Bioherbicidal activity and putative mode of action of Dittrichia graveolens against Amaranthus palmeri and A. retroflexus

Abstract

Challenges in controlling Amaranthus weeds and rising cases of herbicide resistance necessitate the development of alternative weed control strategies. Plant-derived extracts may serve as valuable sources of phytotoxic chemicals. Nevertheless, their species-specific effects on germination and early seedling development require investigation. This study evaluated the allelopathic activity of aqueous extracts of Dittrichia graveolens on Amaranthus palmeri and A. retroflexus. Aqueous extracts of D. graveolens were evaluated against A. palmeri and A. retroflexus under controlled laboratory conditions using a concentration gradient of 0, 6.25, 12.5, 25, 50, and 100%. The final germination, root and shoot lengths, inhibition in germination and growth, and germination timing indices were analyzed. Species-specific dose–response models were used to estimate IC50 and IC90 values, and comparative statistical analyses were conducted to assess interspecific variations in sensitivity. LC–MS/MS profiling was used to determine the phytochemical makeup of the extract. Molecular docking was conducted to predict potential interactions between the identified compounds and selected germination-related enzymes. The LC–MS/MS profiling indicated a phenolic-rich extract, with quinic acid as the major chemical component (23.472 mg g⁻¹ extract), followed by fumaric acid (0.577 mg g⁻¹), salicylic acid (0.254 mg g⁻¹), protocatechuic acid (0.238 mg g⁻¹), and piceid (0.176 mg g⁻¹). The aqueous extract induced a significant dose-dependent suppression of germination and seedling development in both weed species. Higher final germination values were recorded under control and 6.25% extract treatments, decreased to 72% at 12.5%, and further reduced to 44–46% at 25%. Nevertheless, extract doses of 50% and 100% completely inhibited germination (< 5%). Root development exhibited significant sensitivity. Root inhibition reached 87.6% in A. retroflexus, compared with 56.0% in A. palmeri at a 25% extract concentration. A comparative dose–response analysis indicated that A. retroflexus was more sensitive than A. palmeri, as shown by lower IC50 values for germination inhibition (16.4% compared to 20.8%), root inhibition (11.4% vs. 20.5%), and shoot inhibition (13.1% relative to 27.8%). Germination timing indices indicated that higher extract concentrations decreased germination speed, delayed mean germination time, and altered germination synchrony. Molecular docking predicted comparatively favorable binding energies between selected flavonoids and evaluated germination-related enzymes. Luteolin and apigenin showed the most favorable predicted binding energies for phytase, whereas amentoflavone showed favorable predicted binding energies for catechol oxidase and β-glucanase. The aqueous extract of D. graveolens exhibited concentration-dependent allelopathic effects on A. palmeri and A. retroflexus, with A. retroflexus showing greater sensitivity overall. The extract inhibited germination and seedling development and altered germination timing. Phytochemical profiling identified candidate phenolic and flavonoid compounds, while molecular docking generated testable hypotheses regarding their possible interactions with germination-related enzymes. Biochemical, greenhouse, and field validation is required before their mechanistic involvement or the practical bioherbicidal use of the extract can be established.