Abstract
Abstract Amaranthus viridis L. is an invasive weed that competes with crops, thereby causing food security issues. In the present investigation, leaf extracts (aqueous and methanolic) of Brassica juncea (L.) Czern. were evaluated for allelopathic effects on Amaranthus viridis. The extracts (5%, 15% and 25%) inhibited seed germination, seedling growth (length of root and shoot and fresh weight), light-harvesting pigments (chlorophyll a, b and carotenoids), photosynthetic oxygen evolution and PSII photochemistry (Fv/Fm) with rising concentration of extracts, and the methanolic extract inhibited more strongly. Contrastingly, 5% aqueous extract increased growth attributes (shoot length 6%, root length 3% and fresh weight 2%), photosynthetic pigments (Chl a 8%, Chl b 5% and Car 10%) and photosynthesis (O2 evolution 10%). Both extracts disturbed redox homeostasis as evidenced by increased oxidative biomarkers O2. −, H2O2 and MDA contents, and also enhanced the respiratory activity. Accelerated activity of enzymatic antioxidants superoxide dismutase, peroxidase, catalase and glutathione-S-transferase could not keep oxidative biomarkers under limit. As a result, aqueous and methanolic extracts at 25% caused stunted growth, showing 45% and 61% decrease in fresh weight, respectively. Gas Chromatography Mass Spectrometry (GC-MS) analysis identified 47 secondary metabolites in the methanolic leaf extr -act of B juncea. Some phytochemicals are closely related to herbicidal activities showing inhibitory action on seed germination (2-methoxy-4-vinylphenol), plant growth (loliolide) and pigment synthesis (2-pyrrolidinone) and reduction of PS II maximum quantum yield (Fv/Fm) by generating severe oxidative stress (isothiocyanates). The present study establishes the allelopathic potential of Brassica juncea leaf extracts for the management of the prolific weed Amaranthus viridis. However, extensive field validation and toxicity analysis are crucial for its formulation as a bio-herbicide for sustainable agricultur. Graphical abstract