**3.3 Iron nanoparticles (Fe2O3 NPs)**

Iron is a key micronutrient that is essential for plant growth and development and its reduction causes chlorosis in plants [67]. Thus, iron absorption in plants experiencing drought stress may play a crucial role in their ability to withstand drought [68]. Fe2O3 NPs applied topically to *Carthamus tinctorius* leaves, reduced the negative effects of drought stress while simultaneously promoting yield, growth, and development metrics [69]. Plants under drought stress have a substantial impact of Fe2O3 NPs on the number and weight of seeds, number of bolls yield, and oil percentage in cotton. Drought stress in sunflower was significantly reduced by the application of maghemite nanoparticles (a member of iron oxide) [70]. The activity of zerovalent (nZVI) iron nanoparticles is hypothesized to activate the proton pump ATPase (H+ -ATPase) of the plasma membrane in leaves of *Arabidopsis thaliana* plants, which in turn contributes to their ability to withstand drought by increasing the stomatal aperture [71]. They retained the drought sensitivity and boosted CO2 uptake, the rate of the stomatal opening was accelerated, which in turn increased the plant biomass and chlorophyll content [72].
