Abstract <p>Mechanical stresses such as intermittent bending due to flexure wood formation make substantial losses to poplar (<i>Populus</i>) cultivation. In this work, biochemical changes in two-year-old seedlings of <i>Populus</i> <i>alba</i> along with formation of flexure wood under intermittent bending, NPK fertilizer and gibberellin 3 (GA) treatments in field conditions were studied. Poplars treated with intermittent bending often had a higher content of cellulose and lower content of hemicellulose and lignin than straight plants particularly in simultaneous application of NPK and GA fertilizers. A high degree of crystallinity was accompanied with an increase in the cellulose, indicating flexure wood formation in intermittent bent plants. Compared to straight plants, the enhanced activity of superoxide dismutase, catalase, guaiacol peroxidase and ascorbate peroxidase was observed in intermittent bending treatment and this increase was more in plants under simultaneous application NPK and GA. Higher defense responses were accompanied with a decrease in total soluble protein content in intermittent bending treatment, and this decrease was more in plants under simultaneous application NPK and GA. Under such a status, the effect of intermittent bending was much greater than the effect of NPK and GA fertilizers. Although high temperatures in August enhanced antioxidative activities, the increase in defense responses was more pronounced under intermittent bending treatment than under summer heat conditions. Significant correlation analysis showed simultaneous responses among chemical and biochemical characters to cope against stress. Results suggest that inducing flexure wood formation following intermittent bending was related to flexibility and rapid antioxidative responses over long-term structural reinforcement (e.g., lignin deposition) under field conditions.</p>