Spatial heterogeneity in copper distribution changed antioxidant capacity of container-grown Camphora officinarum roots under excessive copper stress.

Coating high concentrations of copper (Cu) on the inner wall of containers prevents root entanglement by inhibiting root tip elongation. However, it remains uncertain whether roots near and far from the container wall differentially absorb Cu, thereby triggering varied defense responses. Two-year-old Camphora officinarum were planted in containers coated with 120 (T1) and 200 (T2) g L Cu(OH) with latex as the carrier. After six months of treatment, obvious root entanglement in containers only coated with latex (T0) and control containers was observed, while no entanglement in T1 and T2. Cu(OH) treatment increased soil and root Cu concentration, which exhibited significant position differences (near-wall vs. far-wall) in T1 and T2. Root Cu concentration near the container wall in T1 and T2 was 3.5 and 3.0 times higher than that far from the container wall, and 19.3 and 32.1 times higher than that in the control. Oxidative stress biomarkers increased with increasing root Cu concentration with the highest levels near the container wall in T2. Excessive Cu increased antioxidant enzyme activities and non-enzymatic antioxidant contents with higher superoxide dismutase, glutathione reductase, glutathione peroxidase, ascorbate peroxidase, ascorbate acid, and reduced glutathione in T1 and higher dehydroascorbate reductase and monodehydroascorbate in T2. Overall, antioxidant enzyme activity in roots was higher near the container wall in T1 while far from the container wall in T2. Excessive and uneven Cu distribution and oxidative stress biomarkers effectively inhibited peripheral root elongation and entanglement. During this process, although antioxidant defense responses were induced, defense capacity was impaired by supra-optimal Cu.