Nanotechnology in Plant Growth Promotion and Protection. Группа авторов

of ZnO NPs at 2000 mg/L caused a significant reduction in the growth rates of mung bean and chickpea seedlings (Mahajan et al. 2011). Studies indicated that coatings on Zn NPs can enhance the efficiency of Zn supplement. Research conducted in agar gel media by Moghaddasi et al. (2017) reported a better Zn supply through humic acid‐coating nanoparticle (ZnO) at 1 mg/L with a substantial increase in root and shoot biomass in cucumber.

      Several recent studies reported the beneficial effect of Zn NP when added to the hydroponic system. Venkatachalam et al. (2017) reported the application of Zn NPs (25–200 mg/L) in combination with 100 mM phosphate in a hydroponic system increased cotton (Gossypium hirsutum L.) biomass by 131% over control. Tirani et al. (2019) reported a similar effect of ZnO NPs (25 nm) in the growth promotion of Nicotiana tabacum when supplied to the hydroponic system together with nutrient solution. The results obtained revealed that these NPs positively affected the growth of N. tabacum, that is, root length, shoot length, and plant biomass. However, the phytotoxicity of Zn NPs in the hydroponic system has also been reported. Ghodake et al. (2011) reported that ZnO NPs (5, 10, and 20 μg/mL) can inhibit onion bulbs (Allium cepa) root elongation in the hydroponic system because of severe accumulation of ZnO NPs in both the cellular and the chromosomal modules. Further investigations are needed to understand the interactions between Zn NPs with plants in a hydroponic system to optimize the beneficial effect of Zn NPs on plants.

      3.3.2.3 Effects Zn NPs Through Soil Application

The crop plants are less sensitive to the toxic effects of Zn NPs at higher concentrations if supplied through soil (Table 3.2). In the meantime, soil application at a lower concentration below 100 mg/kg was not found beneficial or benign for most of the crops. Therefore, higher Zn NPs concentration (>100 mg/kg) is often used for soil application for different crop species. In a greenhouse experiment, the application of ZnO NPs to cucumber (Cucumis sativus) at 400 and 800 mg/kg caused a 10 and 60% increase in plant root dry mass, respectively, and a significant increase in the growth, yield, and fruit quality (carbohydrate, protein, and mineral content) (Zhao et al. 2014).

      For legumes, Zn NPs used through soil application can show significant enhancement in root growth without negative effects on shoot growth. Medina‐Velo et al. (2017) observed positive effects on kidney beans root growth with soil application of 60–500 mg/kg ZnO NPs. Mukherjee et al. (2014) reported a similar effect on root growth promotion in green peas with Zn NPs application at 125–2000 mg/kg. Priester et al. (2012) reported a substantial enhancement in both root and shoot growth of soybean with 50–500 mg/kg ZnO NPs treatments. Table 3.2 shows the effects of Zn NPs on plant growth and promotion applied through different application modes.

      3.3.2.4 Effects of Zn NPs on Plant Physiological and Biochemical Changes

      Studies performed in the past have observed beneficial physiological and biochemical changes for crop growth after Zn NPs application. Tarafdar et al. (2014) observed significant increase in activities of enzymes such as acid phosphatase (76.9%), alkaline phosphatase (61.7%), phytase (322.2%), and dehydrogenase (21%) in the rhizosphere of pearl millet due to foliar spray of Zn NPS (10 mg/L) after six weeks. García‐López et al. (2019) reported high phenolics and antioxidant content in habanero peppers with Zn NPs application at 2000 mg/L. According to Helaly et al. (2014), adding ZnO NPs supplements to MS media to culture banana plant promoted somatic embryogenesis, shooting, regeneration of plantlets, and also increased proline synthesis, the activity of superoxide dismutase, catalase, and peroxidase, thereby improving tolerance to biotic stress. ZnO NPs at 0.2–25 μM concentration positively affected the growth of tobacco plant (root and shoot length/dry weight), leaf surface area and its metabolites (auxin, phenolic compounds, flavonoids), leaf enzymatic activities including catalase, ascorbate peroxidase, superoxide dismutase, peroxidase, guaiacol peroxidase, polyphenol oxidase, and phenylalanine ammonia lyase (Tirani et al. 2019). Venkatachalam et al. (2017) also observed an increase in cotton growth (dry weight), pigment concentration (chlorophyll and carotenoids), and enzyme activities (superoxide dismutase and peroxidase) by more than 100% due to the application of phytomolecule coated ZnO NPs (5 nm).

3.4 Zn NPs in Crop Protection

      3.4.1 Improvement on Disease Resistance

ZnO NPs are well‐known for their antimicrobial activity. Zn based nano‐fungicides are considered ecofriendly in crop disease management. However, studies of its application to control plant diseases are limited. Application of Zn NPs at right concentration range promotes plant growth and increases stress and disease tolerance. ZnO NPs (<50 nm, 100 mg/L) found to reduce spore germination and mycelial growth of some pathogenic fungi such as Alternaria alternata, Botrytis cinerea, Monilia fructicola, Verticillium dahliae, Colletotrichum gloeosporioides, Fusarium oxysporum f. sp. radicis‐lycopersici, and Fusarium solani strains (Malandrakis et al. 2019). A recent study also reported that chitosan encapsulated Zn NPs facilitated disease (Curvularia leaf spot) control in maize through fortifying of plant innate immunity by increasing antioxidant and defense enzymes, balancing of reactive oxygen species (ROS), and enhancing lignin accumulation (Choudhary et al. 2019). Ogunyemi et al. (2019) reported a linear decrease in the population of Xanthomonas oryzae pv. oryzae with the increase in the application rate of biologically synthesized ZnO (40.5–124 nm) up to 16 μg/ml and the maximum result of Zn NPs is to reduce the bacterial population by 68%.

      3.4.2 Enhancement of Stress Tolerance

      ZnO NPs affect the availability of secondary metabolites which play vital roles in biotic and abiotic stress tolerance, e.g. phenolic, anthocyanin, and flavonoids compounds. Application of ZnO NPs with MS media increase phenolics and flavonoids concentration in Lilium ledebourii (Chamani et al. 2015). For Lupine, seed priming with ZnO NPs reduced the negative effects of salinity stress (150 mM NaCl), increased plant biomass, phenol content, pigments (chlorophyll‐a, chlorophyll‐b, and carotenoid), and stimulated the production of antioxidant enzymes like superoxide dismutase, peroxidase, and ascorbate peroxidase (Latef et al. 2017). Also, Hussein and Abou‐Baker (2018) recorded that spraying of Zn NPs to cotton at 100–200 mg/L can decrease plants’ Na/Ca and K/Ca ratios, while increase Ca/(K + Na) ratio. Torabian et al. (2018) also reported that foliar spray application of ZnO NPs at 2000 mg/L promotes sunflowers growth under salt stress (100 mM NaCl) without adverse effects. Watson et al. (2015) grew wheat in acidic and alkaline soils with Zn NPs amendment and revealed the positive effects of Zn NPs on wheat growth in alkaline soil. Under drought stress, the application of ZnO could promote crop productivity through higher germination and production of vigorous seedlings. For example, under polyethylene glycol‐induced drought stress, the effects of ZnO treatment increased soybeans seeds germination percentage and rate over control (Sedghi et al. 2013). Dimkpa et al. (2020) reported under drought, facile coating of urea with low‐dose ZnO NPs reduced panicle initiation time by five days and increased grain yield. Besides, studies have also shown Zn NPs applied to soil or hydroponic system can reduce negative effects of heavy metals such as lead, cadmium, or arsenic (Wang et al. 2018; Sharifan et al. 2019; Faizan et al. 2020).

3.5 Conclusions

      This chapter summarizes the plant application methods of Zn NPs and their positive effects on plant growth. The seed application of Zn NPs can improve seed germination percent, radicle growth, and plumule growth. If applied to a standing crop, the Zn NPs can enhance leaf area, root growth, shoot growth, pigment content in leaf, yield, and quality of production. At the molecular level, Zn NPs affect plant metabolic processes like the production of plant hormones, and the activation of antioxidants and reductases. Zn NPs can also enhance plants' tolerance level to abiotic stresses, such as soil salinity, drought, or heavy metals in the environment.

The studies reviewed in this chapter demonstrate the beneficial effects of Zn NPs for plant growth, however, there is a remarkable lack of information on some key aspects, which prevents a better application