Evaluating the phytotoxicities of two Irish red seaweeds against common weed species

Synthetic chemical herbicides available to farmers and foresters are known to be hazardous, and their overuse, without possible alternatives, have resulted in some weed species developing resistance. Therefore, seaweed species were explored due to their known bioactive capability to see if they would potentially be effective as an alternative, novel bioherbicide. From a phytotoxic screen against Lactuca sativa (lettuce) seeds, the ethyl acetate extract of two Rhodophyta species, Mastocarpus stellatus (MEE) and Porphyra dioica (PEE) were found to be most active in reducing lettuce seedling growth. Through conducting pre- and post-plant emergence assays in lab trials, the phytotoxicities of both extracts were further evaluated against weed species: broad-leaf weed, Trifolium repens (white clover) and grass, Lolium multiflorum (Italian ryegrass). At 5 mg mL⁻¹, MEE produced stronger pre-emergence phytotoxicities than PEE, significantly inhibiting seed germination and seedling growth of white clover by 77.3% and 97.2%, respectively, compared to the solvent control. For ryegrass seeds, comparing the phytotoxicities of both extracts to the solvent control, MEE inhibited germination and seedling growth by 30.2% and 66.8%, respectively, whereas PEE inhibited seedling growth by 21.1%, but had no inhibitory effect on germination. On the other hand, the post-emergence assay revealed stronger phytotoxic activities for PEE at the same concentration (5 mg mL⁻¹). The overall growth of ryegrass and white clover plants, respectively, were reduced by 42.7% and 35.6%, due to PEE treatment, and 13.5% and 30.0%, due to MEE treatment, in comparison to the solvent control. MEE and PEE were found to contain significantly higher quantities of flavonoids compared to the more polar extracts of the same seaweeds, but it is unlikely that these mostly water-insoluble flavonoids played a major role in the observed phytotoxic effects. These findings are indicative of the presence of phytotoxins in MEE and PEE, and could possibly lead to the development of novel, greener and sustainable bioherbicide sources, to replace or augment synthetic herbicides.