Mechanisms of ethylenediurea (EDU) induced ozone protection: Reexamination of free radical scavenger systems in snap bean

Ethylenediurea (EDU), N[2-(2-oxo-l-imidazolidinyl) ethyl]-N'-phenylurea is known to prevent ozone (03) damage to leaf tissues. However, the mechanisms of protection are unclear. We tested the hypothesis that EDU protects against 03 damage by scavenging hydroxyl free radicals (OH). An in vitro study involving the use of high-performance liquid chromatography equipped with an electrochemical detector (HPLC-EC) showed that EDU does not serve as an antioxidant to remove OH free radicals. Effects of 03 and EDU (soil drench) on leaf antioxidant scavenger systems (AOSS) were also studied. The first fully expanded trifoliate leaves of O3-sensitive snap bean (Phaseolus vulgaris cv. Bush Blue Lake 290) was examined. Measurements were made before and after a single 03 exposure (0.30 #11-1 03 for 3 h). Pretreatment with EDU 48 h before exposure protected against O3-induced necrosis and chlorosis. EDU pretreatments did not alter superoxide dismutase (SOD), guaiacol-peroxidase (GPX), ascorbate peroxidase (APX) and glutathione reductase (GR) activities. However, O3-fumigated plants (no EDU) showed elevated SOD activity with decreased GR activity. EDU-treated plants exposed to 03 stress showed no measurable loss of GR activity. These tissues maintained high levels of total glutathione [i.e. reduced glutathione (GSH)+ oxidized glutathione (GSSG)] contents, and had higher GSH/GSSG ratios than the controls at the end of 3 h exposure to 03. These data suggest that EDU protection against 03 damage in plants do not necessarily involve the direct stimulation or induction of antioxidative enzyme defense mechanisms. Instead, protection may result from a more general retention of chlorophyll and maintenance of GR and GSH levels during 03 exposure. © 1997 Elsevier Science B.V.