ALLELOPATHIC POTENTIAL OF SELECTED SHADE TREE SPECIES TO CONTROL WEEDS IN TEA PLANTATIONS

Abstract

The detrimental effects of synthetic herbicides on the environment and human health have spurred increased interest in eco-friendly alternatives for weed control, including allelopathy. A bioassay and a pot study were conducted to assess the allelopathic potential of four common shade tree species (Gliricidia sepium, Calliandra calothrysus, Erythrina lithosperma, and Grevillea robusta) to control weeds in tea plantations in Sri Lanka. The study also examined the most effective concentration of the leaf extract and the extraction techniques. Aqueous extracts were prepared using dry powders of mature leaves at four concentrations (2, 4, 6, and 8% w/v) using hot and cold-distilled water. Bioassays were performed using lettuce as the indicator plant. The effect of different leaf extracts on seed-germination parameters depended on their concentration (P<0.05). There was no significant difference between hot and cold water extraction (P>0.05). G. sepium exhibited the highest allelopathic effect, evidenced by the lowest values for germination percentage (17.8%), germination index (0.4), seedling vigor index (0.1), and the highest inhibition in radical (83.3%), and hypocotyl (89%) elongation followed by G. robusta both at 8% concentration. Most phytotoxic extracts (G. sepium, G. robusta, E. lithosperma each at. 6, and 8% concentrations (w/v)) were further tested on three weed species (Cleoma aspera, Bidens pilosa, Ageratum conyzoides) planted in pots in three replicates. G. sepium recorded the highest weed growth suppression, followed by G. robusta at 8% concentration. G. sepium at 8% recorded the significantly highest dead weed count (C. aspera, 100%; B. pilosa and A. conyzoides, 66.6%), followed by G. robusta at 8%. In conclusion, leaf extracts of G. sepium and G. robusta at 8% can potentially be used to control broad-leaf weeds in tea lands. Further investigations under field conditions are warranted.