Hormone–nanoparticle synergy: Co-application of melatonin and nano-biochar enhances soybean resilience to arsenic stress via modulation of proline metabolism and oxidative stress markers

The primary objective of this study was to evaluate the protective effects of nano-biochar (nano-BC) and melatonin on soybean (Glycine max) plants subjected to arsenic stress. Exposure to arsenic (150 µM) markedly impaired soybean growth, photosynthetic efficiency, and carbohydrate metabolism. Arsenic stress also disrupted nutrient homeostasis, leading to substantial reductions in leaf nitrogen (46 %), phosphorus (41 %), potassium (44 %), manganese (42 %), and iron (35 %) contents. Moreover, oxidative stress intensified under arsenic exposure, as evidenced by a 79 % increase in hydrogen peroxide accumulation and a 67 % elevation in lipid peroxidation. In contrast, exogenous application of nano-BC (1 % w/w) through soil and melatonin (100 µM) as a foliar spray significantly alleviated arsenic-induced oxidative damage by suppressing reactive oxygen species (ROS) accumulation. Both treatments, individually and in combination, enhanced the activity of key antioxidant enzymes involved in ROS detoxification. Notably, the combined application of nano-BC and melatonin improved plant height, Fv/Fm, photosynthetic efficiency, carbohydrate metabolism, nutrients uptake, and protein content under arsenic stress. These benefits are attributed to the ability of nano-BC to improve soil quality, immobilize arsenic, and enhance nutrient availability, while melatonin modulates antioxidant defense and stress signaling pathways. Furthermore, the combined treatment increased proline by 34 %, starch by 23 % and sucrose by 19 %. These findings highlight the synergistic potential of nano-BC and melatonin in mitigating arsenic toxicity in soybean by improving soil-plant interactions, enhancing nutrient uptake, and strengthening physiological and antioxidant responses.