Allelopathic Potential of Invasive Nicotiana glauca Aqueous Extracts on Germination and Seedling Growth of Invasive Prosopis juliflora

Laboratory and greenhouse experiments were conducted to investigate the allelopathic potential of aqueous leaf and bark extracts of invasive Nicotiana glauca on seed germination and seedling growth of invasive Prosopis juliflora. Extracts at 0, 10, 40, 70, and 100 g L−1 were evaluated in two-factor factorial experiments under growth-chamber and greenhouse conditions. Electrical conductivity, pH, and phenolic, tannin, and flavonoid contents were characterized, while germination, growth, root development, and physiological parameters were assessed. Phytochemical levels varied according to extract concentration and tissue source. Aqueous extracts significantly affected P. juliflora germination (p ≤ 0.001), reducing the germination percentage (GP), coefficient of velocity of germination (CVG), and germination rate index (GRI), while increasing the mean germination time (MGT). At the highest extract concentration, GP was reduced by 89% with leaf extracts and 75% with bark extracts compared with the control. Plant height, leaf number, and leaf surface area were also substantially reduced, accompanied by marked alterations in root architecture. For example, root volume decreased from 3.71 cm3 in the control to approximately 1.30 cm3 under high-concentration leaf extracts, while root tips declined from 1452 to 356. Physiological responses further indicated stress, with net photosynthesis decreasing from 6.10 to 1.40–2.00 µmol m−2 s−1 under leaf extracts at 40–100 g L−1, whereas proline accumulation increased from 21.31 to 51.77 µmol g−1. These results demonstrate substantial phytotoxicity affecting germination, seedling growth, root development, and physiological functioning, with generally stronger effects at higher extract concentrations. Overall, N. glauca extracts provided evidence of allelopathic potential against P. juliflora under controlled laboratory and greenhouse conditions, rather than confirming allelopathy under natural conditions. The observed responses may involve multiple potential mechanisms associated with phytochemicals and extract physicochemical properties; however, their relative contributions remain uncertain. Because the experimental concentrations were not intended to represent field leachate levels, further field-based studies are needed to determine whether these effects occur under natural conditions and to establish their ecological relevance.