Allelopathic activity of Combretum lanceolatum and Hyptis suaveolens extracts as weed control

In this work, the allelopathic activity of hexane and ethanoic extracts of Combretum lanceolatum and Hyptis suaveolens plants were evaluated as pre and post-emergent herbicide on Sida rhombifolia weed. These were then to be screened in pre- and post-emergent herbicide assays for the parameters photosynthesis, plant growth and germination inhibition. Among the evaluated extracts, the Combretum lanceolatum ethanolic extract (Cl-eth) demonstrated the best activity as photosynthesis inhibitor, decreasing the PI(abs) parameter by 39, 65 and 66% at 100, 200 and 300 ppm, respectively. The reduction of PI(abs) parameter indicates a damage in the photosynthetic apparatus due to interrupt the electron flow that occurs in photosynthesis. In order to localize the action site of Cl-eth, the parameters of electron absorption, entrapment and transport (ABS/Cs0, TR0/Cs0, ET0/Cs0) were analysed, and the reduction of these parameters by 48, 56 and 60% at 300 ppm, demonstrates the inhibition of electron flow on photosynthesis process at the site of the primary quinone A (QA) of the photosystem II (PSII) [2]. Additionally, the reduction of active reaction centres of PSII (RC/CS0) by 52 % suggests the protein D1 was degraded [3]. The Cl-eth at 300 ppm showed the same behaviour on in vivo assay after sprayed on plants, decreasing the PI(abs) parameter by 47, 48, 24 and 47% at 3, 24, 48 and 72h, respectively. Furthermore, Cl-eth decreased the germination index and increased the germinated rate index compared to the control. Although the activity of Cl-eth does not inhibit the germination rate, it can make difficult the germination process, leading to a decrease in the germination index, possibly damaging the seedlings’ structure. Thus, these results demonstrate that the Cl-eth extract has an herbicide allelopathic activity by inhibiting the photosynthesis and germination process.