Fruit biomass variations under different cadmium exposures
All eggplants grew normally under CK, low-Cd, and high-Cd treatments, and the average fruit biomass (FW) was 153.36, 115.23, and 91.36 g, respectively, and no dystrophy symptoms of Cd toxicity were observed, despite the significant biomass difference between treatments (p < 0.01). According to the results of fruit BRS (Figure 1), five cultivars had significantly higher fruit biomass under high-Cd treatment, 15 cultivars showed similar fruit biomass between high-Cd and low-Cd treatments, and the other 10 cultivars had lower fruit biomass than the low-Cd treatment. Among these eggplant cultivars, ZHCQ and LDCQ had the highest fruit BRS values, whereas ZCQ, HQYH, and DLQZ had the lowest fruit BRS values, showing that different eggplant cultivars had different tolerance to Cd, and most of them were Cd tolerant cultivars.
Fruit cadmium accumulation in eggplants
Significant differences (p < 0.05) in fruit Cd concentrations were found among the 30 cultivars in each treatment. The average fruit Cd concentrations (fresh weight, FW) of CK, low- and high-Cd treatments were 0.018 (0.038-0.004 mg/kg), 0.084 (0.148-0.005 mg/kg), and 0.194 (0.282-0.112 mg/kg) mg/kg, respectively (Figure 2). In addition, the maximal differences in fruit Cd concentrations among the cultivars were 3.88-, 3.22-, and 2.52-fold under CK, low- and high-Cd treatments, respectively (Figure 2). Fruit Cd concentrations of all tested cultivars under CK treatment were lower than the Codex ML for Cd (0.05 mg/kg), while the fruit Cd concentrations of total cultivars exceeded the Codex ML for Cd under high-Cd treatment. However, only MYCQ, ZGQ, and TGZHCQ met the standard of Codex ML for Cd under low-Cd treatment, and these three cultivars also accumulated less Cd than the other cultivars under high-Cd treatment (Figure 2B and C). In addition, the fruit Cd concentrations of BXGZ, BGJDQ, and WCCQ were consistently higher than most of the other cultivars under both low-Cd and high-Cd treatments (Figure 2B and C). These results indicate that eggplant cultivars accumulate Cd easily, even when planted in soils with slight Cd contamination.
Correlative analysis showed that some eggplant cultivars had high consistency in fruit Cd accumulation under low-Cd and high-Cd treatments (p < 0.01, Figure 3), suggesting that the Cd accumulation abilities of eggplant cultivars are stable and genetically determined. Based on the results of fruit Cd accumulation abilities and correlation analysis, MYCQ and ZGQ were selected as candidates for low-Cd cultivars and BXGZ and WCCQ were selected as candidates for high-Cd cultivars for further investigation.
Leaf, stem, and root cadmium concentrations of the selected cultivars
Under low-Cd and high-Cd treatments, leaf and stem Cd concentrations of the high-Cd cultivars (BXGZ and WCCQ) were always 2–3 times significantly higher (p < 0.05) than those of low-Cd cultivars (MYCQ and ZGQ) (Table 2). However, Cd concentrations in the roots of the low-Cd cultivars were always significantly higher (p < 0.05) than those of the high-Cd cultivars. The leaves, stems, and roots of the BXGZ and WCCQ cultivars showed almost the same Cd accumulation ability (p > 0.05), regardless of the Cd treatment. In addition, the high-Cd cultivars, MYCQ and ZGQ, also showed no Cd concentration difference (p < 0.05) under each Cd treatment, except that the leaf Cd concentration of WCCQ was higher (p < 0.05) than that of BXGZ in the high-Cd treatment (Table 2). These results demonstrated that the selected eggplant cultivars had stable characteristics of Cd accumulation, which should be cultivar-dependent. The different fruit Cd concentrations of eggplant were not determined by the root Cd concentrations, but by the differences in Cd transport capacity.
Cadmium accumulation of different tissues and the net uptake of roots
The average total amount of Cd accumulated in eggplant was 3.001 (2.761–3.301) µg, 53.967 (49.243–58.079) ug, and 94.972 (88.490–100.660) µg under CK, low-Cd, and high-Cd treatments (Figure 4). The average root Cd accumulation of low-Cd cultivars (MYCQ and ZGQ) was 1.18- and 1.15-fold higher than that of high-Cd cultivars (BXGZ and WCCQ) under low- and high-Cd treatments, even though it was slightly lower under CK. The amount of Cd accumulated in the roots of MYCQ were the highest (p < 0.05) under CK (1.79 ug), low-Cd treatment (35.71 ug), and under high-Cd treatment (64.33 ug), while the lowest (p < 0.05) root Cd accumulation was observed in BXGZ, with values of 2.12, 27.36, and 54.13 ug under CK, low- and high- treatments (Figure 4). The average stem, leaf, and fruit Cd accumulations of low-Cd cultivars were always lower than those of high-Cd cultivars, among which MYCQ and BXGZ were always the lowest (p < 0.05) and highest (p < 0.05) than the other three selected cultivars, respectively (Figure 4).
Among the selected eggplant cultivars, root Cd net uptake was also significantly different (p < 0.05) (Table 3). The average root Cd net uptake of the high-Cd cultivars was higher than that of the low-Cd cultivars. Interestingly, MYCQ always possessed the lowest root Cd net uptake compared to the other three selected cultivars (p < 0.05), while root Cd net uptake of ZGQ displayed no significant difference (p > 0.05) from those of the high-Cd cultivars under low- and high-Cd treatments (Table 3). In addition, no obvious difference in root Cd net uptake was observed (p > 0.05) between the high-Cd cultivars (Table 3). Based on the results of fruit Cd accumulation and root Cd net uptake, MYCQ and BXGZ were selected for further study.
Subcellular distributions of cadmium in the selected cultivars
The Cd concentrations in different subcellular fractions of the fruit, leaf, stem, and root of MYCQ and BXGZ showed the same trend; that is, with the increase in Cd treatment pressure, the Cd concentrations of each subcellular fraction increased correspondingly (Table 4). As for each subcellular fraction, the mean Cd concentrations of F1 in the fruits, leaves, stems, and roots of both selected cultivars were higher than those of F2, F3, and F4 (p < 0.05). The F1 Cd concentration in the roots of MYCQ was higher than that of BXGZ, and an almost 1.5-fold (p < 0.05) difference was observed under all treatments. However, the average Cd concentrations of F1 in the fruit, leaf, and stem of MYCQ were lower than that of BXGZ, especially in the fruit, wherein the Cd concentration difference were 2.1 to 2.6 times. The F2, F3, and F4 in the fruit, leaf, stem, and root of MYCQ and BXGZ were almost the same as F1, with different Cd concentration fold changes from 1.2 to 2.0 times (p < 0.05), except for the F2 of fruit and F3 of fruit and leaf under CK (p > 0.05) (Table 4).
The Cd proportions of F1 in both selected eggplant cultivars were always the highest in all tissues under CK and low- and high-Cd treatments (Figure 5). MYCQ (52–64%) showed a higher Cd proportion in F1 than in BXGZ (44–62%) in all tissues, regardless of the Cd treatments. In addition, the Cd proportions of F1 in the fruit and leaf of both selected cultivars were higher than those of stems and roots, which is consistent with the results of Cd concentrations in different subcellular fractions.
F2 Cd proportions were always highly accounted for in all tissues of BXGZ (10–17%) than that of MYCQ (7–15%), especially in the stem and root of the high-Cd treatment. In all tissues, the F2 Cd proportions of MYCQ remained stable except for the decrease in roots under high-Cd treatment. However, the F2 Cd proportions of BXGZ were higher in the stems and roots than in the fruits and leaves under high-Cd treatment (Figure 5).
F3 Cd proportions in BXGZ (9–17%) were always higher than that of MYCQ (5–10%) in stems and roots, regardless of whether low- or high-Cd treatment existed. No evident difference in F3 Cd proportions was observed in the fruits and leaves of either cultivar. In addition, the F3 Cd proportions of MYCQ were slightly higher in fruits and leaves than in stems and roots, while the opposite changes were observed in BXGZ (Figure 5).
The Cd proportions of F4 were always the second highest among all the subcellular fractions in both selected cultivars. In roots, the Cd proportions of F4 were higher than those in the stems, leaves, and fruits. The Cd proportions of F4 increased with the increase of Cd treatment in roots, while it showed the opposite trend in the other tissues. In addition, the Cd proportions of F4 in MYCQ (24–30%) were obviously higher than that of BXGZ (18–22%) under low- and high-Cd treatments in roots, and the F4 Cd proportions of fruit, leaf, and stem in MYCQ (18–20%) were always lower than that of BXGZ (21–22%) under high-Cd treatments (Figure 5).