Germination growth response of different plant species to the allelochemical L-3,4-dihydroxyphenylalanine (L-DOPA)

The aim of this study was to investigate the seed germination response of different plant families to L-3,4-dihydroxyphenylalanine (L-DOPA), one of the strongest allelochemicals in nature. Three types of responses in terms of colouration changes on filter paper were obtained; black and gray (Gramineae and Compositae), no change (Leguminosae, Brassicaceae, and Cucurbitaceae) and an obstructed-circle around the seeds with black colouration on the outer side of the circle (Hydrophyllaceae) when L-DOPA solution was applied during seed germination. Radicle growth in the Gramineae and Leguminosae families was inhibited less by a single treatment of L-DOPA solution (250 g/ml) than in the other families. However, continuous treatment with L-DOPA demonstrated that the Gramineae family was less affected in terms of the inhibition of radicle growth than the Leguminosae family. When more seeds were added to the L-DOPA solution less inhibition of radicle growth was observed in all plants tested. The EC₅₀ of L-DOPA for bluebell (Hydrophyllaceae), white clover (Leguminosae), and lettuce (Compositae) was approximately 200, 100, and 50 g/ml, respectively. However, in perennial ryegrass (Gramineae) no EC₅₀ was observed even at 250 g/ml L-DOPA. In the Gramineae family, addition of more seeds into the L-DOPA solution increased the colouration on the filter paper. These results demonstrated that each seed functions to oxidize or dissolve L-DOPA. In the Gramineae, Leguminosae, Compositae, and Hydrophyllaceae, increasing the number of seeds imbibed in the L-DOPA solution increased the rate of L-DOPA disappearance from the petri-dish. Of the Grammaceous plants tested, only perennial ryegrass, which showed fairly weak allelopathic activity, metabolised L-DOPA to dopamine. Although the relationships between the changes in colouration of the filter paper and the inhibition of radicle growth in these experiments are still unknown, there appears to be a strong response in each species to protect the cell from L-DOPA damage.