorophyll content, and proline accumulation. An increase in the accumulation of proline, free amino acids, content of nutrients, antioxidant enzymes activity results due to the SiO2-NPs, thereby improving the tolerance of plants to abiotic stress [107–110]. Wang et al.  found silica coated with quantum dots promoted markedly increased rice root growth. SiO2-NPs enhance the plant growth and development by increasing gas exchange and chlorophyll fluorescence parameters [108, 111]. Similarly, Noji et al.  reported that a nano-mesoporous silica compound (SBA) bound with photosystem II (PSII) induced stable activity of a photosynthetic oxygen-evolving reaction, indicating the light-driven electron transport from water to quinone molecules, and suggested that the PSII-SBA conjugate might have properties to develop for photosensors and artificial photosynthetic systems.
4 Concluding remarks and future directions
The world population is expected to reach 9.6 billion or more in 2050. To feed this ever-increasing population of the world, more pressure will be on land, which is not extendable. The conventional fertilizers might not be helpful under this situation as these have become expensive due to high energy requirements and being environmentally unsafe. Recent advancement in the field of nanotechnology has revolutionized the world with special NPs and NMs that could serve not only a source of macro- and micronutrients but also act as carriers for them, and improve growth and productivity of crops. It is evident from compiled information that the effect of NPs varies from plant to plant and depends on their mode of application, size, morphology, and concentrations. So, the following are some future directions that may be kept in mind while planning future research:
More and more efforts might be focused on N- and P-NFs as these have the largest application rate and also availability problem. The research might be focused on the comparison of Ca-NPs from CaCO3 with other Ca sources like CaCl2 or CaSO4 or multinutrient comparison like that of Ca- and N-NPs using soluble Ca(NO3)2 as control.
In case of micronutrients, comprehensive research might be conducted to elucidate the effect of factors affecting their availability under field conditions. Their role in biofortification in comparison to conventional fertilizers may also be tested. Application strategies through fertigation may also be tested in comparison to commercially available micronutrient fertilizers. Nanotoxicity is the main problem with these types of fertilizers, so optimum dose calculation for each and every crop without any toxic effects may also be investigated.
In the case of nanocarrier-based fertilizers, there is still no solid report about their role in increasing the FUE through improvement in transportation of nutrients into the plant tissues/cells or reducing environmental risks associated with traditional fertilizer use. There is a possibility to find certain NMs like SiO2-NPs, Fe2O3-NPs, and CNTs, which are economical and more efficient compared to previously available carriers like zeolite. These advances would help in the slow uptake of active ingredients, thereby reducing the amount of inputs to be used and also the waste produced. Nanocarriers could be designed in such a way that these can anchor the plant roots or to the surrounding soil structure and organic matter. This can only be possible through the understanding of molecular and conformational mechanisms between the delivery nanoscale structure and targeted structures and matters in soil .
It has been clearly found out that higher concentrations of various NPs/NMs are toxic for plant growth, which is ultimately dependent on their particle size. So, a checkpoint might be worked out in future studies to elucidate the critical concentration of certain NPs/NMs with a particular size and possible combinations might be searched out.
More studies are required to elucidate the responsible mechanisms behind the improved growth and productivity of crop plants caused by the application of TiO2-NPs, Ag-NPs, SiO2-NPs, or CNTs.
In the future, there is vast scope for greener nanonutrition of crop plants keeping in view the nanotoxicological effects of NMs/NPs reported. So, for environmental sustainability, green NMs/NPs could also be utilized as a source of nutrients for the crops and could play a key role in greener nanonutrition.
About the authors
Allah Ditta is currently an Assistant Professor at the Department of Environmental Sciences, PMAs, Arid Agriculture University, Rawalpindi, Pakistan. He received his BSc (Hons) Agriculture in 2007, MSc (Hons) Agriculture Soil Science in 2009, and PhD Soil Science (Environmental Microbiology) in 2014 from University of Agriculture, Faisalabad, Pakistan. During his PhD, he went for IRSIP fellowship at University of Western Australia, Australia. His research focuses on nanonutrition for sustainable crop production and carbon sequestration through algal biochar.
Muhammad Arshad (TI and DNP) is currently a Tenured Professor in the Institute of Soil and Environmental Sciences and Dean, Faculty of Agriculture at University of Agriculture, Faisalabad. He received his PhD in Soil Microbiology from University of California, Riverside, USA. He has received a number of national and international awards. His work focuses on the development of organic fertilizers, pesticide biodegradation, novel biofertilizers, and industrial wastewater treatment technology and renewable energy system (biofuels) and published over 180 peer-reviewed papers.
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