Ameliorative role of apricot shell-derived biochar in modulating photosynthetic pigments, osmolytes, and secondary metabolites in chromium stressed Brassica juncea.

Chromium (Cr), a redox-active heavy metal, induces oxidative stress in plants by generating reactive oxygen species (ROS) such as hydrogen peroxide, superoxide anion, and hydroxyl radicals, which reduce plant productivity and yield. This study evaluated the potential of biochar (BC) derived from apricot kernel shells to alleviate Cr toxicity in Brassica juncea. The BC was characterized as amorphous, alkaline (pH 7.84), with a zeta potential of -22.3 mV, and elemental composition of C (60.70%), O (29.58%), H (2.65%), and N (0.77%). FTIR analysis revealed multiple oxygen-containing functional groups, suggesting its capacity to reduce Cr mobility. The alkaline nature and porous structure of the BC further support its immobilization potential. Seeds of B. juncea were germinated in soil treated with 0.5 mM and 0.75 mM Cr, with or without 1% BC (10 g/kg soil). Cr exposure significantly reduced photosynthetic pigments, with total chlorophyll declining by 30.8-49.1% and carotenoids by 25.7-50% compared to control, while pheophytin content increased by 13.9-45.3%. Application of BC improved chlorophyll content and reduced pheophytin accumulation. Secondary metabolites and osmolytes were enhanced under BC and stress treatments, with total phenols increased by up to 45% and total carbohydrates by up to 34%. Growth and germination parameters were also negatively affected by Cr, but BC treatment effectively mitigated these effects, improving root and shoot development. Overall, apricot kernel shell-derived BC alleviated Cr phytotoxicity by reducing metal availability and enhancing plant growth, photosynthetic performance, and metabolic activity. These findings highlight its potential as an eco-friendly and sustainable strategy for mitigating heavy metal stress in plants.