Agronomic traits and fresh weight of kenaf cultivars in Cd-contaminated paddy soil
The heights of the kenaf plants were between 2.5 and 4.0 m in the two growing seasons. Most of cultivars were grew higher in 2016 than in 2015, expect for Hongma No.1 and Fuhong 952. The average plant height in the growing season in 2015 and 2016 were 3.1 and 3.3 m, respectively. The stem diameter of kenaf plants was between 13.0 and 19.1 mm in the two growing seasons. All of cultivars had a higher diameter in 2016 than in 2015. The average diameter in the growing season in 2015 and 2016 were 15.0 and 17.0 mm, respectively. The bark thickness of the kenaf plants was between 0.84 and 1.48 mm in the two growing seasons. Most of cultivars had thicker bark in 2016 than in 2015, except Hongma No.1 and Fuhong 952. The average bark thickness during the growing season in 2015 and 2016 was 1.02 and 1.22 mm, respectively. The fresh weights the of root, xylem, and phloem for 10 plants were respectively between 0.18 and 0.32 kg, 0.94and 2.40 kg, and 0.62 and 1.60 kg, respectively. Among the 13 cultivars, the fresh weights of the root, xylem, and phloem for Fuhong 952 were more stable in the two seasons and relatively higher than those of the other cultivars (Fig. 2).
Cd concentration in rhizosphere soil and different tissues of kenaf plants
The Cd concentration in the rhizosphere soil in 39 experimental pots was between 0.71 mg·kg-1 and 1.50 mg·kg-1, with a mean concentration of 1.15 mg·kg-1 (Fig. 3). The Cd concentrations in the rhizosphere soil of Hongzong No.1, Hongyou No.2, Fuhong 952, Fuhong 992, and Fuhong R1 were significantly higher than those of the other cultivars. The Cd concentrations in kenaf tissues are presented in Table 2. The Cd concentration ranges in the root, xylem, and phloem of 13 kenaf cultivars were 1.50–4.38 mg·kg-1, 1.00–2.96 mg·kg-1, and 2.39–5.18 mg·kg-1, respectively, with mean concentrations of 2.67, 2.05, and 3.87 mg·kg-1, respectively. The mean accumulation of Cd in the three tissues of kenaf were decreased in the following order: phloem ﹥ root ﹥ xylem, and the mean concentrations of Cd in the phloem of the 13 cultivars were 1.1–2.9 times higher than those in the root and xylem. The top three kenaf cultivars with the highest Cd concentrations were Hongzong No.1, Fuhong 992, and Fuhong 952.
Bioconcentration and translocation of Cd in kenaf tissues
BF was used to evaluate the ability of the kenaf plant to accumulate Cd in roots of the 13 kenaf cultivars. BFs of roots of the 13 kenaf cultivars were ranged from 1.74 to 2.96, with a mean of 2.35. No significant difference was found among cultivars, except for Fuhong 992 and GGMN, which showed the highest and lowest BFs in the 13 kenaf cultivars, respectively. TF was used to evaluate the ability of a kenaf plant to transfer Cd within its tissues. TFrxs and TFrps of kenaf cultivars were ranged from 0.50 to 1.44 and 1.10 to 2.57, respectively. TFrps were higher than TFrxs in all 13 cultivars, suggesting the phloem had a greater ability to transport Cd that of the xylem. There were significantly higher differences among the TFrps of the cultivar than among the TFrx and BF of the roots, indicating a difference in the Cd transport capacity of the phloem among kenaf varieties. TFrps of cultivars with an unlobed leaf shape (DGG, G4, GGMN, and GGS) were significantly higher than those of cultivars with other leaf shapes (Fig. 4).
Biomass of kenaf planting in Cd-contaminated paddy soil and assessment of Cd uptake for the 13 kenaf cultivars.
The biomass of the root, xylem, and phloem of 10 plants among the 13 cultivars were 0.06–0.10 kg, 0.17–0.36 kg, and 0.13–0.36 kg, respectively. The biomass of the xylem was higher than that of other tissues in all 13 kenaf cultivars, accounted for 46%–55% of the total biomass. Phloem biomass was lower than xylem biomass but higher than root biomass. The percentage of phloem biomass ranged from 29% to 38%, and the root biomass ranged from 14% to 19%. Fuhong 952 (0.72 kg) and G4 (0.37 kg) had the highest and lowest of total 10 plants biomass (Fig. 5).
In the actual agricultural operations, only the aerial parts of kenaf plants are harvested. According to the data of fresh weight and dry weight by experimental plots, and the Cd concentration in different tissues, we assessed Cd uptake of each cultivar for a hectare area. Because all leaves had fallen off during harvest, the biomass of leaves was not calculated in the total aerial biomass. Fuhong 952 and Fuhong R1 had the highest Cd phytoextraction capacity and uptake exceeding 60 g·of Cd from every hectare of soil. For most cultivars, except for Fuhong R1, phloem phytoextraction accumulated more Cd than xylem. The percentage of Cd uptake by the phloem ranged from 47% to 61%, and that by the xylem ranged from 38% to 53% (Fig. 6).
Comprehensive evaluation of the 13 kenaf cultivars in the phytoremediation of Cd-contaminated soil
To comprehensive evaluate the 13 kenaf cultivars in the phytoremediation of Cd-contaminated soil, we normalized and clustered the data using the zero-to-one method in the heat map (Fig. 7). Fuhong 992, Fuhong 952, and Fuhong R1 exhibited better performance than other cultivars in most of the traits, except for the TFs of the xylem and phloem, and thus, they can be used as candidate plant materials for soil phytoremediation.