Absorption, translocation and metabolism of -DOPA in barnyardgrass and lettuce: their involvement in species-selective phytotoxic action

-DOPA (-3,4-Dihydroxyphenylalanine) is one of the most highly active allelochemicals. -DOPA is exuded from the roots of velvetbean Mucuna pruriens (L.) DC. var. utilis] into soil, and causes growth inhibition of other species. In order to highly clarify the phytotoxic mechanism of -DOPA, its effect on 32 species was surveyed, and absorption, translocation and metabolism in a tolerant species, barnyardgrass (Echinochloa crus-galli L.), and susceptible species, lettuce (Lactuca sativa L. cv. Great lakes 366), were examined at the germination stage. The species tested showed distinctly different responses to -DOPA, with root elongation being more suppressed than that of shoots. Barnyardgrass was 77-fold more tolerant than lettuce based on the GR₅₀ values determined 5 days after treatment. Absorption of -DOPA in barnyardgrass and lettuce increased continuously during a 5 day exposure period, however,barnyardgrass absorbed a larger amount of -DOPA than lettuce. The translocation of radioactivity derived from ¹⁴C--DOPA to shoots was greater in lettuce than in barnyardgrass 3 and 5 days after treatment. Although the -DOPA absorbed was metabolized in the roots of both species, the percentage of radioactive ¹⁴C--DOPA increased in lettuce continuously but decreased in barnyardgrass over 5 days. In lettuce roots, a continuous increase of -DOPA but not other metabolites was observed. However, the concentration of -DOPA was higher in barnyardgrass roots compared with lettuce roots throughout the exposure period. These results suggest that -DOPA itself is the active form, and the species-selective phytotoxicity of -DOPA is at least partly due to metabolism and not due to absorption or translocation.