It is widely accepted that elevated levels of surface ozone (O3) negatively affect plants. Ethylenediurea (EDU) is a synthetic substance which effectively protects plants against O3-caused phytotoxicity. Among other questions, the one still open is: which EDU application method is more appropriate for treating fast-growing tree species. The main aims of this study were: (i) to test if chronic exposure of Salix sachalinensis plants to 200–400 mg EDU L− 1, the usually applied range for protection against O3 phytotoxicity, is beneficial to plants; (ii) to evaluate the effects of chronic exposure to elevated O3 on S. sachalinensis; (iii) to assess the efficacy of two methods (i.e. soil drench and foliar spray) of EDU application to plants; (iv) to investigate the appropriate concentration of EDU to protect against elevated O3-induced damage in S. sachalinensis; and (v) to compare the two methods of EDU application in terms of effectiveness and EDU consumption. Current-year cuttings grown in infertile soil free from organic matter were exposed either to low ambient O3 (AOZ, 10-h ≈ 28.3 nmol mol− 1) or to elevated O3 (EOZ, 10-h ≈ 65.8 nmol mol− 1) levels during daylight hours. Over the growing season, plants were treated every nine days with 200 mL soil drench of 0, 200 or 400 mg EDU L− 1 or with foliar spray of 0, 200 or 400 mg EDU L− 1 (in two separate experiments). We found that EDU per se had no effects on plants exposed to AOZ. EOZ practically significantly injured S. sachalinensis plants, and the impact was indifferent between the experiments. EDU did not protect plants against EOZ impact when applied as soil drench but it did protect them when applied as 200–400 mg L− 1 foliar spray. We conclude that EDU may be more effective against O3 phytotoxicity to fast-growing species when applied as a spray than when applied as a drench.
Keymessage: Soil-drenched EDU was ineffective in protecting willow plants against O3-induced injury, whereas foliar-sprayed EDU was effective even at the concentration of 200 mg L− 1.