We investigated potential alleviation effects of homobrassinosteroid (HBR) hormone on sunflower (Helianthus annuus L. cv. TR003) grown under copper (Cu) stress conditions. Seeds were grown under Cu (30 mu M and 40 mu M) and also HBR (2 mu M) treatments to analyse primary root lengths, protein contents, antioxidant system enzymes' activities (superoxide dismutase-SOD and catalase-CAT) and expressions of these enzymes at 48 h and 72 h. When compared to control, Cu applications decreased primary root lengths. On the other hand, HBR application increased primary root lengths of Cu-stressed samples at 48 h and 72 h. Depending on concentration and timing, protein contents and enzyme activities showed varying results in Cu-treated and Cu+HBR-applied samples. In addition to enzyme activities, SOD and CAT expression profiles were also investigated by RT-PCR. Expected band sizes for SOD (309 bp) and CAT (248 bp) were observed in all samples. These results indicate a positive role of HBR on Cu-stressed sunflower for agricultural applications.