Abstract
Plant growth-inhibitory effects were found in the aqueous extract of manilagrass shoots. A methanol extract of dried manilagrass leaves was used for the isolation of phytotoxins. To isolate the growth inhibitors, methanol extract of manilagrass shoots was partitioned into ethyl acetate soluble (AE), neutral (NE) and aqueous (AQ) fractions. AQ and NE fractions were inhibitory to seed germination and seedling growth of livid amaranth. AE fraction was also inhibitory but stimulatory at lower concentrations. The relative inhibitory effects of these fractions were higher than that of the original extract. Column chromatographies, HPLC and gas chromatography were the principal methods used to identify the phytotoxins. The active fractions from ODS-HPLC were derivatized (Me/TMS), and then analyzed by gas chromatography-mass spectrometry. The identities were finally confirmed by the comparison with known authentic compounds by GC-MS. Eleven compounds were identified as phytotoxins from manilagrass. Nine fatty acids (C10 to C18) and two phenolic compounds (p-coumaric and p-hydroxybenzoic acids) were the major compounds and were known to have growth regulatory activities.Bioassays were also conducted using the standard chemicals to determine whether similar phytotoxic effects were produced. The results showed that all of the fatty acids and phenolic compounds found in manilagrass significantly inhibited root growth of livid amaranth seedlings. p-Coumaric and p-hydroxybenzoic acids were highly inhibitory to the root growth of livid amaranth even at 62.5 ppm. The others reduced root growth significantly at concentrations between 250-500 ppm. In general, the bioassay results with the standard compounds were similar to those obtained with the plant-derived phytotoxins, and thus the identities of these phytotoxins were confirmed.
