*2.4. Determination of Biomass*

Thirty days after the treatment, the peach seedlings were uprooted, washed, and dried off with an absorbent paper. Then, the fresh weights of the aboveground and belowground parts were measured. The dry weights of the aboveground and belowground parts were measured after drying the samples in an oven at 80 ◦C for 30 min and at 60 ◦C to a constant weight.

#### *2.5. Analysis of Leaf Ultrastructure*

On the sixth day after treatment, mesophyll tissue (2 mm × 2 mm) was cut with a blade (avoiding the vein), immersed in 2.5% glutaraldehyde, vacuum-dried, and stored at 4 ◦C for 24 h. The samples were dehydrated with alcohol, soaked in resin, and heated. The sections were stained and observed under an electron microscope (JEOLTEM-1230EX, Tokyo, Japan) [20].

#### *2.6. Determination of Cu Element Contents*

On the sixth day after treatment, the powdered peach seedling sample was digested with a combination of nitric acid and perchloric acid, boiled to a constant volume, and filtered through a filter membrane (0.22 μm). The Cu element content in the final sample was analyzed with an atomic absorption spectrometer [21].

#### *2.7. Determination of Evans Blue Assay and Reactive ROS*

On the sixth day after treatment, the leaf blades and roots were cleaned with ultrapure water, dried with an absorbent paper, and soaked in an Evans blue dye solution (0.25%) for 24 h. The stained leaf blades were washed with ultrapure water, dried, and boiled in a mixture of anhydrous ethanol and glycerin (9:1) to remove the chlorophyll entirely. Images were captured. Then, the cell viability of the roots was quantified with Evans blue using a standard curve. Fluorescence was measured at 600 nm [22].

The production rate of superoxide radicals (O2 •−) and the content of hydrogen peroxide (H2O2) were determined as described previously [23]. The production rate of O2 •− was determined by analyzing the formation of nitrite (NO2-) from hydroxylamine in the presence of O2 •−. The leaf sample was homogenized and centrifuged. The supernatant was mixed and incubated. The absorbance of the supernatant was read at a wavelength of 530 nm, and a standard curve with NO2- was used to calculate the production rate of O2 •−. The content of H2O2 was determined by monitoring the formation of the hydrogen peroxide–titanium complex. The sample was homogenized with cold acetone. Then, a titanium reagent (15% Ti(SO4)2) was added to a final concentration of 4%. To precipitate the peroxide–titanium complex, 0.2 mL of concentrated NH4OH was added per 1 mL of the reaction mixture. After centrifugation (5 min at 10,000× *g*), the pellet was washed twice with acetone and solubilized in 2 mL of 2 N H2SO4. The absorbance of the solution was measured at 410 nm.
