However, the non-significant differences in Zn concentrations of grain and shoot between the N2 and N3 (Table 1 and Table 2) suggest that the N2 treatment has already provided sufficient Zn to satisfy plant Zn nutrition. The lack of greater shoot Zn concentrations with increasing N supplies from N2 and N3 treatments may be due to Zn detoxification by the plant, thereby excluding Zn from the shoot. AtMTP3, predominantly expressed in roots, is thought to be a key protein for plant metal tolerance.