# The Accumulation Characteristics of Different Heavy Metals in Sea Rice
## 4 Discussion  
Plant phytoremediation can be categorized as plant extraction, plant volatilization, plant stabilization, and root filtration [24]. Plant extraction is based on the concept of hyperaccumulator plants, which transfer and store heavy metals from soil in the shoot parts of plants, and allows remediation of heavy metal-contaminated soil. The general criteria for hyperaccumulator plants are as follows [25]: (1) Zn content \(\geq\) 10,000 mg/kg; Pb, Cu, and As content, \(\geq\)1000 mg/kg; and Cd content \(\geq\)100 mg/kg; (2) the content of heavy metals in the upper part of the plant is higher than that in the shoot part; and (3) the transport coefficient is >1, and the enrichment coefficient is >1. Heavy metal uptake and accumulation in rice were mainly affected by rice varieties and field environmental conditions (heavy metal bioavailability was affected by agronomic management measures such as water, fertilizer, and conditioner). The root growth of HH12 was inhibited under single Cd, Zn, and Cu stress. The root biomass decreased significantly with an increase in stress, which was due to the toxic effect of heavy metals on the root tip mitosis index and root length [22; 26]. However, the growth of shoots and spikes of HH12 was slightly inhibited by the stress of 1-2 mg/kg Cd but promoted at a high concentration of 8 mg/kg. [PERSON] et al. [27] reported that the rice cultivar Xiangwanxian 12 experienced an inhibitory effect when planted in the soil with 3.7 mg/kg Cd. [PERSON] [28] demonstrated that Cd stress of 0.2-1 mg/kg inhibits the growth of Zhongjiazao 17 and Taiyou 390. A possible reason for different experimental results is the variation in the Cd tolerance of rice varieties. Sea rice HH12 may be more tolerant than the aforementioned three rice varieties. [PERSON] [29] showed that 100-300 mg/kg Zn treatment could significantly promote rice growth, which was consistent with the present study results. Studies by [PERSON] [30] and [PERSON] [31] have shown that Cu stress of 400-1000 mg/kg significantly inhibits rice growth. In this study, the growth of HH12 was severely inhibited or even HH12 died with the 400 mg/kg Cu treatment. According to heavy metal absorption of HH12, a positive correlation was observed between the Cd content in different HH12 parts and the soil Cd content. The Cd contents of shoot and spikes of HH12 were the highest at 8 mg/kg Cd, which were 89.02 and 45.37 mg/kg, respectively. These content levels did not meet the content standard of Cd-enriched plants. However, when the Cd concentration was \(\leq\)2 mg/kg, Cd distribution in the plant body was in the order of shoot > root > spike. This means the transport coefficient of Cd in the shoot was >1. At the 1 mg/kg concentration, the transport coefficient of Cd in the shoot was as high as 1.36. This was possibly because of the high cumulative risk of HH12 under low Cd pollution. According to [PERSON]'s [22] comparative analysis of Cd subcellular distribution between low and high Cd accumulation rice varieties and HH12, sea rice exhibited higher Cd accumulation characteristics than the other two rice varieties. However, Cu distribution in HH12 was in the order of root > shoot > spike. At 200 mg/kg Cu, the Cu contents of shoot and spike were 82.85 and 46.97 mg/kg, respectively, which did not meet the content standard of Cu-enriched plants. The Cu transport capacity of HH12 was in the order of spike > shoot. Under the Cu stress of 100-300 mg/kg, the Cu transport coefficients of shoots were all <0.5 and decreased as the Cu concentration increased. This may be explained by the stronger Cu fixation ability of the root cell wall and increased with an increase in stress to inhibit Cu migration to protoplasts [32]. Zn distribution in HH12