barrier to the absorption of Zn from ZnO than from ZnSO4; therefore, ZnO show less toxicity. Raskar and Laware (2014) also reported for onion seeds Zn NPs treatment (less than 40 mg/L) substantially increased antioxidant enzymes, including guaiacol peroxidase (GPx), catalase (CAT), SOD, and glutathione reductase (GR). Besides, Sedghi et al. (2013) stated that Zn NPs could play a vital role in the biosynthesis of auxin and gibberellin, resulting in a higher germination rate. However, there is optimal concentration for Zn NPs application, concentration too high can reduce the activity of antioxidant or reducing enzymes. Israel García‐López et al. (2018) found that ZnO NPs (18 nm) applied during imbibition and incubated for 72 hours at 100–400 mg/L induced a significant increase in production of peroxidase and ascorbate peroxidase; however, at 500 mg/L these enzymes were reduced in Capsicum. Therefore, it is crucial to determine the optimal Zn NPs concentration and treatment duration for seed treatment.
3.3 ZnO NPs in Enhanced Plant Growth
ZnO NPs are one of the most effective sources to prevent Zn deficiencies and to improve crop production and quality (Naderi and Danesh‐Shahraki 2013; Dimkpa et al. 2015). Understanding the effects of Zn NPs on the plant life process is critical to find a sustainable application method for crop growth enhancement. The positive effects have been attributed to the supplement of Zn as an essential component for mitotic cells and several metabolic enzymes such as alcohol dehydrogenase, carbonic anhydrase, and Cu‐Zn‐SOD (Römheld and Marschner 1991; Hafeez et al. 2013). Also, Zn is actively involved in the biosynthesis of proteins, carbohydrates, lipids, and nucleic acids in plants (Tarafdar et al. 2014). For example, Raliya et al. (2015) reported that Zn can act as a cofactor for P‐solubilizing enzymes such as phosphatase and phytase, and nano‐ZnO increased their activity in the soil. Bioengineered ZnO NPs made from ZnO and algal extract can interact with meristematic cells thereby altering biochemical pathways that are contributing to the better accumulation of biomass (Venkatachalam et al. 2017).
3.3.1 Application Methods
Zn is taken up by plant roots, as Zn cations or Zn‐organic molecule complexes, then translocated by the protein transporters through the xylem to different parts of the plant (Figure 3.2). Protein transporters for heavy metals are mainly responsible for Zn transport from rhizodermal and cortex cells into the xylem (Hussain et al. 2004). Without a fully developed Casparian strip Zn can also transport through the extracellular apoplastic pathway (White et al. 2002). But protein transporters involved in Zn translocation through the shoot to the assimilation are still not fully identified (Deinlein et al. 2012).
Plants' leaves can absorb Zn through foliar application. There are several critical factors controlling the adsorption capacity of leaves, including the thickness of the waxy protective layer, the chemical composition and structure of the cuticle, the density of stoma and trichomes, and the physiological state of the plant (Schönherr 2006; Eichert and Goldbach 2008). Environmental factors such as humidity, temperature, and lightning also play important role in Zn adoption by plant leaves (Fernández and Eichert 2009).
Due to the sorption capacity of both plant roots and leaves, Zn NPs can be supplied in different ways (Figure 3.2). Foliar spray application means spraying of NPs suspension over plant leaves, NPs suspension can be sprayed over crop at different intervals depending on crop spices. When Zn NPs are sprayed over leaves, Zn is absorbed by leaves and then distributed to lower parts through the phloem. The substrate mediated application includes the application of NPs to the root zone in soil culture and soilless culture. In soil culture, nano‐fertilizers are mixed with soil after broadcasting. In soilless culture or hydroponic system, Zn NPs are often supplied through mixing with nutrient solution. Zn applied in the root zoon is first adsorbed by roots then translocated to upper parts through xylem and phloem.
3.3.2 Effects of Zn NPs on Plant Growth Promotion
3.3.2.1 Effects of Zn NPs Via Foliar Application
The aerial application of Zn NPs shows crop specie dependent effects at different concentration levels. Based on crop species the effective dose of Zn NPs varies (10–1000 mg/L) for foliar spray (Table 3.2). A study by Prasad et al. (2012) revealed that ZnO NPs at 1000 mg/L is the most effective dose for foliar spray in peanut, which provided the maximum positive effects on chlorophyll content, flowering, dry mass accumulation, plant height, and yield. ZnO NPs can be applied at 500 or 1000 mg/L as a precursor for tryptophan and biofortification agents to increase leaf yield and nutritional quality of spinach (Kisan et al. 2015). In peanut, maximum positive effects on flower initiation and chlorophyll content were observed at 1000 mg/L ZnO (25 nm) through the foliar spray at 35 and 70 days. In a recent study, ZnO NPs (12–24 nm) at 1000 mg/L yielded positive effects on Capsicum biomass, yield, and chlorophyll content, however, 2000 mg/L showed negative effects on plant biomass accumulation (García‐Gómez and Fernández 2019).
Figure 3.2 Translocation and major physiological and biochemical responses of Zn NPs in plants.
Additionally, Zn NPs can also be responsive at a very low concentration level. Typically, if the application concentration is reduced, then application time should be increased to obtain significant results (Table 3.2). Tarafdar et al. (2014) observed that 10 mg/L Zn NPs applied to pearl millet can substantially improve shoot length, root length, root area, chlorophyll content, total soluble leaf protein, plant dry biomass, and yield by 15.1, 4.2, 24.2, 24.4, 38.7, 12.5, and 37.7% respectively. Similarly, Rossi et al. (2019) got positive effects on coffee plants by application of ZnO NPs (15–20 nm) at 10 mg/L in 14 days intervals. But for maize, the optimal ZnO NPs (25 nm) foliar spray concentration was found to be 100 mg/L which provided maximum plant growth, production, and higher grain quality (Subbaiah et al. 2016). A study revealed that foliar application of ZnO NPs (18 nm) in onion, three times in each 15 day, promoted early crop growth from 10 to 30 μg/mL and enhanced early flowering with higher thousand seed weight up to 40 μg/mL (Laware and Raskar 2014). A study conducted in chickpea showed similar results, where the foliar spray (irrigated daily for 15 days) of ZnO NPs (20–30 nm) promoted dry matter production at 1.5 mg/L, but inhibited dry matter production at 10 mg/L (Burman et al. 2013).
Table 3.2 Effects of Zn NPs on plant growth and promotion applied through different application modes.
| Size and coatings (nm) | Crop | Concentrations | Mode of application | Effects | References | ||
|---|---|---|---|---|---|---|---|
| Root growth | Shoot growth | Yield | |||||
| 35.5 | Maize | 50–2000 mg/L | Foliar spray, two times (tasseling and milking stage) | + | + | + | Subbaiah et al. (2016) |
| 20 | Sunflower | 2000 mg/L |
Foliar spray, two times (6 leaf stage and 1 week later), under salt
|