Allelochemics impacts of leaf extracts from Vernonia amygdalina and Chromolaena odorata on germination of rice

Abstract

Allelopathy from invasive and medicinal weeds can affect crop establishment, yet species- and concentration-specific impacts on rice remain poorly quantified. This study evaluated the concentration-dependent allelopathic effects of aqueous leaf extracts of Vernonia amygdalina Delile and Chromolaena odorata (L.) R.M. King and H. Rob. on germination and early seedling growth of Jamila rice (Oryza sativa L.). A completely randomised design with five replicates was used. Four concentrations of fresh leaf aqueous extracts were tested for each species: 0% (control), 25%, 50%, and 100% w/v. Radicle and plumule lengths were measured every 3 days for 21 days. One-way ANOVA analysed data and means separated using Duncan’s Multiple Range Test at p ≤ 0.05. C. odorata caused a linear, concentration-dependent inhibition of both radicle and plumule growth. At 21 DAP, 100% C. odorata reduced radicle length by 34% and plumule length by 12% relative to the control (p ≤ 0.05). V. amygdalina exhibited a biphasic hormetic response. The 25% extract stimulated radicle growth by 18% and plumule growth by 5% compared to the control at 21 DAP, while the 100% extract caused 31% inhibition of radicle and 12% inhibition of plumule at 2 DAP (p ≤ 0.05). Inhibitory effects were most pronounced during early ontogeny (2–11 DAP) and became transient by 21 DAP. Both extracts are concentration-dependent modulators of rice seedling growth. C. odorata is consistently inhibitory, while V. amygdalina can be stimulatory at low concentrations and inhibitory at high concentrations. These findings suggest potential for low-dose V. amygdalina as a bio-stimulant and high-dose extracts of both species as natural herbicides, pending field validation. Comparative allelopathic assessment: Investigated and compared the concentration-dependent allelochemics impact of fresh leaf aqueous extracts from Vernonia amygdalina (bitter leaf) and Chromolaena odorata (Siam weed) on the germination dynamics of Jamila rice (Oryza sativa). Biphasic (Hormetic) Response of V. amygdalina: Discovered that V. amygdalina exhibits a dual agronomic nature; lower concentrations (25% w/v) significantly stimulated radicle and plumule elongation, whereas high concentrations (100% w/v) exerted severe phytotoxic inhibition (V0 > V{50%} > V{100%}). Linear Inhibition by C. odorata: Demonstrated that C. odorata extracts cause a strictly linear, concentration-dependent suppression of early seedling growth parameters, following a distinct statutory decline pattern (C0 > C{25%} > C{50%} > C{100%}) for both radicle and plumule development. Transient chronological effects: Revealed that the allelopathic inhibitory stresses of both botanical extracts are highly prominent during early ontogeny (2 to 11 days after planting) but gradually exhibit a transient nature, showing no statistically significant growth differences (p ≥ 0.05) by the conclusion of the 21-day bioassay. Agricultural application insight: Highlights the critical threshold dynamics required when harnessing these bioactive weeds as sustainable biofertilizers or botanical pest-control agents without risking autotoxic or non-target crop suppression in rice-cropping systems.