ZnO NPs are an amphoteric semiconductive material with a wide band gap (Eg = 3.37 eV) [36]. Because of their unique properties, such as high binding energy, refractive index, thermal conductivity, piezoelectric nature, high absorbance of UV light, and antibacterial properties, these are widely used in various applications [37]. Moreover, as an added advantage, the above-mentioned properties are highly tuneable. Their size can be altered from a few nanometres to the upper limit of nanoparticle size definition (100 nm), and their shape can be easily adjusted by selecting the appropriate method of synthesis [37]. Different synthesis techniques have been used to produce ZnO NPs, including mechanochemical processes, controlled precipitation, sol-gel, solvothermal, hydrothermal methods, methods using emulsions and microemulsions, growing from a gas phase, pyrolysis spray methods, and others [37]. A broad range of shapes, such as flowerlike structures, nanorods, nanotubes and spherical or oblong nanoparticles, can be easily synthesised [37,38]. The surface of ZnO NPs is often modified to enhance their stability in colloidal suspension, to improve their positive effects on plants and to reduce their potential toxicity. The modification of their surface can be obtained by treatment with the inorganic compounds such as SiO₂, Al₂O₃, etc., simple organic compounds, e.g., silanes or organic acids, and by more complex polymeric matrices [37]. Often biosynthesis of ZnO NPs is selected for agricultural applications since it is anticipated to create eco-benign nanomaterial [39]. Bare and surface-modified ZnO NPs were used in the laboratory, greenhouse, and field experiments on crop plants due to their UV protective, and antimicrobial properties besides their nutritional role as slow releasing Zn source for plants [31,38,39,40,41,42]. ZnO NPs easily dissolve compared with some other ENPs [43], such as TiO₂ NPs, which affect plant health partly by their nano-specific properties, but also in larger part by the release of the Zn, which is essential to many processes on the cellular level [44]. In addition, ZnO NPs are reported to have an ability to decrease the effect of environmental stresses on plants, such as drought [45], temperature [46], metals, metalloids [47,48], and salt [49]. When applied at suitable concentrations, ZnO NPs increase plants’ seed germination [50], growth [51], the activity of antioxidants and protein production [52,53], chlorophyll content [54] and photosynthesis [55], production of oils and seeds [31,32], and uptake of essential elements [56].

TiO₂ NPs are insoluble semiconductive material with a high refractive index, UV absorption, photocatalytic, and antimicrobial properties. These have highly tuneable properties partially because these ENPs exhibit diverse crystal symmetries represented by mineral phases such as anatase, brookite, or rutile. Each crystal structure has unique features that can benefit its application; most commonly, the suitable mineral form is selected for its lower or higher photocatalytic ability [57]. The size can be adjusted from a few nanometres up to 100 nm in any dimension, and the shape of TiO₂ NPs can be tuned during their synthesis to obtain both nanorods and spherical nanoparticles [58]. Different types of synthesis protocols have been used for the production of TiO₂ NPs to create nanomaterials of specific properties, e.g., sol, sol-gel, micelle, solvothermal, and hydrothermal methods, vapour deposition, and many others [59]. Because of their properties, TiO₂ NPs have a wide range of applications in diverse fields of human activity, including agriculture. Similar to ZnO NPs, surface properties of TiO₂ NPs are often modified to help with their stability or to increase their positive effects and decrease their toxicity [57,58,59]. Their environmental applications include water purification, degradation of pollutants, antimicrobial coating, biosensing, and drug delivery [60,61,62,63,64]. TiO₂ NPs have been applied to protect seeds, enhance plant growth and germination, control crop diseases [65], degrade pesticides and detect their residues [66]. In addition, these NPs have been reported to increase root and shoot growth, seed or produce yield, and improve plant health. An increase in chlorophyll production, soluble leaf protein [67], and carotenoid content [68], and an increase in uptake of several essential elements [69] was also reported. Environmental stresses, such as drought in wheat [70] and high Cd levels in maize [71], were also alleviated significantly with the use of TiO₂ NPs.