Chromium (Cr) contamination in agricultural soils is a growing concern, impacting the ecological balance and threatening sustainable crop production worldwide [44]. Industrial activities such as coal combustion, petroleum refining, electroplating, and leather tanning contribute to excess Cr deposition in soils [50]. Chromium toxicity poses significant health risks, including respiratory diseases, kidney and liver damage, and various cancers [69]. In areas like Kasur, Punjab Province, Pakistan, Cr levels in residential water sources are as high as 2.12 mg/L due to industrial waste [80], exceeds the U.S. Environmental Protection Agency (EPA) drinking water standard of 0.1 mg/L for total Cr (https://www.epa.gov/sdwa/chromium-drinking-water). Chromium accumulation in soil affects crop physiology, reducing seed germination, plant biomass, and photosynthetic efficiency, while increasing electrolyte leakage and disrupting micronutrient uptake [17], [47]. Wheat, a crucial staple crop in Pakistan, is particularly vulnerable to Cr toxicity due to its limited natural resistance [1]. In wheat, Cr accumulation in root and shoot tissues leads to cellular damage, disrupts stomatal conductance, reduces chlorophyll content, and alters root and leaf anatomy. It also increases oxidative stress by elevating hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) levels, impairs nutrient uptake, and compromises cell membrane integrity, ultimately hindering photosynthesis and overall plant growth [5], [7]. Thus, adopting effective mitigation strategies is essential to sustain wheat production under Cr stress. Previous studies have demonstrated promising results in this regard, such as the application of polymer-modified biochar to reduce Cr(VI) toxicity and enhance seedling growth [58] and the supplementation of silicon to improve Cr detoxification by enhancing physiological resilience and reducing oxidative stress [63]. Biomass allocation in higher plants is a key adaptive response to abiotic stress, as plants dynamically regulate resource partitioning between roots and shoots to optimize survival under unfavorable conditions. This allocation pattern is influenced by various environmental stressors, including heavy metal contamination, drought, and salinity, which affect the carbon and nutrient distribution in plant tissues. Chromium stress has been shown to alter biomass partitioning by reducing root-to-shoot translocation of essential nutrients while triggering enhanced root proliferation as a compensatory mechanism [20]. The ability of wheat to modulate biomass allocation under stress is critical for its tolerance and recovery, making it a key aspect of studies on abiotic stress mitigation.

Wheat (Triticum aestivum L.) is one of the most widely cultivated cereal crops, providing food security for a significant portion of the global population. It is grown on more land area than any other crop and serves as a staple for over 20 % of the world’s population [22], [43]. Due to its adaptability, wheat can thrive in diverse environmental conditions, but its productivity is significantly impacted by abiotic stressors such as drought, salinity, and heavy metal toxicity [71]. In Pakistan, wheat is an essential component of agricultural sustainability and economic stability, contributing substantially to food security [38]. However, exposure to heavy metals such as Cr disrupts its growth and productivity by affecting physiological and biochemical processes, including photosynthesis, nutrient uptake, and oxidative defense mechanisms. Understanding wheat's response to abiotic stress and its tolerance mechanisms is critical for developing strategies to enhance its resilience under challenging environmental conditions.

Environmental stressors significantly impact plant growth, development, and yield, often leading to substantial crop losses [82]. Chromium toxicity first manifests through its detrimental effects on seed germination, and it further disrupts root architecture by reducing root diameter and cortex thickness, while also compromising membrane permeability and causing chlorophyll degradation [41]. These physiological disruptions are largely driven by oxidative stress, as Cr exposure induces an overproduction of reactive oxygen species (ROS) such as H₂O₂ and MDA, leading to oxidative damage at the cellular level [15], [79]. To counteract this oxidative burst, the plant's antioxidant defense system plays a critical role. Key antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), glutathione peroxidase (GPX), and ascorbate peroxidase (APX), work in synergy with non-enzymatic antioxidants such as carotenoids, ascorbic acid (AsA), glutathione (GSH), and phenolics to scavenge harmful oxidative species [74]. However, Cr stress often leads to the downregulation of the crucial AsA-GSH cycle, further weakening the plant’s antioxidant response (Kumar [39]). Ionomics, the study of the plant's elemental composition and ion homeostasis, is essential for understanding the mineral interactions and nutrient transport mechanisms affected by Cr toxicity. Chromium interferes with the uptake and transport of vital nutrients such as calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), and zinc (Zn), disrupting essential physiological processes [4]. By hindering the plant's mineral balance, Cr not only compromises nutrient assimilation but also impairs overall stress tolerance, making ionomics a crucial area of focus in Cr stress studies as studied by [9].

Nano-remediation is an effective strategy for mitigating Cr stress in crops [16]. Studies have demonstrated that various nanoparticles, such as silicon oxide, iron oxide, and copper (Cu), can alleviate Cr toxicity in plants, including wheat and sunflower [48], [54]. Application of nanoparticles with nanopriming technique is more reliable as it prevents accumulation of nanoparticles in plant tissues, enhance the electron exchange and surface reaction capabilities, and improve seedlings growth [53], [59]. Nanopriming, which prevents nanoparticle accumulation in plant tissues, enhances electron exchange, improves surface reactions, and promotes seedling growth, has shown reliable results in combating abiotic stress [60]. Manganese dioxide (MnO₂) nanoparticles (NPs), due to their high cation adsorption capacity and large surface area, have proven to be particularly effective in heavy metal removal while exhibiting low phytotoxicity [10], [85]. Manganese dioxide nanoparticles are less toxic to plants as compared to Mn nutrient in reducing plant abiotic stresses [85]. Our hypothesis is that MnO₂ NPs will mitigate Cr-toxicity in wheat by reducing Cr uptake, enhancing antioxidant defenses, and stabilizing ion homeostasis, thereby improving overall physiological resilience and growth under metal stress conditions. The novelty of this study lies in demonstrating how MnO₂ NPs not only mitigate Cr toxicity but also improve plant metabolic responses, offering a novel approach for enhancing wheat productivity under Cr stress. The main objectives are to (1) investigate the effect of MnO₂ NPs on Cr uptake and distribution in wheat, (2) assess their impact on growth, ionomics, and the antioxidant system, and (3) evaluate the structural changes in roots and leaves under Cr stress to provide insights for sustainable crop production.