As shown in Figure 2 and Supplemental Figures S1 and S2, an increase in Pi application limited photosynthesis and reduced the PET chain in the leaves. These results implied that ROS production is stimulated under P toxicity conditions (Cakmak, 2005; Takagi et al., 2016a). Based on this consideration, the ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities were measured as key candidates for ROS-scavenging enzymes. The total APX activities, including the cytosolic and chloroplastic APX activities, tended to be higher in the 2.4 and 3.0 mM Pi treatments than in the other Pi treatments (Figure 5a). The cytosolic APX activity did not differ among the Pi treatments, but the chloroplastic APX activity was significantly increased in the 3.0 mM Pi treatments (Figure 5a). The increase in chloroplastic APX activity in the 3.0 mM Pi treatments was also observed on the leaf Chl content basis (Figure 5b). In contrast, a higher Pi application significantly decreased the total SOD activities, including the Fe-, Mn-, and Cu/Zn-SOD activities (Figure 5c). A decrease in the total SOD activities under excessive Pi application was also reported by Cakmak and Marschner (1987). The decrease in total SOD activities was due to the significant decrease in the Cu/Zn-SOD activity rather than a decrease in the Fe-, Mn-SOD activities (Figure 5c).