Xylem development, cadmium bioconcentration, and antioxidant defense in Populus × euramericana stems under combined conditions of nitrogen and cadmium

•Xylem development was suppressed by combination of N deficiency and Cd exposure.•Phytohormones and antioxidants decreased due to poor N nutritional status in stem.•Adequate N application promoted antioxidant defense and Cd tolerance in stems.•The suppression on xylem development was partially alleviated by N application.•N icnreased stem Cd accumulation via promoting Cd tolerance and xylem development.