*3.3. Cd Accumulation*

The concentrations of PTEs in the lettuce and pak-choi at three different growth periods (D35, D42, and D49) are shown in Figures 1 and 2. The Cd concentrations in the roots of LSC cultivars grown in CK for 35–49 days were in the range of 3.1–11.0 mg/kg, which were 1.4 to 6.4 times higher than that accumulated in the roots of LSS (Figure 1). In the shoots of different growing periods of lettuce, LSC also accumulated higher concentrations of Cd compared with LSS. The Cd concentrations in the shoots of LSC and LSS grown in CK of D35–D49 were in the ranges of 2.1–6.9 and 1.8–3.1 mg/kg, respectively. The LSC accumulated higher concentrations of Cd both in roots and shoots compared with LSS. Relative to LSS, the accumulated Cd concentrations in the edible parts of LSC were 1.7–3.5 times higher, which revealed that LSC can be regarded as a high-Cd-accumulation cultivar of lettuce. The PCW cultivar accumulated higher concentrations of Cd in the roots and shoots in comparison with PCC in general (Figure 2). The accumulated Cd concentrations in the roots of PCW and PCC growing for D35–D49 were in the ranges of 6.6–32.3 and 2.9–12.7 mg/kg, respectively. In the shoots of PCW and PCC, the Cd concentrations of D35–D49 were 3.4–8.9 and 3.4–7.8 mg/kg, respectively.

**Figure 1.** Effect of husk biochar treatment on the Cd accumulation in the roots and shoots of lettuce. The meanings of abbreviations are the same as in Table 1. The same lowercase letter indicates no significant difference between treatments for the same growth period.

**Figure 2.** Effect of husk biochar treatment on the Cd accumulation in the roots and shoots of pak-choi. The meanings of abbreviations are the same as in Table 1. The same lowercase letter indicates no significant difference between treatments for the same growth period.

Except for LSS-D35, in comparison with CK, the BC treatments decreased or significantly (*p* < 0.05) decreased the accumulation of Cd in the roots and shoots of lettuce (Figure 1) and pak-choi (Figure 2). This phenomenon was especially true for the LSC variety, which had high accumulating capacity compared with LSS. Relative to CK, the Cd concentrations accumulated in the roots and shoots of LSC under BC treatments decreased 50–64% and 18–52%, respectively, in comparison with CK. Whether treating with BC or not, higher Cd concentrations were found in the roots and shoots of pak-choi compared with lettuce at different growth periods. Under the BC treatments, most of the Cd concentrations in the roots and shoots of pak-choi decreased 24–77% and 16–60%, respectively, in comparison with CK.
