Differences of Cadmium Extraction Characteristics between Sedum alfredii H. and Sedum plumbizincicola

doi:10.21203/rs.3.rs-3229569/v1

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Differences of Cadmium Extraction Characteristics between Sedum alfredii H. and Sedum plumbizincicola

https://doi.org/10.21203/rs.3.rs-3229569/v1

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[Aims & Methods] In order to compare the differences of cadmium (Cd) extraction characteristics between Sedum alfredii H (S.A.) and Sedum plumbizincicola (S.P.), a two-years field experiment was conducted in Cd-contaminated soil, plant’s phenophase and Cd extraction efficiency were studied.

[Results] Cultivation trials revealed that S.A.'s phenophase commenced about 20 days prior to S.P.'s. The shoots Cd content of S.A. increased rapidly at first, slowly at vigorous growing stage; the root and stem had the lowest and highest contribution rate of Cd extraction, respectively. In contrast, S.P.'s shoot Cd content steadily increased with leaf Cd extraction contributions surpassing stems, flowers, and roots. Under identical conditions, the shoots dry biomass, Cd content, bioconcentration factors and theoretical Cd extraction amount of S.P. were significantly higher than those of S.A. However, S.A.'s faster radication, superior survival rate, and lower disease susceptibility resulted in its measured Cd extraction ranging from 173.58 -174.00 g ha^-1, while that of S.P. was 137.71-199.91 g ha^-1 with significant inter-annual differences. The optimal harvest time with the highest Cd extraction efficiency of S.A. and S.P. was the early final flower period (early May and mid-May) and the entire final flower period (early June), respectively.

[Conclusions] In conclusion, both S.A. and S.P. have good phytoremediation effects on Cd-contaminated soil, and the phytoremediation potential of S.P. is greater than that of S.A. Proper cultivation measures can promote the improvement of phytoremediation efficiency. This study provides theoretical and technical support for the phytoremediation of Cd-contaminated soil by these two Sedum species.

Sedum alfredii H.

Sedum plumbizincicola

Cd-contaminated soil

Cd extraction

With industrial development, soil pollution caused by heavy metals has become increasingly prominent, and its potential threat to human health has attracted the attention of countries around the world (Rascio and Navari-Izzo, 2011; Wuana and Okieimen, 2011). Phytoremediation technology is highly regarded for its cost-effectiveness and environmental friendliness, and the use of hyperaccumulator plant to extract and remove heavy metals has been widely applied worldwide (Rascio and Navari-Izzo, 2011; Chen and Li, 2018; Huang et al., 2020). Cadmium (Cd) is a highly toxic, non-degradable, and bioaccumulative heavy metal, Cd pollution ranks first among major inorganic pollutants in China, with a point source exceeding rate of 7.0% (Huang et al., 2017). Sedum alfredii H. (S.A.) (Ni and Wei, 2003) and Sedum plumbizincicola (S.P.) (Wu et al., 2006) are Cd and Zn hyperaccumulators with advantages such as large biomass, wide distribution, multiple harvests, barren and drought resistance. They are ideal materials for studying the hyperaccumulation mechanism, detoxification mechanism, and practical research of phytoremediation in farmland (Huang et al., 2020; Peng et al., 2020). Under natural conditions, the shoots Cd content of S.P. accounts for 0.04% of the biomass (Peng et al., 2017a), and under hydroponic conditions, the Cd bioconcentration factors (BCF) of S.P. and S.A. can reach 150.6 (Li et al., 2010) and 22.5 (Yang et al., 2004), respectively. Previous studies have found that multiple plantings do not affect the extraction efficiency of Cd by Sedum plumbizincicola, and the more polluted and fertile the soil, the higher the Cd extraction efficiency of S.P. (Zhou et al., 2018; Fan et al., 2019). The addition of organic acids (Bai et al., 2020) and bamboo shoots shells (Wang et al., 2022) to the soil can effectively increase the Cd extraction efficiency of S.P. and S.A., and exogenous spraying of appropriate concentrations of ABA (Lu et al., 2020) and IAA (Chen et al., 2021b) can also increase the Cd extraction efficiency of S.A. In addition, shading (Chen et al., 2021a), mulching (Yin et al., 2019), or intercropping with maize (Chen et al., 2019), Excoecaria cochinchinensis (Guan et al., 2021), Trichosanthes kirilowii Maxim. (Li et al., 2022) are commonly used cultivation measures in production to improve extraction efficiency. However, different cultivation measures mean different extraction effects, and the two Sedum species respond differently to different cultivation measures (Dai et al., 2022). Studies have shown that after shading or intercropping with other crops, the photosynthesis rate, transpiration rate and water use efficiency of S.P. reduced with decreasing light intensity, leading to a decrease in the plant's ability to accumulate and transport Cd and Zn (Chen et al., 2019; Chen et al., 2021a). In contrast, the shoots biomass and Cd content of S.A. increased after intercropping with oilseed crops (Tong et al., 2021). Most of the current studies on the extraction effect of the two Sedum species are focused on one species, and there are few comparative studies on their extraction efficiency under the same environment (Zhu et al., 2019). Clarifying the growth and development rules of hyperaccumulator in field condition is the basis for research to improve their heavy metal removal efficiency. In this study, the growth and development processes and Cd accumulation and redistribution patterns of S.A. and S.P. were comparatively studied through field experiments, as well as the growth patterns of the two Sedum species, the differences in Cd accumulation characteristics of each organ at different developmental stages were clarified. This study will provide scientific basis for the application of phytoremediation technology in Cd contaminated farmland.

1.1 Overview of the Experimental Site

The experimental site is located in a fallow area of Hunan Province, China (113°03′26.30″E, 28°26′19.02″N). The area has a subtropical monsoon climate with distinct seasons, characterized by abundant heat, sufficient rainfall, and ample sunshine. The temperature ranges from 15/25℃ in spring to 18/36℃ in early autumn, and 5/15℃ from late autumn to winter. The annual precipitation ranges from 1000 mm to 1200 mm.

1.2 Experimental Design

The seedlings of S.A. and S.P. were obtained from the Breeding Base of Environmental Restoration Plant Seedlings of the Key Laboratory for Agro-Environment in Midstream of Yangtze Plain, Ministry of Agriculture and Rural Affairs, P.R.China. The quality of the seedlings reached the standards of Grade I seedlings (Dai et al., 2022). Both species were cultured and transplanted at the same time, with transplanting dates in November 2020 and 2021. The planting distance was 30cm × 10cm, with a density of 3.0 × 10⁵ plants/ha. Each species planted around 1500 m², and the field management measures were consistent. The soil properties of the test plots are shown in Table 1.

Table 1

Basic properties of soil in test plot
Years	pH	Total cadmium mg kg^− 1	Effective cadmium mg kg^− 1	Total nitrogen g kg^− 1	Total phosphorus g kg^− 1	Total Potassium g kg^− 1	Alkaline nitrogen mg kg^− 1	Available phosphorus mg kg^− 1	Available potassium mg kg^− 1	Organic matter g kg^− 1
202–2021	5.0	0.84	0.42	2.23	0.80	35.20	184.25	23.13	120.25	38.53
2021–2022	4.9	0.75	0.38	2.31	0.70	31.13	156.67	30.53	102.67	39.37

1.3 Sampling and processing

After transplantation, plant samples of each Sedum species were collected every month before the budding stage, and every half-month (in 2021) or every week (in 2022) starting from the budding stage, with no fewer than 10 plants per sample. The samples were washed with deionized water and separated into four (roots, stems, leaves and flowers) or two parts (flower branches and leaf branches). Flowering branches referred to the branches that grow independently from the stolon of S.A. or from the top of the main stem of Sedum plumbizincicola, flowering but without leaves. Leaf branches referred to other branches that have leaves. All samples were dried at 105℃ for 30min and at 75℃ until a constant weight was reached, and then weighed to calculate the biomass of each part. The samples were finely ground to pass through a 0.25mm sieve and stored in a sealed container for future use. Soi total Cd was extracted by microwave digestion with HNO₃-H₂O₂-HF, then the total Cd content in the solution was measured by ICP-MS and the available Cd content was measured by the DTPA extraction method. Other basic physical and chemical properties of the soil were determined according to the Soil Agricultural Chemistry Analysis Method. The total Cd in the plant samples was extracted with HNO₃-HClO₄, and measured by ICP-MS.

Phenological period observation methods. Seedling stage referred to the period from transplanting a plant to its rapid growth. Vigorous growing stage referred to the period from 50% of the plant growth rapidly in the field until the plant height no longer increases and the crown width no longer expands. Budding stage referred to the period from when 30% of the plants in the field develop flower buds and continues until all the buds have appeared and only a few flowers have bloomed. Initial flowering stage referred to the period from a few plants blooming to 30% of the plants blooming in the field. Full flowering stage referred to the period when 30% of the plants in the field bloom until all the flowers are in full bloom, and a small portion of the flowers begin to brown and wither. Flower withering stage was defined as more than 30% of petals in the field browning to wilting.

Yield measurement method. In this study, the harvest time in 2021 was obtained based on the experience of the previous authors (Zhu et al., 2019), and in order to determine the harvest time of the two Sedum species more precisely, the harvest time in 2022 was delayed by one month compared with 2021. The best harvest time for the two Sedum species was found to be the respective flower withering stage, so the yield measurement in 2021 was the last harvest (May 17, when S.A. had entered the terminal flowering stage, and S.P. was in full flowering stage), and the yield measurement in 2022 was the respective terminal flowering stage (May 25 for S.A. and June 17 for Sedum plumbizincicola).

1.4 Data processing and analysis

Cd extraction proportion of each organ (%) = [organ dry biomass (g) × Cd content of the organ (mg kg^− 1)] / [shoots dry biomass (g) × Cd content of shoots (mg kg^− 1)] × 100%.

Theoretical Cd extraction (g Ha^− 1) = shoots dry biomass (g plant^− 1) × Cd content of shoots (mg kg^− 1) × 300,000 (plants Ha^− 1) / 10⁶.

Measured Cd extraction (g Ha^− 1) = measured yield (kg Ha^− 1) × Cd content of shoots (mg kg^− 1) / 10⁶.

Theoretical yield (kg Ha^− 1) = plant dry biomass (g plant^− 1) × 300,000 (plants Ha^− 1) / 10³.

Measured yield (kg Ha^− 1) = shoots dry biomass per unit area (kg m^− 2) × 10,000.

The data obtained from the experiment were analyzed using Excel 2016 and SPSS 20.0 software, and Origin 2018 was used to create graphs. The least significant difference (LSD) method and T-Test were used to analyze the significant differences between the mean values.

2.1 Phenophase

When transplanted at the same time in winter, the seedling stage of S.A. was shorter than that of S.P., mainly manifested by the significant increase of shoots biomass of S.A. in early March, while that of S.P. appeared in late March, indicating that S.A. entered the vigorous growing stage earlier than that of S.P (Table 2). In addition, the duration of the vigorous growing stage of S.P. is shorter than that of S.A., which can last for about three months, while the vigorous growing stage of S.P. is less than two months. The budding stage of S.A. was from early to mid-April, and the flowering stage was from late April to late May, lasting about 1 month; the budding stage of S.P. was from late April to early May, and the flowering stage was from mid-May to mid-June, also lasting about 1 month. Compared with the S.P., the budding and flowering stages of S.A. were about 20 days earlier (Table 2). Due to inter-annual climate change, there were fluctuations of about 10 days in the phenological period of each Sedum.

Table 2

**Comparison of phenological periods between the two Sedum(MM-DD)**
Sedum	Seedling stage	Vigorous growing stage	Budding stage	Flowering stage
Sedum	Seedling stage	Vigorous growing stage	Budding stage	Initial flowering stage	Full flowering stage	Flowering withering stage
S.A.	Transplanting date-02/20	02/20 − 05/15	04/05 − 04/20	04/21 − 04/25	04/26 − 05/05	05/06 − 05/25
S.P.	Transplanting date − 03/20	04/05–05/20	04/25 − 05/10	05/11 − 05/14	05/15 − 05/25	05/26 − 06/15

2.2 Dynamics of Dry Biomass and Cd Content

In the practical application of Cd-contaminated farmland remediation using Sedum, only the shoots of the plants were harvested and removed because of their fine scattered root system, small root biomass and low root Cd removal. Therefore, this study mainly focused on the shoots of the plants. S.A. entered the vigorous growing stage in late March until mid-May when it entered the terminal flowering stage, marking the end of the vigorous growing stage. The shoots dry biomass of S.A. showed a trend of increasing and then decreasing, reaching a maximum at flower withering stage. After that the leaves began to senesce and drop in a large area with time, and the biomass gradually decreased. In contrast to S.A., the vigorous growing stage of S.P. began in early April and ended in late May, followed by a slow growth period; after the end of the flowering stage, S.P. did not show a large-scale leaf aging and falling phenomenon, so its dry biomass did not decrease. In this experiment, the harvest time was different for two years, which were May 17, 2021, and June 18, 2022. Comparing the shoots dry biomass of both Sedum species between the two years (Fig. 1), it was found that in 2021, only S.A. had entered the flower withering stage, and its dry biomass reached the maximum for that planting season, while S.P. had just entered the full flowering stage, and its dry biomass had a greater potential for increase. In 2022, the harvest period was delayed by one month, and it was clear that the plant of S.A. began to decline and its dry biomass decreased significantly after the flower withering stage, while the shoots dry biomass of S.P. remained at a relatively high level (Fig. 1).

The plants Cd content of both Sedum species gradually increased with planting time (Fig. 2). The Cd content of S.A. showed a trend of "rapid - slow - rapid growth", with the first rapid growth period occurring during the seedling stage. With the arrival of the growing season, the Cd content growth was no longer significant and continued until the budding stage. The second rapid growth period for Cd content of S.A. began from the initial flowering stage until the end of the vigorous growing stage. After the end of the flowering withering stage, the Cd content of S.A. no longer increased. The Cd content of S.P. increased continuously with planting time. During the harvesting period, the Cd content of S.P. was significantly higher than that of S.A.

2.3 Agricultural Traits and Cd Content of Organs

In the comparison of Cd content between S.A. and S.P in the two years, it was found that the Cd content of the harvested S.P. was significantly higher than that of the S.A. To analyze the reasons for the difference in Cd content between the two species of Sedum plants, the dry matter and Cd content of roots, stems, leaves, and flowers were further analyzed. Figure 3 shows the dry mass and Cd content of organs of the two Sedum species during the harvest period. There were no significant differences in the dry mass of roots, stems, leaves, and flowers, as well as the stem Cd content between the two Sedum species. However, the Cd content in the roots, leaves, and flowers of the S.P. was significantly higher than that of the S.A., with the differences in the Cd content of the leaves and flowers reaching extremely significant levels. The Cd content in the leaves and flowers of the S.P. was 6.2 and 1.5 times higher than that of the S.A., respectively. Since there were significant differences in the Cd content of the roots, leaves, and flowers of the Sedum species, the contribution rates of Cd extraction by the roots, leaves, and flowers also showed significant differences between the species, and all reached extremely significant levels. The contribution rate of Cd extraction by the root system of both Sedum species was the lowest, ranging from 1.23–1.56%, indicating that the aboveground part of Sedum is the main contributor to Cd extraction. Considering the difficulty of harvesting, the aboveground part of the plants should be mainly harvested during the harvesting period. The stem had the highest contribution rate to Cd extraction in the S.A. (68.13%), followed by the flower (19.91%) and the leaf (10.41%). For the S.P., the organ with the highest contribution rate to Cd extraction was the leaf (45.97%), followed by the stem (39.03%) and the flower (13.77%).

To investigate the causes of the differences in Cd extraction from each organ of the two Sedum species, the agronomic traits of the two Sedum plants were analyzed in this study. As shown in Fig. 3, there were no significant differences in the dry mass and Cd content of stem between S.A. and S.P., but there were extremely significant inter-species differences in leaves and flowers. The differentiation, attachment site, and morphology of the flowering and leaf branches were all different between the two species. Under the condition of one-year field cultivation, the main stem of S.A. during the seedling stage was an erect stem with attached leaf branches, and a large number of cluster buds were differentiated from the base of the stem. After entering the vigorous growing stage, these buds gradually developed into leaf branches (mostly) and flowering branches (fewer). As branching increased, the main stem gradually became prostrate and grew adventitious roots (Fig. 4). The number of leaf branches gradually increased with plant growth, ranging from a few to more than ten, and the number of leaf branches increased with better plant nutrition. The number of flowering branches was usually fewer than the number of leaf branches, and unlike leaf branches, the number of flowering branches did not increase after differentiation was completed. The difference in the number of flowering and leaf branches reached an extremely significant level with plant growth (Fig. 5). No leaves were attached to the flower branches, only inflorescences were differentiated at the top. After entering the budding stage, the dry mass of leaf branches of S.A. gradually increased with plant growth, reaching the maximum value at the end of flowering. Then the leaves on the lower end of leaf branches began to decline, and the leaves began to fall off in a large area until the leaves of some branches were completely detached, leaving only the bare stems. Although the number of leaf branches continued to increase at this time, their dry mass gradually decreased, and the plant gradually declined. This indicates that for S.A., the shoots biomass was the largest at the flower withering stage, which was the best time for harvesting.

Different from the stoloniferous main stem of S.A., the main stem of S.P. grew horizontally and was stronger (Fig. 4). The leaf branches were attached to the main stem, and their number and dry mass increased as the plant grows, while the main stem became stronger. Affected by the fleshy stem's folding characteristics, the main stem often broke as the dry mass of the leaf branches increases, resulting in a significant reduction in its biomass and harvest index. The top of the main stem differentiated into flowering branches, with 1–2 branches and no leaves, and the dry mass of the flowering branches reached its maximum during the full flowering stage and then gradually decreased.

After entering the flowering stage, the Cd content of the flowering and leaf branches of both Sedum species gradually increased with the planting time (Fig. 6). The Cd content of the flowering and leaf branches of S.A. reached its maximum value at the end of the flower withering stage, and then tended to be stable and no longer increased. Moreover, the Cd content of the flowering branches in S.A. was 1.11–1.69 times higher than that of the leaf branches. On the contrary, S.P. showed the opposite trend, and the Cd content of the leaf branches was 1.20–1.78 times that of the flowering branches. Interspecific comparison found that the Cd content in the leaf branches of S.P. was significantly higher than that of S.A. in the same sampling period.

The Cd extraction amount of flowering branches was higher than that of leaf branches in S.A. The Cd extraction amount of S.A. showed a trend of initially increasing and then decreasing with the planting time, and the highest Cd extraction amount occurred during the flower withering stage. Affected by the reduction of dry matter quality caused by leaf shedding and inflorescence decay, the maximum amount of Cd extraction lasted for about ten days, and then gradually decreased. For S.P., the Cd extraction amount increased gradually with the planting time, and the Cd extraction amount in the leaf branches were significantly higher than that in flowering branches during the same period. Furthermore, it was observed from Fig. 6 that the optimal harvest time for both species was during the flower withering stage. Although the Cd extraction amount of S.A. did not reach its maximum value during this period, its shoots biomass was the largest, which led to the highest Cd extraction amount. The Cd extraction amount and shoots biomass of S.P. continued to increase during the flower withering stage, but the growth rate was slow. Considering the time cost, the flower withering stage was the best time for harvesting.

2.5 Yield, extraction efficiency and BCF

During the harvest period, the Cd content (71.82–82.04 mg kg^− 1) and BCF (85.17–109.39) of S.P. were significantly higher than those of S.A. (52.59–49.01 mg kg^− 1 and 62.36–66.41, respectively). The Cd content and BCF of S.P. planted in 2021–2022 were higher than those planted in 2020–2021, but the difference of S.A. between the two years was not significant. Therefore, the theoretical yield of S.P. (3845.50–3887.59 kg ha^− 1) was higher than that of S.A. (3573.00–3719.46 kg ha^− 1), and the theoretical Cd extraction amount (276.17–318.95 g ha^− 1) was also significantly higher than that of S.A. (187.89–182.30 g ha^− 1) (Table 3).

Table 3

Inter-annual remediation effects analysis between *S.A.* and *S.P.*
Year	Sedum	Cd content (mg kg^− 1)	BCF	Yield (kg ha^− 1)		Extraction amount (g ha^− 1)
Year	Sedum	Cd content (mg kg^− 1)	BCF	Theoretical value	Measured value	Theoretical value	Measured value
2020–2021	S.A.	52.59b	62.36b	3573.00b	3308.94a	187.89b	174.00a
2020–2021	S.P.	71.82a	85.17a	3845.50a	1917.54b	276.17a	137.71b
2021–2022	S.A.	49.01b	66.41b	3719.46a	3541.46a	182.30b	173.58a
2021–2022	S.P.	82.04a	109.39a	3887.59a	2436.65b	318.95a	199.91a
The different letters in the same column indicate the significant differences between varieties within the same year at p < 0.05.

Two years of field cultivation experiments showed that S.A. had strong stress tolerance, high survival rate, good plant growth and high yield with the measured yield (3308.94–3719.46 kg ha^− 1) was 92.61% − 95.21% of the theoretical value. The measured Cd extraction amount showed little difference between the years, with values of 174.00 and 173.58 g ha^− 1, respectively. Although the measured Cd extraction amount was slightly lower than the theoretical value (187.89–182.30 g ha^− 1), the difference was not significant.

Compared with the S.A., the S.P. had weaker stress resistance. Due to excessive rainfall in winter and spring during the planting season, some S.P. seedlings showed water-soaking rot on the roots and stems, wilted leaves, then the leaf stalks and even the entire plants were rotten and withered. This resulted in a reduced survival rate and measured yield that only accounted for 49.86–62.68% of the theoretical yield. Although the S.P. had a high Cd content, due to the low measured yield, the measured Cd extraction amount (137.71–199.91 g ha^− 1) were significantly lower than the theoretical values (276.17–318.95 g ha^− 1). Among them, the measured Cd extraction rate of the S.P. planted in the 2020–2021 season (137.71 g ha^− 1) was the lowest, while the highest was obtained in the 2021–2022 season (199.91 g ha^− 1). This was due to the high survival rate (62.68%) and the largest plant Cd content in the S.P. planted in 2020–2021, resulting in the highest measured Cd removal rate.

Phenology is a key indicator that reflects the dynamic changes in crop growth patterns and is directly related to yield (Richardson et al., 2013). In production, scientific field management is often based on phenology. In this study, the growth stages of S.A. were about 20 days earlier than those of S.P. The peak growth period of S.A. and S.P. started in late March and early April, respectively. To ensure the nutrient supply of the Sedum group during the vigorous growing stage, fertilizers should be applied before the start of the vigorous growing stage. At the flower withering stage, the biomass of S.A. and S.P. reached its maximum, and then the leaves of S.A. began to fall off on a large scale, and the biomass was also reduced, but not in the case of S.P. Therefore, from the perspective of yield, the best harvest time for S.A. is beginning of the flower withering stage (mid and late May), and for S.P., it is the entire flower withering stage (early June).

The different growth patterns of the two Sedum species are the main reasons for the differences in the dynamic changes in Cd content in their shoots. The Cd content and extraction amount in the leaves of S.A. were significantly lower than those in the stems. During the vigorous growing stage, the stem-leaf ratio of S.A. was lower than that of the flowering stage which led to a slow increase in Cd content during this period. The Cd distribution ratio in each organ of S.P. was less affected by phenology, its Cd content increased with increasing planting time and was less affected by phenology, too. This result is consistent with the study of Song et al. (2022). However, Yin et al. (2019) supposed that the Cd content in the shoots of S.P. showed a low-high-low trend with growth time, which is different from the results of this study. There may be two reasons for this. First, the Cd content of S.P. seedlings in that study was higher than 100 mg kg^− 1, but in the current study the seedling quality reached the Ⅰ grade standard and the Cd content was less than 5 mg kg^− 1. The Cd content of seedlings was significantly negatively correlated with the extraction amount of plants (Dai et al., 2022). Second, in that study, shading treatment was carried out in the later growth stage, which reduced the light intensity, and the photosynthetic rate, transpiration rate, and water use efficiency of plants decreased, leading to a significant decrease in Cd content in stems and leaves (Chen et al., 2021a; Guan et al., 2021). In addition to ensuring the survival rate and yield, the improvement of the plant's ability to extract and accumulate heavy metals is the key to the selection of cultivation measures for hyperaccumulators. Improper selection of seedlings with high Cd content and cultivation practices will result in reduced restoration efficiency and increased costs.

Previous studies have found that the expression levels of genes encoding metallothioneins (HMA2, HMA4, ZIPs, NRAMP3, YSLs, and MTL) in the leaves of S.P. were higher than those in S.A., indicating that the S.P. is more suitable for Cd storage and accumulation (Peng et al., 2017b). Consistent with previous studies, the Cd content in the leaves of S.P. was significantly higher than that in S.A. in this study. The dry mass and Cd content in root of both Sedum species were much lower than those in shoots, and the contribution of root Cd extraction was only 1.23%-1.56%, while the contribution of shoots Cd extraction was greater than 98%. However, the contribution of Cd extraction from each organ of the two Sedum species differed greatly, where the contribution of Cd extraction from S.A. was in the order of stem > flower > leaf > root, and that of S.P. was leaf > stem > flower > root (Fig. 3). The different growth patterns and Cd storage locations were the reasons for the large interspecific differences in the contribution rate of Cd extraction from each organ. From the budding stage, the dry mass and Cd content of the flowering branches of S.A. continued to increase and were greater than the increase in the leaf branches, indicating that the stems and flowers were in a state of continuous growth and were superior to the leaf branches during this period. Since Cd in S.A. is preferentially stored in young tissues (Hu et al., 2019), the stems and flowers with continuous growth obtained the priority for Cd storage, and thus had a greater Cd extraction rate than the leaves. The Cd in S.P. is mainly stored in parenchyma cells such as cortex in stem and mesophyll in leaf (Hu et al., 2015). After entering the flowering stage, the dry mass and Cd content of the leaf branches of S.P. continued to increase, and the continuous growth of stems and leaves provided more space for Cd storage. At the same time, the high abundance of metallothionein-encoding genes in S.P. ensured the mobility of Cd ions, promoted long-distance transport, and increased the Cd tolerance (Peng et al., 2017a; Peng et al., 2020; Zhang et al., 2022), thus, the S.P. stems and leaves contributed the highest contribution rate of Cd extraction. Therefore, it was inferred that promoting flowering branch differentiation and stem dry mass accumulation in S.A. or promoting leaf branch differentiation and leaf dry mass accumulation in S.P. through appropriate cultivation measures, could be an effective method to improve extraction efficiency in practice.

In this study, both Sedums were planted in autumn, and their shoots biomass grew slowly during the winter and spring seedling stage, mainly focusing on root growth. The S.A. entered the vigorous growing stage earlier, which may indicate that the S.A. roots and sprouts earlier than the S.P. in the early spring, and the S.A. is better adapted to lower temperature environments. (Zhu et al., 2019) studied the phytoremediation of hyperaccumulators S.A. and S.P. on Cd/Zn contaminated soil and contrasted the effects. They found that the Cd content of S.P. was significantly higher than that of the S.A., consistent with the results of this study. However, the results of previous field experiment in which S.P. had better adaptability, better growth ability and higher biomass production than that of S.A. were contrary to the current study. The different survival rates and harvest periods were the main reasons for the differences in the two study results. In this study, the survival rate of the S.P. was only 49%-62%, far lower than the 90% or more of the S.P. Therefore, although the theoretical yield and extraction values of the S.P. were greater than that of the S.A., the measured values were lower than that of the S.A. Secondly, in previous study, when the yield was measured, the S.A. had entered the late period of the flower withering stage, and its leaves began to fall off and rot extensively, which resulted in low measured values of S.A. This once again proved that harvesting in the early period of the flower withering stage could ensure a higher shoots dry mass and extraction efficiency of the S.A. In addition, the fertilizer application was consistent for both Sedums in current study, but this two Sedums fertilizer requirements were different. Therefore, there were phenomena such as the main stem of the S.P. being easily broken and the S.A. declining in the late stage of the flower withering stage. Therefore, it is necessary to strengthen the research on the nutrient requirements of the two Sedums separately, and timely and properly supplement nitrogen, potassium, and other elements to increase the stem thickness of the S.P. while improving its flexibility and delaying the decline period of the S.A.

In this study, the low survival rate of S.P. may be due to infection with Plectosphaerella cucumerina. After infection with Plectosphaerella cucumerina, the shoots and stems first showed water-soaked decay, leaf wilting, and finally the petioles and even the entire plant rotted and withered, with a field incidence rate of 20%-50% (Chen et al., 2020). But this symptom was not observed in S.A. After surviving the crisis during the seedling stage, the rapid increase in shoots biomass of S.P. could easily cause the fleshy main stem to break, resulting in a lower harvestability factor, and the wound caused by the main stem breakage could easily lead to pathogen invasion. High incidence of disease and easy breakage of the main stem were the main reasons for the low shoots biomass and Cd extraction rate of S.P. in this study. The shoots Cd content of S.P. accounts for 0.07%-0.08% of its biomass, which was higher than that of S.A. (0.05%), and the theoretical yield was also higher than that of S.A. (Table 3), indicating that S.P. has a higher potential and effectiveness for Cd extraction and remediation than S.A. However, its low survival rate and harvest coefficient are urgent problems that need to be addressed. S.A. has fast rooting, high survival rate, and is not susceptible to disease, suggesting that its adaptability and stress resistance may be stronger than that of S.A. and require further study.

The growth stages of the S.A. were about 20 days earlier than those of the S.P., and the best harvest time of the two Sedum were the early period of the flower withering stage (mid to late May) for S.A. and the whole flower withering stage (early June) for S.P. Under the same cultivation conditions, the shoots biomass, Cd content, and BCF of S.P. were significantly higher than those of S.A. The Cd content of S.A. showed a "fast-slow-fast" trend of growth with planting time, and the contribution rate of Cd content and Cd extraction amount of each organ was stem > flower > leaf > root. The Cd content of S.P. gradually increased with the planting time, and the contribution rate of Cd content and Cd extraction amount of each organ was leaf > stem > flower > root. S.A. had fast rooting, high survival rate, and was less susceptible to disease. The measured Cd extraction amount was 173.58–174.00 g ha^− 1, accounting for 92.61%~95.21% of the theoretical value, while the measured Cd extraction amount of S.P. varies greatly between years, ranging from 137.71-199.91 g ha^− 1, accounting for 49.86%~62.68% of the theoretical value. The remediation potential of S.P. is greater than that of S.A.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2022YFD1700104-2), Natural Science Foundation of Hunan Province (2022JJ40212), Research Project of Hunan Provincial Department of Ecology and Environment (HBKT-202222), Hunan Provincial Science and Technology Talent Recruitment Project (2023TJ-N19).

Declaration interest statement

The authors declared that they have no conflicts of interest to this work.

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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Differences of Cadmium Extraction Characteristics between Sedum alfredii H. and Sedum plumbizincicola

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Abstract

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Introduction

Materials and Methods

1.1 Overview of the Experimental Site

1.2 Experimental Design

1.3 Sampling and processing

1.4 Data processing and analysis

Results

2.1 Phenophase

2.2 Dynamics of Dry Biomass and Cd Content

2.3 Agricultural Traits and Cd Content of Organs

2.5 Yield, extraction efficiency and BCF

Discussion

Conclusion

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References

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