Evaluation of the allelopathic potential of five South African mesic grassland species

The recent success of Aristida junciformis Trin. and Rupr. and Eragrostis curvula Schrad. Nees in dominating South African grasslands has posed agricultural and ecological threats. With the objective of examining if the current success of the species is related to allelopathy, the present study assessed the allelopathic potential of five common perennial grassland species (Poaceae). Aqueous extracts prepared by soaking separately 2, 10, 25 and 40 g of leaf and root material in 100 mL of distilled-water for 36 h were bioassayed using a biotest species i.e. Grand Rapids lettuce seeds (Lactuca sativa L. cv.). Species, dilution level and plant-part had a highly significant (P < 0.001) effect on germination percentage, as well as shoot and root elongation of the biotest species. At higher concentrations, the leaf extracts of all species showed significant inhibitory activities which declined with increases in dilution level. A. junciformis, E. curvula and Megathyrsus maximus (Jacq.) BK Simon and SWL Jacobs. demonstrated the highest degree of inhibition in germination percentage and subsequent seedling length; with complete inhibition recorded at dilution levels a parts per thousand yen25 % w/v. At high concentration levels, root extracts of all species, except Themeda triandra Forssk. exhibited significant inhibitory effects on germination; with Hyparrhenia hirta (L.) Staph being most inhibitory. Relatively speaking, E. curvula, A. junciformis and M. maximus showed the highest degree of inhibition. These findings suggest that these three species possess strong allelopathic potential. Such an allelopathic supremacy may contribute to their dominance in natural grasslands. Further studies, however, are still required to isolate responsible phytochemicals and to better understand if allelopathic advantage is the key factor contributing to population dynamics in the field.