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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.

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.	Increase in shoot and root length, and dry biomassIncrease in phosphorus contentChanges in soil organic matter	Zahra et al. (2019)
30	Pisum sativum, Grown for 28 days in contaminated soil	n.a.	n.a.	80, 800 mg/kg	Reduction in the availability of important soil mineral nutrients (Mn, Fe, P)Reduction in root lengthIncrease in shoot length	Bellani et al. (2020)

Application of TiO₂NPs on seeds may help with germination through better water uptake and oxygen absorption (Zheng et al. 2005; Feizi et al. 2013a,b) and plant protection due to antimicrobial and antifungal effects of TiO₂NPs (Navarro et al. 2008). This may be a good approach for plants having seed germination problems such as medical herbs (Feizi et al. 2013b).

Throughout the various growth stages of plants, both foliar and soil applications of TiO₂NPs were shown to improve the plant growth, through improvements in root and shoot elongation, root and shoot biomass, increased number of root nodules, increased yield of seeds, fruits, or other plant parts involved in food production. Besides, other improvements in plant health, like an increase in chlorophyll production, production of soluble leaf protein, and several amino acids were also recorded. At right concentrations, an increase in essential macronutrients (N, P, K, Na, and Ca), and micronutrients (Fe, Mn, and Zn) was observed (Kužel et al. 2003; Yang et al. 2006; Raliya et al. 2015a,b; Tan et al. 2017). This may be beneficial for agriculture and the environment, since it decreases the requirement of fertilizers. Plants were also shown to have increased resistance to draught after the application of TiO₂NPs. A foliar spray may also help in the reduction in uptake of toxic elements such as Cd (Lian et al. 2020). Another avenue of TiO₂NPs application is its potential pesticidal properties. TiO₂NPs were used to protect plants against cotton leafworms (Shaker et al. 2017), Cercospora leaf spot, brown blotch, Curvularia leaf spot, and bacterial leaf blight (Owolade and Ogunleti 2008).

Although TiO₂NPs can be used alone to inhibit crop pests, its composite formulae can be promisingly utilized to protect plants against various crop diseases. These nanopesticides utilize nanoparticles in several ways. The nanoparticles are used as carriers that help with distribution, dispersity, and wettability of plant surface and they increase pesticide's adhesion (Rajwade et al. 2020). The improved effect of pesticides by nanoformulations with TiO₂ is not the only benefit these composites provide. Degradability in the soil of these nano‐pesticides is also an important aspect and photocatalytic properties of TiO₂NPs were utilized to increase the decomposition of these pesticides in soil (Yan et al. 2005; Guan et al. 2010; Mohamed and Khairou 2011).