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

Abstract

Populus species feature remarkable phytoremediation of cadmium (Cd) contamination. The influence of nitrogen (N) availability on Cd accumulation and tolerance in stems was investigated in a clone of Populus × euramericana. Secondary xylem development in stems was dramatically suppressed by the combined conditions of N deficiency and Cd exposure, but such suppression was partially alleviated by N application. Under adequate N condition, the Cd bioconcentration factor (BCF) in stems at moderate and high Cd exposures reached 3.07 and 0.91, respectively, whereas the Cd content in stems at high Cd exposure reached 164 μg Cd plant−1. However, Cd bioconcentration and accumulation in stems were sharply decreased due to N deficiency. The levels of phytohormones (abscisic acid, jasmonic acid, and salicylic acid) and antioxidants (free proline and peroxidase) in stems decreased in low N treatment because of poor tissue N concentration, thereby attenuating antioxidant defense and Cd tolerance. The decreased Cd tolerance and inhibited xylem development under low N condition led to the dramatic decrement of Cd accumulation in stems. Adequate N application increased the phytohormone and antioxidant levels in stems, thereby promoting antioxidant defense, as indicated by the low malondialdehyde (MDA) level. Moreover, both the improved antioxidant defense and appropriate N nutrient status contributed to xylem development. Consequently, the capacity for Cd bioconcentration and accumulation in Populus stems was promoted by N application, thereby facilitating the phytoextraction of Cd from Cd-contaminated soils.