## 3 Results
### Amaranth Biomass Affected by the Combined Pollutants of Cd and BDE-153  
The changes in the fresh weight of amaranth under the combined effect of Cd and BDE-153 are shown in Figure S1. It can be seen from the figure that the fresh weight of amaranth under the condition of single Cd pollution, with an increase in the Cd concentration, increased first and then decreased. The variation in the amaranth's fresh weight ranged from 8.82 to 13.96 g. For BDE-153 pollution alone, an increase in the BDE-153 concentration had no significant effect on the fresh weight of amaranth. Studies have shown that the proliferation activity of stem cells will be enhanced when stimulated by oxidative stress [32], DNA damage [33], and heat stress [34], so our previous studies suggested that this may be due to the anti-oxidative stress of plants at low concentrations of BDE-153 [3]. In addition, BDE-153 is difficult for amaranth to absorb due to its hydrophobicity, which may also be one of the reasons why BDE-153 has less effect on the fresh weight of amaranth [35].  
Figure 3: Changes in Cd and BDE–153 concentrations in amaranth roots, stems, and leaves over time under combined pollution conditions. (**a**,**b**) represent the concentration changes in Cd and BDE–153 in amaranth, respectively. (**c**,**d**) represent the logarithmic growth rates of Cd and BDE–153 in amaranth, respectively.  
Under the composite pollutant condition, when the Cd concentration reached the highest concentration of 5 mg/L (Cd\({}_{3}\)), the biomass of the amaranth decreased significantly with the increase in BDE-153 concentration (Figure S1). This may be due to the fact that the high concentration of Cd disrupted the plasma membrane structure of the amaranth root system, making it easier for BDE-153 to enter the cells and increasing the toxicity of the composite pollutant in the amaranth. In addition, under the condition of combined pollution, the fresh weight of the amaranth increased first and then decreased, which was the same as the results under single Cd pollution, indicating that a low concentration of Cd (0.1 mg/L) could stimulate the growth of the amaranth. When the concentration of Cd increased to 1 mg/L and 5 mg/L, the toxic effect of the Cd on the amaranth increased, resulting in a significant decrease in the biomass of the amaranth.  
The synergistic and antagonistic effects of organic matter and heavy metals on plants have been reported in some studies. For example, it has been reported that combined pollution of low concentrations of PAH (50 + 50 mg/kg with phenanthrene + pyrene in 1:1 proportion) and Cd (10 or 50 mg/kg) can reduce the toxicity of Cd in Juncus subsecundus and promote its growth compared with Cd pollution alone [36]. In addition, studies have found that the germination rate of wheat under the combined pollution of lead (100 mg/L) and low amounts of fluoranthene (0.2 mg/L) is higher than that of low levels of fluoranthene (0.2 mg/L) pollution alone [37].  
However, from our present study, it was found that the synergistic and antagonistic effects of organic matter and heavy metals on the plants depend on the concentration of these pollutants. When the concentration of Cd was low, the addition of BDE-153 had little effect on the fresh weight of plants, while at high concentrations of Cd, BDE-153 showed a synergistic effect in its toxicity in the plants.