Allelopathic Effects of Mexican sunflower (Tithonia diversifolia (Hemsl) A. Gray) Aqueous Extract on Rice (Oryza sativa L.) Test Plants Germination and Early Growth

Aims: This study aims to assess the allelopathic effects of Mexican sunflower (Tithonia diversifolia (Hemsl) A. Gray) on rice seed germination as test plant, determine the optimal extract concentration for inhibiting vegetative growth of rice (Oryza sativa L.), and establish the IC₅₀ value through a laboratory bioassay. Study Design: The experiment used a Completely Randomized Design (CRD) with five levels of Mexican sunflower extract concentration (0%, 2.5%, 5%, 7.5%, and 10%) conducted in both laboratory and greenhouse. Time and Location Site: This study was conducted from November 2024 to January 2025 at the Agronomy Laboratory and Medan Baru Experimental Farm in Kandang Limun, Muara Bengkulu District, Bengkulu City, Indonesia, about 15 m above sea level. Methodology: The study consisted of two stages. In the laboratory, 25 rice seeds were placed in Petri dishes containing different concentrations of Mexican sunflower extract to observe germination and early growth for 7 days. In the greenhouse, germinated seedlings were transplanted into soil-manure mixtures and grown for 60 days to evaluate vegetative parameters. Data were analyzed using ANOVA, followed by polynomial regression and LSD tests. IC₅₀ values were calculated to determine phytotoxic potency. Results: Mexican sunflower extract significantly suppressed rice germination and growth, especially at higher concentrations. The most sensitive response was abnormal seedling percentage (IC₅₀ = 5.59%), followed by radicle length (7.82%) and plumule length (8.81%). At 10% extract concentration, there was a notable reduction in plant height, leaf area, chlorophyll content, and biomass. Radicle growth was more affected than plumule, indicating root sensitivity to allelochemicals. Conclusion: Mexican sunflower shows strong allelopathic effects on rice, especially at higher extract concentrations. Radicle development is more sensitive than plumule growth, indicating early root exposure to allelochemicals. These findings highlight Mexican sunflower's potential as a natural bioherbicide. Further studies using weed species as test plants are recommended to explore its practical application in sustainable weed management.