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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 pratense) in a 28‐day exposure experiment (Moll et al. 2016).

Table 2.1 Influence of TiO₂ nanoparticles on plants, seed treatment.

Size (diameter in nm)	Plant species, length of exposure	Effect of concentration	Impact	References
No effect	Positive	Negative
n.a.	Spinacia oleraces, 48 h	n.a.	250–4000 mg/L	4000−8000 mg/L	Increased germination, germination index, seedling dry weight, vigor indexDecreased germination, germination index, seedling dry weight, vigor index	Zheng et al. (2005)
5	Spinacia oleraces, 48 h	n.a.	300 mg/L	n.a.	Increase in plant fresh and dry weightIncrease in amount of Rubisco activase	Gao et al. (2008)
n.a.	Oryza sativa, 24–72 h	100, 500, 1000 mg/L	n.a.	n.a.	Slight decrease in root length at 2‐ and 3‐day exposure	Boonyanitipong et al. (2011)
<50	Cucumis sativus, 6 days	n.a.	n.a.	100–5000 mg/L	Decrease in germination, germination index	Mushtaq (2011)
<100	Vicia narbonensis, Zea mays 24 h	n.a.	n.a.	200–4000 mg/L	Decrease in root elongationDecrease in mitotic indexIncrease in aberration index	Ruffini Castiglione et al. (2011)
21	a Triticum aestivum, 8 days	1, 100, 500 mg/L	2, 10 mg/L	n.a.	Mean germination time loweredIncrease in shoot length	Feizi et al. (2012)
21	a Foeniculum vulgare, 14 days	80 mg/L	5, 20, 40, 60 mg/L	n.a.	Mean germination time loweredGermination percentage, germination value, vigor index and mean daily germination improved	Feizi et al. (2013b)
21	a Salvia officinalis, 21 days	5, 20, 40, 80 mg/L	60 mg/L	n.a.	Mean germination time loweredGermination percentage improved	Feizi et al. (2013a)
15, 25, 32	Linum usitatissimum, 24 h seed germination, 48 h root biomass, 72 h root biomass, and length	n.a.	100 mg/L (25 and 32 nm only)	0.01−100 mg/L	Inhibition of germinationDecrease in root length and biomassIncrease in root growth and germination (25 and 32 nm only)	Clément et al. (2013)
27	Solanum lycopersicum, 48 h	50–5000 mg/L	n.a.	n.a.	No effect on germination	Song et al. (2013)
35	Pisum sativum, 24 h	100, 250, 500, 750, 1000 mg/L	n.a.	n.a.	No effect on the germination	Fan et al. (2014)
25	a Nicotiana tabacum, 21 days	n.a.	n.a.	1000, 10 000 mg/L	Decrease in root lengthChanges in microRNA expression	Frazier et al. (2014)
25	a Hordeum vulgare 7 days	500, 1000 mg/L	n.a.	2000 mg/L	Elevated reactive oxygen species within plant	Mattiello et al. (2015)
23	a Allium cepa, Avena sativa, Brassica oleracea capitate, Cucumis sativus, Daucus carota, Glycine max, Lactuca sativa, Lolium perenne, Solanum lycopersicum, Zea mays, 24 h + variable time period	250, 500, 1000 mg/L	250 mg/L (B. oleracea) 500 mg/L (B. oleracea, A. cepa) 1000 mg/L (A, sativa, C. sativus, A. cepa)	250 mg/L (C. sativus, G. max, Z. mays) 500 mg/L (A. sativa, Z. mays) 1000 mg/L (C. sativus, Z. mays)	Decreased or increased germination (4 species)Decreased or increased cotyledon presence (4 species)Decreased or increased average root length (6 species)	Andersen et al. (2016)
21, 10 nm and 10 000 nm nanowire	Sinapis alba, 72 h	10, 100, 1000 mg/L	n.a.	n.a.	No effect on germination	Landa et al. (2016)
<10, <100	Vicia faba, 72 h	n.a.	50 mg/L (<10 nm)	50 mg/L (<100 nm)	Stimulation of germination process (<10 nm)Oxidative stress, genotoxicity (<100 nm)	Ruffini Castiglione et al. (2016)

n.a: Not available

^a Both seed treatment and prolonged exposure.

Table 2.2 Influence of TiO₂ nanoparticles on plants, hydroponic exposure.

Size (diameter in nm)	Plant species, length of exposure	Effect of concentration	Impact	References
No effect	Positive	Negative
30	Zea mays, 72 h	n.a.	n.a.	300–1000 mg/L	Inhibition of leaf growth and transpiration via physical effects on root water transport	Asli and Neumann (2009)
25	Salix schwerinii x viminalis, 72 h	1–100 mg/L	n.a.	n.a.	No observable effect	Seeger et al. (2009)
50	Vicia faba, 48 h	5, 25, 50 mg/L	n.a.	n.a.	Oxidative stress response in root at all concentrations	Foltête et al. (2011)
14, 22, 25, 36	Triticum estivum, 1 week	10 mg/L (14, 22 nm), 10–50 mg/L (36 nm) 10–100 mg/L (25 nm)	50, 100 mg/L (14, 22 nm), 100 mg/L (36 nm)	n.a.	Increased root elongationno effect on germination, evapotranspiration, and plant biomass	Larue et al. (2012a)
14, 25	Brassica napus, Triticum estivum 1 week	n.a.	100 mg/L	n.a.	Increased root elongationno effect on germination, evapotranspiration, and plant biomass	Larue et al. (2012b)
27	Cucumis sativus, 15 days	n.a.	100, 250, 500, 1000, 4000 mg/L	n.a.	promotion of root elongationhigher nitrogen accumulation in roots	Servin et al. (2012)
27	Solanum lycopersicum, 15 days	50, 100, 1000, 2500, 5000 mg/L	n.a.	n.a.	No effect on root elongation	Song et al. (2013)
21, 50	Allium cepa 18 h	10, 100 mg/L (21 nm)	n.a.	10, 100, 1000 mg/L (50 nm), 1000 mg/L (21 nm)	Increase in genotoxicity with concentration	Demir et al. (2014)
35	Pisum sativum, 24 hours	n.a.	n.a.	100, 250, 500, 750 mg/L	No effect on root length, stem length, and leave surface areaDiminished secondary lateral roots, diminished nutrient transportdisrupted Rhizobium–legume symbiosis system and nitrogen fixation	Fan et al. (2014)
90–98	Allium cepa, 4 h	n.a.	n.a.	12.5, 25, 50, 100 mg/L	Increased damage from reactive oxygen speciesIncrease in genotoxicity with concentration	Pakrashi et al. (2014)
4	Lactuca sativa, 7 days	10–1000 mg/L	n.a.	n.a.	Higher accumulation of Fe, P, S, and Ca in the root epidermisDecreased concentrations of Fe, P, S, and Ca in both the parenchyma and vascular cylinder	Larue et al. (2016)
29, 92	Trifolium pratense, 7‐day old plants for 28 days	n.a.	n.a.	12.5, 25 mg/L	Decrease in nodule formationDecrease in growth	Moll et al. (2016)
10–30	Hydrilla verticillata, 24 h	0.01, 0.1, 1 mg/L	n.a.	10 mg/L	Increase in catalase and glutathione reductase activityIncrease in H2O2 at 10 mg/L	Okupnik and Pflugmacher (2016)
25, 33, 41, 40	Oryza sativa, 35‐day old plants for 7 days	n.a.	10, 1000 mg/L	n.a.	Reduced concentration of Pb in plantNo negative effect on plants	Cai et al. (2017)
8	Hordeum vulgare, 7 days	100 mg/L	n.a.	150–1000 mg/L	Decrease in root length	Kořenková et al. (2017)
21	Triticum aestivum, 20 days	n.a.	n.a.	5, 50, 150 mg/L	Impaired light‐dependent and ‐independent phases of photosynthesisDecreased chlorophyll a content, maximal and effective efficiency of PSII, net photosynthetic rateDecreased transpiration rate, stomatal conductance, intercellular CO2 concentration, and starch content	Dias et al. (2019)

The experiments performed with plants grown in soil contaminated with nanoparticles, either in pots in laboratory, greenhouse, or in field conditions represent the realistic scenarios of exposure to nanoparticles in the environment to a greater degree. From these experiments, the right concentration of TiO₂NPs can be more objectively selected and later used to enhance plant growth in agriculture. Most of these experiments need to be performed for a longer duration than that of hydroponic growth and hence, it can help to get a better understanding of the effects of long‐term exposure of nanomaterials in plants (Table 2.3). Usually, two modes of nanoparticle application were preferred, (1) contamination of soil where plants were growing and (2) foliar application at the important stages of development of plants (Servin et al. 2013; Raliya et al. 2015b; Rezaei et al. 2015; Marchiol et al. 2016; Pošćić et al. 2016; Moll et al. 2017; Rafique et al. 2018; Giorgetti et al. 2019; Zahra et al. 2019; Bellani et al. 2020). In case of soil contamination, it was observed that the higher concentrations of TiO₂NPs in soil may have negative effects on the growth of plants (Du et al. 2011; Song et al. 2013; Marchiol et al. 2016; Pošćić et al. 2016; Tan et al. 2017; Rafique et al. 2018; Giorgetti et al. 2019; Bellani et al. 2020). However, the most concerning thing is that even concentrations as low as 100 mg/kg may have a negative long‐term effect on the growth and nutritional quality of crop plants (Du et al. 2011; Rafique et al. 2018; Bellani et al. 2020). The positive effects of TiO₂NPs applied to soil were recorded at concentrations between 25 and 500 mg/kg (Servin et al. 2013; Rafique et al. 2018; Zahra et al. 2019). The concentration range with positive effects for a plant species is narrower and the differences between studies may largely depend on the composition of the soil. Higher amounts of fine particles in the soil led to a higher concentration of TiO₂NPs needed to enhance plant growth (Zahra et al. 2019). In addition, it was reported that the concentrations of TiO₂NPs that enhance growth in plants may vary between plant species (Andersen et al. 2016). Moreover, foliar application of TiO₂NPs may become the preferred method of application on plants. A single application at the right growth stage may have a positive effect on the plant growth (Rezaei et al. 2015) and even concentrations as low as 10 mg/L in the form of a spray can improve the plant growth (Raliya et al. 2015b). Foliar application has the benefit of using lower amounts of nanoparticles that lead to lower contamination of soil and thus are more sustainable.