Differential impacts of iron oxide nanoparticles and zinc on ion homeostasis, antioxidant defense, and growth performance of cotton under salinity stress

Salinity is one of the most detrimental abiotic stresses, significantly impairing plant growth, nutrient balance, and productivity. In recent decades, nanotechnology-based approaches, such as the use of iron oxide nanoparticles Fe(II,III) oxide nanoparticles, and micronutrient supplementation like zinc (Zn) foliar applications, have emerged as promising strategies to mitigate salinity-induced damage. These approaches are gaining importance due to their potential to enhance ion homeostasis, antioxidant defenses, and physiological functions, thereby improving plant resilience under saline conditions. This study aimed to evaluate the effects of Fe(II,III) oxide NPs and Zn on macro- and microelement accumulation, enzymatic antioxidant activity, total phenolic and flavonoid contents, and growth performance of cotton (Gossypium hirsutum L.) subjected to salinity stress. In this regard, we designed the two different salinity conditions assigned to the main plots (0 dS m−1 control and 13 dS NaCl m−1) and the foliar application to the sub-plots, which included distilled water, zinc (Zn) (80 ppm) and iron oxide nanoparticles Fe(II,III) oxide NPs; 300 ppm). The study demonstrated that Fe(II,III) oxide NPs significantly enhanced cotton growth under salinity stress, increasing dry leaf weight (DLW) to 0.74 g and fresh leaf weight (FLW) to 4.96 g. Foliar application of zinc (Zn) notably improved relative water content (RWC) to 83.90% and SPAD value to 43.53. Both foliar treatments markedly influenced the levels of flavonoids, proline, and antioxidant enzymes (CAT, POD, and SOD) in cotton leaves under saline conditions. A substantial increase in proline content and antioxidant enzyme activities was observed, with proline levels reaching 3.19 and 3.18 μmol g⁻1 FW for Zn and Fe(II,III) oxide NPs, respectively. Catalase (CAT) activity also increased significantly under salinity stress, rising to 54.86% with Zn and 55.72% with Fe(II,III) oxide NPs compared to non-saline conditions. Another notable finding was a 384.7% increase in leaf Zn content with Zn application under salinity stress. Nutrient accumulation exhibited organ-specific variations, with Fe(II,III) oxide NPs affecting affecting K, Mg, Na, Fe2⁺/Fe3⁺, and Zn whereas Zn treatments enhanced K, Mg, Na, Zn, Cu, and Mn concentrations. Additionally, Zn, Cu, and Mg concentrations were higher in leaves, while Na, Zn, Cu, and Mn were more pronounced in roots. The results indicate that foliar applications of Zn and Fe(II,III) oxide NPs enhance cotton growth and stress tolerance by promoting ion homeostasis, strengthening antioxidant defenses, and supporting key physiological processes. This study highlights the potential of nanotechnology as a sustainable strategy for nutrient management in saline environments, contributing to improved agricultural productivity and resilience.