In this study, the generation patterns of endogenous nitric oxide (NO) and reactive oxygen species (ROS) and the changes in root activity of Bidens pilosa
L. seedlings under Pb stress were investigated, with the ameliorating effects of NO on oxidative damage also analyzed. Results showed that the content of NO in the leaves increased with the increase in Pb concentration within the range of 0-1000 mg/L, and the content of NO in the roots first increased and then decreased but was still higher than that in the control. In the Pb concentration range of 0-400 mg/L, the content of NO in the stem was the same as that of the control, but was lower than that of the control when the concentration was greater than 600 mg/L. At a concentration of 600 mg/L, Pb significantly increased NO synthase (NOS) and nitrate reductase (NR) activities in the roots, stems, and leaves and increased nitrite ion (NO2-
) and carotenoid (Car) content in the stems and leaves. NOS, NR, NO2-
, and Car significantly influenced NO content in the leaves and NOS was the major pathway for the formation of endogenous NO in the roots. Pb stress significantly increased the superoxide anion (O2·-
) production rate, hydrogen peroxide (H2
) concentration, malondialdehyde (MDA) content, and relative electrical conductivity (REC), resulting in severe membrane lipid peroxidation injury in the B. pilosa
L. seedlings. However, NO induced by Pb stress may have reduced the production of ROS in the roots, promoted root activity in seedlings, and relieved membrane lipid peroxidation damage caused by stress.