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Retardation of Senescence in Red Clover Leaf Discs by a New Antiozonant, N-[2-(2-Oxo-1-imidazolidinyl)ethyl]-N'-phenylurea ¹

Plant Stress Laboratory, Plant Physiology Institute, Beltsville, Maryland 20705, United States Department of Agriculture, Science and Education Administration-Agricultural Research, Beltsville, Maryland 20705

Dark-induced senescence in leaf discs from O₃-sensitive red clover trifoliates (Trifolium pratense L. cv. `Pennscott') was markedly retarded by treatment with N-[2-(2-oxo-1-imidazolidinyl)ethyl-N'-phenylurea (EDU). EDU also protects against acute and chronic foliar O₃ injury when sprayed on intact leaves or supplied to the plants through soil application. Senescence retardation was measured by time-dependent analyses of chlorophyll, protein, and RNA in discs floated on aqueous EDU solutions ranging from 0 to 500 micrograms per milliliter EDU. Chlorophyll degradation, total protein, and nucleic acids were followed over 10-day test periods.

EDU at 500 micrograms per milliliter (50 milligrams per pot), a concentration known to provide optimal protection to intact leaves against O₃ injury, was most effective in preventing chlorosis and in maintaining high concentrations of protein and RNA in the discs. In discs treated with 500 micrograms per milliliter EDU 90% of the chlorophyll was retained after 10 days in the dark. In contrast, lower concentrations (0, 125, and 250 micrograms per milliliter) showed the complete loss of chlorophyll or an intermediate retardation. The intermediate concentrations were similarly less effective in maintaining protein and RNA levels in the dark stressed leaf discs. It is suggested that EDU retards senescence and mitigates O₃ injury through the induction of specific free radical scavenging enzymes and in sustaining RNA and protein synthesis.

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