**2.2 Iron nanoparticles (Fe2O3 NPs)**

Iron oxide nanoparticles are a rich source of Fe for plants. When peppermint was exposed to Fe2O3 NPs, proline content, and lipid peroxidation were decreased significantly in saline soil. Antioxidant enzyme activities (guaiacol peroxidase, CAT, and SOD) declined in plants. They also increased the potassium, zinc, calcium, iron, leaf dry and fresh weight, and phosphorus [23]. Grape softwood showed a prominent increase in protein content and reduced production of hydrogen peroxide, proline, and antioxidant enzyme activities when treated with potassium silicate and Fe2O3 NPs [24]. Under salt stress, the application of Fe2O3 NPs on *Helianthus annuus* increased the activities of POD and CAT [25]. Ajowan (*Trachyspermum ammi*) was treated with Fe2O3 NPs under saline conditions increased antioxidant activities, osmolyte synthesis, and maintained Na+/ K+ ratio. These adaptations help plants to improve leaf pigments, seed yield, membrane stability, and shoot and root growth [9].

### **2.3 Silicon nanoparticles (SiO2 NPs)**

SiO2 NPs are used to help plants by forming a layer in cell walls and maintaining yield. In squash and tomato plants, the antioxidant system is enhanced and seed germination increases due to SiO2 NPs under salt stress [26]. In Basil plants, silica nanoparticles have shown promising results related to morphological and physiological traits under salt stress [27]. SiO2 NPs increased the seedling growth of lentils and seed germination and improved the defense mechanism of plants in saline conditions [28]. They help plants to cope up with salt stress by increasing the fresh weight in maize [29]. Under salt stress, the application of SiO2 NPs on soybean decreased toxic ROS production and Na + level in leaves [30]. Wheat cultivars treated with SiO2 NPs improved biological antioxidant levels and seedling growth under salt stress [31]. Application of SiO2 NPs on the strawberry plant in saline conditions increased the photosynthetic pigment and maintained the carotenoid and chlorophyll content, decreasing the effect on epicuticular wax [32].

### **2.4 Cerium nanoparticles (CeO2 NPs)**

They can be used as fertilizer to stimulate the growth of roots, enhance the antioxidant enzyme activities, and to prevent membrane leakage and peroxidation [33]. Moreover, CeO2 NPs help to preserve cell wall and chloroplast structure [34]. Activation of CeO2 NPs as antioxidants depends upon the pH of surroundings, sub-cellular localization, surface charge, concentration, and particle size. CeO2 NPs increased the growth in *Dracocephalum Moldavica* (a herbaceous plant also called Moldavian balm), by regulating nonenzymatic and enzymatic defense mechanisms under saline conditions [35]. *Brassica Napus* plants treated with CeO2 NPs have efficient chloroplast and biomass under salt stress [36]. Anatomical changes, such as low accumulation of Na + in roots and high Na + flow toward shoots have also been reported [37].

#### **2.5 Silver nanoparticles Ag NPs**

Silver nanoparticles enhanced the sodium, potassium, and chloride to regulate the osmolality level in treated plants under salt stress conditions. The stability of Ag NPs can be easily controlled in aquatic environments as compared to soil conditions [38]. Priming of seeds was carried out with Ag NPs to enhance the seed germination in wheat and the development of tomato plants [39]. The combined effect of Ag NPs with NaCl reduced the thiobarbituric acid reactive substances, electrolyte leakage, and hydrogen peroxide to control the oxidative damage in plants that is linked with the overproduction of ROS [40]. *Triticum aestivum* treated with Ag NPs increased the fresh and dry biomass under saline conditions [41]. Seeds of *Pennisetum glaucum* treated with Ag NPs improved the growth, proline, and relative water content (RWC) and decreased the oxidative damage by increasing the antioxidant enzyme activities under saline conditions [42].
