Nicotian glauca is a noxious invasive shrub in Saudi Arabia, and it is well known for its harmful impact on biodiversity, competing with native plants using various strategies. Among their strategies is their allelochemical activity, i.e., the use of chemicals to dominate and compete. The present study aims to evaluate the allelopathic potential of N. glauca aqueous extracts from leaves, flowers, and twigs on Acacia gerrardi seed germination and seedling growth. Petri dishes containing extracts from N. glauca parts with different concentrations were incubated in a growth chamber. The results indicated that extracts and concentrations negatively impact seed germination and hypocotyl. Relative to the control, the leaf extract with concentrations of 60 and 100 g/L reduced the seed germination rate by 81.11% and 93.33%, respectively. Flower extracts of similar concentrations decreased germination by 81.91 and 92.55%, while the twig extracts declined germination by 79.56 and 95.69%, respectively. The leaf, flower, and twig extracts (100 g/L) decreased hypocotyl radicle by 76.54, 78.05, and 65.75%, respectively. Overall, the concentrations of 20, 60, and 100 g/L showed the lowest growth indices. Generally, aqueous extracts, particularly 100 g/L, impacted the germination and protrusion of A. gerrardii hypocotyl. In conclusion, our study draws attention to the harmful impact of N. glauca on A. gerrardii regeneration.

A greenhouse experiment was conducted to determine the potential of arbuscular mycorrhizal fungi (AMF) to mitigate the allelopathic effects of invasive Nicotiana glauca on the growth of Acacia gerrardii seedlings. Aqueous extracts from N. glauca leaves and flowers (at concentrations of 20, 60, and 100 g/L) and AMF treatment (±AMF) were applied to A. gerrardii seedlings arranged in a factorial experiment with four replicates. After 6 months, vegetative (plant fresh and dry weight, height, leaf number area), physiological (photosynthesis, stomatal conductance, transpiration rate), chlorophyll a and b, proline, and root (length, surface area, diameter, volume, tip number) parameters as well as root distribution in five root diameter classes (0–5 mm, in 1 mm increments) were examined. The presence of AMF increased the seedling growth parameters. The 20 g/L treatment stimulated most of the vegetative growth, root, chlorophyll, and photosynthesis parameters at both inoculant levels for all extract types and concentrations, and the 100 g/L treatment inhibited seedling growth whether inoculated with AMF or not. This decline was alleviated in AMF-treated seedlings. For both extract sources, root length decreased as extract concentration increased, and roots <1 mm accounted for the greatest proportion of total root length, surface area, and tip number, with higher values observed in AMF-treated seedlings compared to their untreated counterparts. Further research is required at the field level to identify AMF strains and their interaction effects in alleviating the allelopathic impact of N. glauca on A. gerrardii seedling growth.