Nickel, a naturally occurring metal, is essential for plant growth in tiny amounts but becomes highly toxic when present in excess. Industrial activities like mining and manufacturing have increased nickel levels in soils worldwide, posing a severe threat to agriculture.
Nickel toxicity disrupts plant metabolism by interfering with enzyme functions, damaging cell membranes, and blocking the uptake of essential nutrients such as magnesium and iron. Watermelon, a vital global crop producing over 110 million tons annually, is particularly vulnerable due to its high water content and sensitivity to soil contaminants.
High nickel concentrations damage roots, reduce nutrient absorption, and disrupt photosynthesis, leading to stunted growth and lower yields. Worse, nickel enters the food chain through contaminated crops, risking human health by accumulating in organs and causing long-term diseases.
Plants under nickel stress produce harmful molecules called reactive oxygen species (ROS), which are unstable chemicals like superoxide radicals (O₂⁻) and hydrogen peroxide (H₂O₂) that damage cells, proteins, and vital structures like chloroplasts.
While plants have natural antioxidant defenses enzymes like superoxide dismutase (SOD) and catalase (CAT) excessive nickel overwhelms these systems, causing irreversible harm. This study explores an innovative solution: green-synthesized zinc oxide nanoparticles (ZnONPs) to protect watermelon from nickel toxicity, improve growth, and ensure food safety.
Eco-Friendly Synthesis of Zinc Oxide Nanoparticles for Agriculture
The experiment began with the eco-friendly synthesis of ZnONPs using sweet wormwood (Artemisia annua) leaf extract. Green synthesis refers to the process of creating nanoparticles using natural materials like plant extracts, which act as reducing and stabilizing agents, avoiding the need for toxic chemicals.
This method is sustainable and environmentally friendly compared to conventional chemical synthesis. The process involved boiling dried leaves to create an extract rich in phytochemicals, mixing it with zinc acetate to form nanoparticles through a reduction reaction, and verifying their structure using advanced tools.
Fourier-transform infrared spectroscopy (FTIR), a technique that identifies molecular bonds based on their absorption of infrared light, confirmed the presence of functional groups like amines and aromatic compounds on the nanoparticle surface.
X-ray diffraction (XRD), which analyzes crystal structures by measuring diffraction patterns, confirmed the nanoparticles had a crystalline structure. Electron microscopy revealed spherical and hexagonal nanoparticles 8–42 nanometers in size—small enough to penetrate plant tissues effectively and interact with cellular components.
Dramatic Growth Recovery in Nickel-Stressed Watermelons
Nickel stress severely stunted growth. Untreated plants showed a 46% drop in fresh weight and a 55% reduction in dry weight compared to healthy plants. Roots became shorter and weaker, with root tips—small projections critical for nutrient uptake—declining by 55%.
However, applying ZnONPs reversed this damage. Treated plants under nickel stress regained 53% of their fresh weight and 51% of their dry weight.
Roots grew 32% longer, and root tips increased by 56%, enabling better water and nutrient absorption.
These improvements highlight ZnONPs’ role in strengthening root systems and enhancing resilience. The nanoparticles likely formed protective coatings on root surfaces or altered soil chemistry to reduce nickel availability, allowing plants to focus on growth rather than defense.
Restoring Photosynthetic Efficiency Under Metal Stress
Photosynthesis, the process by which plants convert sunlight into energy using chlorophyll pigments in chloroplasts, suffered under nickel stress. Chlorophyll levels dropped by 33–55%, and chloroplasts the structures where photosynthesis occurs showed visible damage.
Transmission electron microscopy (TEM), a technique that uses electron beams to create high-resolution images of cellular structures, revealed collapsed thylakoid membranes, which are essential for light absorption and energy production.
Gas exchange metrics, such as CO₂ absorption (net photosynthesis rate) and stomatal conductance, fell by 43–74%.
ZnONPs restored chlorophyll production by up to 82%, repaired chloroplast structures, and boosted photosynthesis rates by 39%. Stomatal conductance and transpiration rates improved by 120–134%, allowing plants to efficiently use water and light. The nanoparticles may have protected chloroplast membranes from oxidative damage or enhanced the activity of enzymes involved in chlorophyll synthesis.
Oxidative Stress Reduction Through Nanoparticle Intervention
Oxidative stress, marked by toxic molecules like hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), surged in nickel-exposed plants. Hydrogen peroxide levels rose by 35%, while MDA, a byproduct of lipid peroxidation that indicates cell membrane damage, jumped by 81%.
Electrolyte leakage, a measure of cell membrane integrity where ions escape damaged cells, increased by 122%. ZnONPs counteracted this damage, reducing hydrogen peroxide by 21%, MDA by 21%, and electrolyte leakage by 34%.
This protection stemmed from a significant boost in antioxidant enzymes: superoxide dismutase (SOD), which converts superoxide radicals into oxygen and hydrogen peroxide, saw activity rise by 39%. Catalase (CAT), which breaks down hydrogen peroxide into water and oxygen, increased by 32%, and ascorbate peroxidase (APX), which uses ascorbate to neutralize H₂O₂, rose by 25%.
These enzymes neutralize ROS, acting as a cleanup crew to prevent cellular damage. The nanoparticles may have directly scavenged ROS or stimulated the plant’s innate antioxidant pathways through signaling molecules.
Sustainable Farming Solutions for Heavy Metal Contamination
Nutrient uptake, disrupted by nickel, rebounded with ZnONP treatment. Essential elements like nitrogen (a key component of amino acids and chlorophyll), phosphorus (vital for energy transfer via ATP), and potassium (regulates stomatal function) increased by 104%, 66%, and 95% in shoots, respectively.
Magnesium and manganese, critical for enzyme activation and photosynthesis, rose by 159% and 109%. Zinc levels doubled in roots, highlighting the nanoparticles’ dual role as both a protector and nutrient source. Zinc is a cofactor for over 300 enzymes, including those involved in DNA synthesis and antioxidant defense.
By blocking nickel absorption, ZnONPs allowed plants to prioritize beneficial minerals, restoring metabolic balance. The nanoparticles might have competed with nickel for transport channels in root cells or formed complexes with nickel ions, rendering them less available for uptake.
Future Directions in Nanotechnology for Agriculture
At the molecular level, ZnONPs activated genes responsible for antioxidant production. SOD gene expression increased by 45%, CAT by 59%, and APX by 31%. Gene expression refers to the process by which genetic instructions are used to synthesize functional products like proteins.
This genetic boost equipped plants to manage future stress more effectively. Additionally, nickel accumulation in leaves dropped by 50%, and root uptake fell by 24%, reducing the risk of toxic metals entering the food chain.
Lower nickel levels in edible parts like fruits make crops safer for consumption, addressing a critical public health concern.
Revolutionizing Crop Protection with Green Nanotechnology
The implications for agriculture are profound. Farmers in polluted regions could use ZnONP sprays to rescue crops, improve yields, and ensure safer food. Traditional remediation methods, like soil replacement or chemical treatments, are expensive and impractical for small-scale farmers.
For instance, removing and replacing contaminated soil can cost thousands of dollars per acre, whereas ZnONP sprays are affordable and easy to apply. A single foliar spray of 100 mg/L ZnONPs applied twice during growth could shield watermelon from nickel toxicity without harming soil health.
- The nanoparticles’ small size and high reactivity allow them to work at lower concentrations than traditional fertilizers, reducing environmental impact.
Future Directions in Nanotechnology for Agriculture
This study also aligns with global sustainability goals. Green synthesis methods, like using Artemisia annua extract, minimize chemical waste and environmental impact. Unlike conventional nanoparticles, which may leave toxic residues, plant-based ZnONPs degrade safely, reducing long-term soil contamination risks.
- The use of renewable resources like plant extracts supports circular economy principles, where waste materials are repurposed into valuable products.
Future research should explore long-term effects, such as how repeated ZnONP applications affect soil microbes—tiny organisms essential for nutrient cycling and plant health over multiple growing seasons. Testing the approach on other crops, like rice or tomatoes, could broaden its applicability to staple foods and high-value vegetables.
Revolutionizing Crop Protection with Green Nanotechnology
Despite promising results, challenges remain. Field trials are needed to confirm greenhouse findings under real-world conditions, where variables like unpredictable weather, pests, and soil variability complicate outcomes.
For example, heavy rainfall might wash away nanoparticles, reducing their effectiveness, while drought could concentrate nickel levels in soil.Economic analyses must assess the cost of large-scale ZnONP production and application, including expenses for raw materials, labor, and equipment.
However, the potential benefits higher yields, safer food, and reduced fertilizer use—make this a compelling solution for nickel-polluted farmlands. Governments and agricultural organizations could subsidize nanoparticle adoption to support farmers in polluted regions.
Conclusion
In conclusion, green-synthesized zinc oxide nanoparticles offer a groundbreaking strategy to combat nickel toxicity in watermelon. By enhancing antioxidant defenses, repairing photosynthesis, and blocking nickel uptake, ZnONPs restore plant health and productivity. With global food security at risk from soil pollution, this nanotechnology-driven approach provides hope for sustainable, resilient agriculture.
Future innovations could combine ZnONPs with organic practices, such as composting or crop rotation, to create holistic solutions for farmers worldwide. For instance, integrating nanoparticles with organic matter could improve soil structure and microbial diversity, further enhancing plant resilience. As research advances, these tiny particles may become a cornerstone of climate-smart agriculture, ensuring food production keeps pace with population growth and environmental challenges.
Power Terms
Nickel (Ni): A metallic element naturally present in soil, essential for plants in trace amounts (0.01–5.0 µg/g) but harmful at higher concentrations. Nickel acts as a cofactor for certain enzymes but becomes toxic when industrial activities elevate its levels in soil. Excess nickel damages plant roots, reduces nutrient absorption, and triggers oxidative stress. For example, watermelon plants exposed to 70 mg/kg soil nickel showed stunted growth and lower yields. Nickel’s toxicity also allows it to enter the food chain, posing health risks like organ damage in humans. Managing nickel levels is critical for agriculture to ensure crop safety and productivity.
