Nanotechnology in Plant Growth Promotion and Protection. Группа авторов
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et al. 2005; Clément et al. 2013; Jaberzadeh et al. 2013). Promotion of root growth may therefore happen not only because of the improved conditions by TiO2NPs but also as an avoidance mechanism arising from the stress caused by nanoparticles (Barrena et al. 2009; Feizi et al. 2013b).
Treatment with TiO2NPs was shown to protect chloroplasts from aging with prolonged stability of chloroplast under illumination. Suggested mechanisms involve a significant increase in catalase, peroxidase, and superoxide dismutase, a decrease in the concentration of reactive oxygen species and the content of malondialdehyde. Moreover, at the molecular level, the reduction rate of ferricyanide, the rate of oxygen evolution in chloroplasts, and the rate of noncyclic photophosphorylation activity of chloroplasts were found to be higher. In addition, treatment with TiO2NPs also showed improvements in chloroplast coupling, and activities of Mg2+‐ATPase and chloroplast coupling factor I (CF1)‐ATPase on the thylakoid membranes (Hong et al. 2005). Similarly, the overall increase in chlorophyll concentration was also observed (Servin et al. 2013; Raliya et al. 2015a).
However, the application of TiO2NPs at high concentrations was found to be toxic. The decreased growth was accompanied by a lowered mitotic index, increase in reactive oxygen species, antioxidant activity, and genotoxicity (Rafique et al. 2015). In microalgae, the genotoxicity is connected with the absence of an intact nucleus (Dalai et al. 2013). Ghosh et al. (2010) simply reported DNA damage in N. tabacum.
TiO2NPs also affect the uptake and homeostasis of essential elements. It was proposed that the accumulation of essential elements like Cu and Fe was significantly higher under influence of pristine TiO2NPs (Tan et al. 2017). Kužel et al. (2003) reported a similar effect on homeostasis in plants grown with dissolved Ti4+ citrate. The proposed mechanism involved suggested that Ti causes apparent Fe deficiency resulting in upregulation of transport of divalent ions and accumulation of Fe, Zn, and possibly also Cu. A higher conversion of inorganic nitrogen to its organic form was observed in spinach (Yang et al. 2006) and a higher accumulation of K and P was also observed in cucumber treated with TiO2NPs (Servin et al. 2013).
2.4 Effect of Different Concentrations of TiO2NPs on Plants
Both dissolved elements and some nanoparticles, including TiO2NPs, have concentration‐dependent behavior. The concentration range for positive or negative effects may be largely affected by the size and surface of particles and the means of application. Different plant species are also more or less tolerant of different concentrations of TiO2NPs. There were few general trends already well established for dissolved elements, a similar pattern was observed in the case of TiO2NPs. The application of low concentrations does not show any observable positive effects. At a certain higher concentration range positive effects show up. However, a further increase in concentration induces toxicity. The toxicity is often dependent on concentration and higher concentrations lead to higher toxicity (Kořenková et al. 2017). There are also some nano‐specific behaviors. The higher concentrations of TiO2NPs may induce enhanced aggregation of particles and the increased size of aggregates may lead to lower toxicity in hydroponic experiments (Clément et al. 2013; Kořenková et al. 2017). In a hydroponic experiment conducted by Kořenková et al. (2017), the concentration of TiO2NPs used between 150 and 600 mg/L led to a significant reduction in root length with concentration. However, at 1000 mg/L, the length increased as compared to plants grown at 400 and 600 mg/L.
Experimental design, such as the choice between hydroponic growth and growth in soil or solid substrate, the pathway of application, and its timing change the concentration range of TiO2NPs at which they have positive or negative effects on plants. Application of nanoparticles on leaves and hydroponic growth tend to induce response at lower concentrations than application to soil or other solid growth media (Kořenková et al. 2017). The type of soil also affects bioavailability. The higher concentration of clay and organic matter decreases the mobility and bioavailability of nanoparticles and thus may also affect the influence that nanoparticles have on plants (Larue et al. 2018).
TiO2NPs are considered to be used as a coating additive to increase the germination of plant seeds. It was observed that certain concentrations have a positive effect on germination (Table 2.1). The selection of the appropriate concentration of TiO2NPs to enhance the early stages of plant growth was found to be species‐specific. Zheng et al. (2005) studied the effect of TiO2NPs at a various concentration between 250 and 8000 mg/L on spinach seeds (Spinacia oleracea). The treatments of seeds with as high as 4000 mg/L concentration had a positive effect on germination, germination index, seedling dry weight, and vigor index. However, not all studies showed a similar pattern of positive effects. A study in Vicia narbonensis and Zea mays by Ruffini Castiglione et al. (2011) showed growth inhibition at similar concentrations (200–4000 mg/L). Similarly, in another study, the germination of Cucumis sativus was found to inhibit at concentrations as low as 100 mg/L (Mushtaq 2011). However, there are several studies that report positive effects of TiO2NPs at various concentrations between 50 and 400 mg/L (Gao et al. 2008; Clément et al. 2013; Haghighi and Teixeira da Silva 2014; Ruffini Castiglione et al. 2016) or 2–60 mg/L (Feizi et al. 2012; Feizi et al. 2013a,b). Andersen et al. (2016) tested the efficacy of TiO2NPs in 10 plant species and found that concentrations between 250 and 1000 mg/L had a positive effect on germination and early plant development for some plant species, while in other plant species negative or no effect at all was observed. However, it was observed that the negative effect is most common in plants treated with concentrations higher than 1000 mg/L (Zheng et al. 2005; Frazier et al. 2014). However, enzymatic activity and water uptake were negatively affected in onion seeds (Allium) at concentrations as low as 40 and 50 mg/L (Laware and Raskar 2014).
Soil structure and chemical composition have a strong influence on the behavior of chemical compounds, including nanoparticles in soils (Šebesta et al. 2017; Šebesta et al. 2020; Urík et al. 2020). Hydroponic experiments are used to evaluate the behavior of chemical compounds without the added influence of soil. Hydroponic toxicity tests were also used in the evaluation of the influence of TiO2NPs on plants (Table 2.2). In hydroponics, no negative effect was observed for TiO2NPs at concentrations between 10 and 100 mg /L in lettuce (Lactuca sativa), however, at concentrations with 1000 mg/L they were found to affect the plant weight significantly but no organ interaction was detected (Larue et al. 2016). The significant toxic effect in barley (Hordeum vulgare) was observed only at concentrations higher than 150 mg/L and expressed by a reduction in root length (Kořenková et al. 2017). Seeger et al. (2009) demonstrated that concentrations of 1–100 mg/L did not induce any significant response in willow tree saplings (Salix schwerinii x viminalis). The experiment performed in petri dishes showed that suspension of TiO2NPs with concentrations as low as 0.01 mg/L exert phytotoxic effects on flax (Linum usitatissimum) (Clément et al. 2013). Wheat (Triticum aestivum) was exposed to differently sized TiO2NPs of two crystalline structures (anatase and rutile) at concentrations of 10, 50, and 100 mg/L. Only nanoparticles having a smaller size (14 nm anatase and 22 and 36 nm rutile) had a significant positive effect at concentrations higher than 50 mg/L (Larue et al. 2012a). However, in rapeseed (Brassica napus), TiO2NPs did not induce any significant physiological response at the same concentrations as shown in wheat (Larue et al. 2012b). TiO2NPs at concentrations 100, 250, 500, or 750 mg/L negatively affected the formation of nodules with symbiotic bacteria and nitrogen fixation in legumes (Fan et al. 2014). Inhibition in nutrient transport was also observed at a concentration higher than 100 mg/L (Asli and Neumann 2009; Fan et al. 2014). Damage from reactive oxygen species and genotoxicity were reported even at concentrations as low as 10 mg/L (Demir et al. 2014; Pakrashi et al. 2014; Okupnik and Pflugmacher 2016). During longer cultivations (20 days) wheat's photosynthesis and other associated parameters were negatively affected by concentrations as low as 5 mg/L (Dias et al. 2019). Similarly, a lower concentration (12.5 mg/L) of TiO2NPs showed a negative effect on root growth of red clover (Trifolium