Allelopathic Effects of Extracts from Different Sorghum Parts on Giant Foxtail

Giant foxtail is a common weed in foxtail millet fields, and continuous cropping of foxtail millet increases its occurrence. Rotating foxtail millet with sorghum is a major practice in the dry farming regions of northern China. Given this situation, this study combined Petri dish germination tests with pot experiments to clarify the allelopathic effects of water extracts from different sorghum parts on giant foxtail. Distilled water (SCK) was used as the control, and treatments included stock solution (S1), as well as 10× dilution (S2), 50× dilution (S3), and 100× dilution (S4) of water extracts prepared from sorghum roots, stems, leaves, and rhizosphere soil. The results showed that water extracts from all sorghum parts exhibited promotion of giant foxtail seed germination at higher dilutions and inhibition at lower dilutions. The S3 treatment of rhizosphere soil extract achieved a germination rate of 62.22%, which was significantly higher than that of the SCK by 40.00%. The germination energy of root extract S1 was only 4.00%, which was significantly lower than that of the SCK by 86.67%. The S3 treatment showed the strongest allelopathic promotion effect. All dilution treatments promoted the plant height, stem diameter, and above ground fresh and dry weight of giant Foxtail, and these effects increased initially and then stabilized over time. The root extract exhibited higher allelopathic activity than the other extracts. The activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) first increased and then decreased as the dilution factor increased, while the S1 treatment inhibited enzyme activities and caused an imbalance in the antioxidant system. Sorghum root water extract had a significant effect on CAT activity in giant foxtail, with an increase of 98.82% under the S3 treatment compared with the SCK at 40 d. Malondialdehyde (MDA) content increased with decreasing dilution factor, and the undiluted extract induced membrane lipid peroxidation damage. In conclusion, the allelopathic effects of sorghum extracts on giant foxtail were closely related to the dilution level. Higher dilutions activated the antioxidant defense system and promoted growth, while lower dilutions disrupted enzyme system balance and exacerbated membrane damage. These findings provide basic data and theoretical support for the development and utilization of sorghum allelochemicals and for the green control of giant foxtail.