Melatonin Enhances Crop Tolerance to Aluminum Toxicity in Acid Soils: A Comprehensive Review

Soil acidity is a major constraint in many agricultural regions, where increased aluminum (Al³⁺) solubility at low pH severely affects plant health by inhibiting root elongation, disrupting nutrient uptake, and inducing oxidative stress. Recent studies have highlighted melatonin, a widely occurring indoleamine with strong antioxidant and stress-modulating properties, which alleviates Al-induced damage in crops. This review synthesizes current physiological, biochemical, and agronomic evidence demonstrating that exogenous melatonin enhances plant tolerance to aluminum toxicity. Across multiple model and crop species, melatonin application has been shown to improve root elongation by 20-45%, reduce lipid peroxidation by 30-60%, and enhance key antioxidant enzymes such as SOD, POD, and CAT by 25-70% under Al stress. Case studies in soybean, wheat, maize, and rice further indicate that melatonin protects root meristems from oxidative damage, stabilizes photosynthetic machinery, and improves nutrient acquisition. In acidic soils (pH 4.5), melatonin-treated soybean exhibited 28% greater biomass and 15-22% higher N and P uptake, while wheat plants demonstrated 10-18% higher grain filling under field-simulated Al stress. Emerging long-term studies show that melatonin also benefits soil health. Multi season experiments reveal that melatonin enhances root exudates that support beneficial rhizosphere microbes, increases soil enzymatic activities (urease, phosphatase) by 20-35%, and lowers exchangeable Al by 12-18%. These improvements contribute to cumulative yield gains of 10-18% over successive cropping cycles. Additionally, genetic approaches aimed at increasing endogenous melatonin levels in plants have demonstrated 12-30% yield improvement in acid soil conditions. This review highlights the need for multi-year, multi-location studies to further clarify how melatonin can support sustainable agricultural practices, enhance soil fertility, and mitigate aluminum toxicity in acid-affected regions.