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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.

2.5 Benefits of Using TiO₂NPs Alone and in Complex Formulations on Plant Growth and Yield

At every stage of plant development, TiO₂NPs may have beneficial effects on the health of plants. On the other hand, there are some concerns, since their application led to the limited transport to fruits or other edible parts of the plant. However, literature has repeatedly shown that the overall uptake of TiO₂NPs is not increased compared to control and there are reports showing no major implication for food safety after whole plant life cycles (Bakshi et al. 2019). TiO₂NPs are, therefore, viable to use either alone or in composite form to increase the health, nutritional quality, and yield of plants or to protect them from diseases and adverse environmental conditions. Two main applications are considered:(1) seed coating to promote germination and (2) foliar or soil application to promote plant growth.

Table 2.3 A influence of TiO₂ nanoparticles on plants grown in soil.

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Size (diameter in nm)	Plant species, length of exposure	Effect of concentration	Impact	References
No effect	Positive	Negative
<100	Triticum aestivum, 7 months in contaminated soil	n.a.	n.a.	91 mg/kg	Reduced biomassInhibition of soil protease, catalase, and peroxidase activities	Du et al. (2011)
27	Cucumis sativus, 150 days in contaminated soil	250, 750 mg/kg	500 mg/kg	n.a.	Enhanced metabolic activity in plant leavesIncreased K and P allocation in fruitNanoparticles transported to fruit	Servin et al. (2013)
27	Solanum lycopersicum, after 35 days of growth foliar application and 7 days of growth	1000 mg/L	n.a.	5000 mg/L	No effect on chlorophyll contentChange in superoxide dismutase activity	Song et al. (2013)
12–15	Vigna radiata, one foliar application of TiO2 at 8.3 mL per plant on the tenth day of germination, 28 days of subsequent growth	n.a.	10 mg/L	n.a.	Increase in shoot and root length, root area and number of root nodulesIncrease in chlorophyll and soluble leaf proteinIncreased population of rhizospheric microbesIncreased activity of dehydrogenase, phytase, acid phosphatase, and alkaline phosphatase in roots	Raliya et al. (2015a)
n.a.	Glycine max, Grown to maturity	n.a.	100, 300, 500 mg/L	n.a.	Increased plant height and biomass	Rezaei et al. (2015)
25	Hordeum vulgare, grown to physiological maturity in contaminated soil	n.a.	n.a.	500 or 1000 mg/kg	Growth cycle 10 days longer	Marchiol et al. (2016)
25	Hordeum vulgare, grown to physiological maturity in contaminated soil	1000 mg/kg	n.a.	500 mg/kg	Lowered kernel quantity and grain yieldIncrease in crude protein and most amino acidsPotential beneficial effects on the nutritional quality of barley grains	Pošćić et al. (2016)
29, 92	Triticum aestivum, Grown for 12 weeks in contaminated soil	1, 100, 1000 mg/kg	n.a.	n.a.	No negative effects on wheat growthNo negative effect on arbuscular mycorrhizal root colonization	Moll et al. (2017)
50	Ocimum basilicum, Grown for 65 days in contaminated soil	n.a.	n.a.	125, 250, 500, 750 mg/kg	Changes in absorption of essential elements, starch, and sugarsDecreases in root length, biomass, and chlorophyllIncrease in oxidative stress	Tan et al. (2017)
<20	Triticum aestivum, Grown for 50 days in contaminated soil	n.a.	20, 40, 60, 80 mg/kg	100 mg/kg	Increase in plant growthIncrease in chlorophyll and phosphorusIncrease in oxidative stress	Rafique et al. (2018)
30	Pisum sativum, Grown for 28 days in contaminated soil	n.a.	n.a.	80, 800 mg/kg	Reduction in root lengthIncreased oxidative stress and cellular damage	Giorgetti et al. (2019)
<40	Triticum aestivum, Complete growth cycle in contaminated soil	750 and 1000 mg/kg	150, 250, 500 mg/kg (loam soil) 25, 50, 150, 250 mg/kg (sandy loam soil)	n.a.