Piper longum L. leaf extracts, a candidate allelopathic plant that suppressed the growth of six test plants, could be a source of potent phytotoxic compounds

 Using allelopathic plants, particularly those with growth inhibitory action, has gained significant consideration worldwide because such plants do not have detrimental effects on the environment. Piper longum L. (Piperaceae) is a medicinal plant that has already been documented for its diverse ethnomedicinal uses but not for its phytotoxic action. Therefore, aqueous methanol extracts of P. longum were evaluated for their allelopathic activity at plant biochemistry laboratory, Kagawa University, Kagawa, Japan. The allelopathic efficacy of different concentrations of P. longum leaf extract was investigated against six plant species (alfalfa, cress, lettuce, Italian ryegrass, barnyard grass, and foxtail fescue), and their growth was recorded after 48 h of treatment. The results showed significant inhibition of seedling growth at 0.03 g dry weight (DW) equivalent P. longum extract, and this inhibition was further increased with increasing extract concentration. The maximum inhibitory effect was noted at 0.3 g DW equivalent P. longum extract per mL. The concentrations required for 50% inhibition of shoot and root growth were 0.003–0.070 and 0.003–0.028 g DW equivalent P. longum leaf extract per mL, respectively. The shoot growth of cress, alfalfa, and lettuce, and the root growth of cress, foxtail fescue, and alfalfa were inhibited the most by the P. longum extracts. Thus, these results indicate the potent allelopathic activity of P. longum, which also implies the presence of allelopathic compounds in the P. longum leaf extract.
