Secreted NA forms stable complexes with Zn(II). A, Metal-NA speciation analysis in A. halleri roots. B, Relative abundances of the identified complexes in A. halleri roots were obtained with normalized peak areas of the complexes using an internal control. C, Root exudates collected from the control (c) treatment revealed Zn(II)-NA and Fe(II)-NA complexes at 364.049 and 356.055 m/z, respectively, in UPLC-ESI-Q-TOF-MS in ESI negative mode. D, Relative abundances of the identified complexes in A. halleri root exudates were obtained with normalized peak areas of the complexes using an internal control. Leu enkephalin (0.2 ppm) was used as an internal control. Samples were collected from the control (c), iron deficiency (−Fe), and excess zinc (+Zn) treatments for 12 d. Data are the mean ± sd of four replicates with three technical repeats. Asterisks denote significant differences compared with the control treatment. ***P < 0.001 by the Student’s t test.