Bioguided isolation and identification of the protodioscin isomers from Brachiaria ruziziensis L. as inhibitors of growth of Euphorbia heterophylla L.

A bioguided identification of the phytotoxic compounds of the Brachiaria ruziziensis L. on the weed species Euphorbia heterophylla L. was conducted. From the butanolic fraction of B. ruziziensis it was separated an active subfraction (Subfraction III) that was demonstrated to contain the steroidal saponins. These substances were determined as protodioscin and protonedioscin by 1H and 13C NMR analysis. b-ethyl glucose was also identified, but it represented only 2.4% of the subfraction III. The effects of pure protodioscin were very similar to those caused by Fraction III or saponins isolated from B. ruziziensis. Root length was the most sensitive variable and the germination indices were little modified. The calculated ID50 for primary root inhibition was 390.1 ± 112.7 µg/ml and 359.5 ± 44.6 µg/ml for saponins and pure protodioscin, respectively. b-ethyl glucose did not cause significant modifications in the germination and seedling growth when assayed at a concentration range of 100 to 1000 µg/ml. These results suggest that protodioscin isomers are the causative compounds of the inhibitory effects on growth of E. heterophylla. These compounds are possibly involved in the reported ability of B. ruziziensis in reducing the emergence and the growth of weed species when used as a cover plants. This finding confirms the potential of the utilization of B. ruziziensis for weed management options in the field.