Abstract
l-DOPA (l-3,4-dihydroxyphenylalanine) is a bioactive secondary metabolite which inhibits growth of many weed species. However, its mode of action is not well elucidated in plants. The present studies were conducted to evaluate the effect of l-DOPA on root growth of cucumber (Cucumis sativus L.) and the possible involvement of quinoproteins (quinone-incorporated proteins) in phytotoxicity of l-DOPA. The results revealed that l-DOPA significantly inhibited root growth, induced cell death, increased polyphenol oxidase (PPO) activity, reduced free cysteine content, and enhanced quinoprotein formation in cucumber roots. The decrease in free cysteine content and increase in PPO activity suggested that quinones covalently bind with cysteine to form quinoproteins. The quinoproteins may be involved in phytotoxic action of l-DOPA in cucumber roots.
