| Cadmium (Cd), accumulated in soils as a consequence of agricultural activities (excessive phosphatic fertilizers application) and industrial activities (mining and smelting of metalliferous ores, electroplating, etc.) is becoming a major problem due to the great toxicity, high mobility from soil to plant, long half-life of Cd and further entry into food chain. Phytoremediation of Cd polluted soils has attracted world-wide attention and research, and in which the hyperaccumulation of Cd in the shoots by plants is the main technique to remove Cd in the contaminated soils. In the present study, the newly discovered Cd hyperaccumulator plant Solanum nigrum L. was grown in hydroponics with increasing Cd concentrations to study the growth, antioxidant system, Nitrogen metabolism, photosynthetic parameters, and Cd accumulation characteristics. Based these, distribution and mobility of Cd in the S. nigrum plant, and subcellular distribution and chemical forms of Cd in the leaves of different position were investigated. Further, the difference of Cd, Zn accumulation between S. nigrum and S. melongena plants subjected to Cd, Zn, and Cd-Zn complex stress was studied by hydroponic culture method. Moreover, the interactions between Cd and macro- and micro-nutrient elements were investigated through a pot experiment. In addition, exogenous SA was applied in the hydroponic system to study the alleviating effects to Cd toxicity in the leaves of Solanum nigrum L.; exogenous chitosan was applied in different textural soils to study the effects on the available contents and vertical distribution of Cd2+. The main results were indicated as follows:①The growth status, physiological characteristics, and Cd accumulation in the hyperaccumulator plant Solanum nigrum L seedlings subjected to Cd stress.Physiological responses of S. nigrum and S. melongena seedlings to Cd stress were investigated by nutrient solution culture method. The results showed that Cd could affect the growth of S. nigrum and S. melongena seedlings, with concentration-dependent and time-dependent manners. Dry shoot biomass of S. nigrum decreased 15.48% when Cd concentration was below 50μM (P > 0.05), but dry root biomass increased 111.11% with the application of Cd (≤100μM, P < 0.05). Root activities of S. nigrum increased at low Cd concentrations (≤50μM) and then decreased. However, dry shoot biomass and root activities of S. melongena decreased significantly in all Cd treatments. The leaves pigment contents of S. nigrum and S. melongena seedlings decreased with the increase of Cd concentration. SOD, POD, and CAT in the leaves of S. nigrum played an important role against Cd stress. With increasing Cd concentration in the solution and along with treatment time, the relative elective conductivities, malondialdehyde (MDA) contents increased, soluble sugar and proline contents also increased, which played an important role of osmotic adjustment.②Response of the ascorbate-glutathione cycle and the alleviating effects of exogenous SA to Cd toxicity in the hyperaccumulator plant Solanum nigrum L.Response of the ascorbate-glutathione cycle and the alleviating effects of exogenous SA to Cd toxicity in the hyperaccumulator plant S. nigrum seedlings under Cd stress were investigated by nutrient solution culture method. The results showed that Cd stress significantly influenced the contents of GSH, GSSG, AsA, DHA and the activities of GR and APX in the leaves of S. nigrum, and have concentration-dependent and time-dependent effects. After 3 d of Cd exposure, the contents of GSH and GSSG decreased with a little extent firstly, and reached maximum after 9 d of Cd exposure. GSSG content presented a rule of"decrease-increase-decrease"subjected to 25μM of Cd, and reached minimum after 12 d of Cd exposure, but when treated with 150μM of Cd, GSSG content presented a reverse rule-"increase-decrease-increase", and reached maximum after 6 d of Cd exposure. And, AsA content increased firstly and decreased along with treatment time subjected to 25μM of Cd, average content increased 48.58% compared to the control. However, AsA content presented a rule of"decrease-increase-decrease"subjected to 150μM of Cd, and reached minimum after 6 d of Cd exposure, but the average content increased 40.88% than the control. DHA content increased with increasing Cd concentration and along with treatment time, and reached maximum after 9 d of 25μM Cd and 12 d of 150μM Cd exposures. GR activity increased firstly and decreased along with exposure time, and reached peak values after 9 d of Cd exposure, further, average GR activities increased 30.90% and 51.67% subjected to 25 and 150μM of Cd, respectively. APX activity presented a different rule with GR activity, and a rule of"increase-decrease-increase"was found when the seedlings subjected to 25μM of Cd, average activity increased 47.37% compared to the control, however, APX activity presented the rule of"decrease-increase-decrease", and reached maximum after 3 d of 150μM Cd exposure, increased 37.81% compared to the control (P < 0.05).Application of exogenous SA significantly increased the contents of leaf pigments, GSH, AsA, and DHA, decreased the MDA contents. Furthermore, increased the activities of GR and APX, after 3 d and 6 d of SA application, GR activities increased 27.50% and 32.08% compared to that of T2 (no SA application, 150μM Cd, P < 0.05), and APX activities increased 23.74% and 22.48%, respectively (P < 0.05).③Effects of Cd stress on the leaf photosynthesis, Nitrogen metabolism, and uptake of nutrient elements in the hyperaccumulator plant Solanum nigrum L.Effects of Cd stress on the leaf photosynthesis and macro- and micro-nutrient elements uptake in the hyperaccumulator plant S. nigrum were investigated through a soil pot experiment. The results showed that net photosynthesis rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr) decreased with increasing Cd concentrations, but intercellular CO2 concentration (Ci) increased. And, Fv/ Fm ratios decreased significantly.Nitrogen metabolism in S. nigrum seedlings under Cd stress were investigated by nutrient solution culture method. The results showed that NO3--N and soluble protein contents, NR and GS activities in the leaves and roots increased firstly and decreased with increasing Cd concentrations in the solutions, and decreased along with treatment time. And, soluble protein contents in the roots decreased with more extent. After 8 d of Cd exposure (25μM), NO3--N content and NR activity in the leaves of S. nigrum reached peak values, but root NO3--N contents reached maximum after 8 d of 50μM Cd exposure, and leaf NO3--N contents were more sensitive to root. However, NH4+-N contents in the leaves of S. nigrum increased with increasing Cd concentrations and along with treatment time, and reached maximum after 16 d of 100μM Cd exposure, which suggested significant NH4+-N accumulation in the leaves of S. nigrum subjected to Cd stress. Furthermore, GDH activities in the leaves and roots increased with increasing Cd concentrations and along with exposure time, and reached peak values after 16 d of 100μM Cd exposure.④Subcellular distribution, chemical forms, and mobility of Cadmium in the hyperaccumulator plant Solanum nigrum L.Cd mainly accumulated in the shoot of S. nigrum seedlings, and Cd concentrations in the plant tissues could be drawn as followed: leaf﹥stem﹥root. The Cd concentrations in leaves of S. nigrum varied with leaf position, the highest concentration was found in the 7th leaf (2373.51μg Cd g-1 DW). And, an increase in Cd concentration from the marginal leaf to the midrib part was observed. When the 7th leaf was wrapped in a transparent plastic bag for 24 h, the Cd concentration of this leaf decreased 76.73% than that in the corresponding leaf of no wrapping. And, the treatment did not affect the distribution of Cd in other leaves. Cd concentrations in the two new leaves (11th and 12th leaves), which had appeared after the cessation of Cd treatment, were extremely low. The mobility of Cd in S. nigrum was investigated in this study by firstly growing seedlings in one-half-strength Hoagland nutrient solution containing Cd, followed by a further growth period in a nutrient solution without Cd. After the plant has been transplanted into Cd-free nutrient solution, Cd concentrations in the older leaves decreased compared to the control, but Cd contents increased. And, the total Cd contents of seedlings remained stable after the two treatments. The distribution pattern of Cd in the S. nigrum did not change, and approximately 37.78% of the Cd content in the roots was translocated to shoots after the external Cd source was removed. Therefore, we suggest that Cd could be readily translocated from roots to shoots along with transpiration stream; however, once it reached the leaves, it could not be remobilized through the phloem to other organs.Subcellular fractionation of Cd-containing tissues indicated that about 62%~66% of Cd was localized in cell walls and 26%~32% in soluble fraction, and the lowest in cellular organelles. The distribution rates of Cd in the cell walls and cellular organelles increased with increasing external Cd levels, and larger distribution rates in the leaves of lower position than upper leaves, but decreased in the soluble fraction. And, results showed that the greatest amount of Cd was found in the extraction of 1 M NaCl and 2% HAC, particularly 1 M NaCl (40.09%~56.09%), which suggested that Cd in the leaves of S. nigrum mainly detoxified as the combination of protein and phosphates, further suggested that Cd mainly localized in cell walls. ⑤Cadmium uptake and interactions with Zinc in two species of Solanum --Solanum nigrum L and Solanum melongena L: A short-time hydroponic study.In the present study, the growth status and metal accumulation of S. nigrum and S. melongena were examined under Cd, Zn, and Cd-Zn complex polluted conditions using nutrient solution culture method. The results showed that high concentrations of Cd (200μM Cd) and Zn (500μM Zn) greatly affected the growth of S. nigrum and S. melongena seedlings, but dry biomass of part Cd-Zn treated plants were higher compared with the control plants under low concentrations of Cd-Zn. With the application of Zn in the nutrient solution, Cd concentration in the roots and leaves of S. nigrum and S. melongena seedlings increased firstly, then decreased, and reached maximum when 100μM Zn and 50μM Zn were applied, respectively (P < 0.05). Cd concentrations in the roots and shoots of S. nigrum and S. melongena seedlings were mainly determined by the Cd concentrations in the Hoagland solution, the effect of Cd-Zn interaction was limited. From the view of total heavy metal contents, shoot Cd contents of S. nigrum were 7~16-fold of roots, but Cd mainly accumulated in the roots of S. melongena seedlings.Cd application significantly influenced the shoots and roots Zn contents of S. nigrum and S. melongena seedlings, shoot Zn contents reached maximum when applied 50μM Cd, especially reached peak value 445.9μg·Plant-1 when Cd/ Zn ratio was 50/ 500μM, however, shoot Zn contents reached peak value 48.3μg·Plant-1 when Cd/ Zn ratio was 25/ 500μM. Zn concentrations in the roots and shoots of S. nigrum and S. melongena seedlings were mainly determined by the Zn concentrations in the Hoagland solution, the effect of Cd-Zn interaction on the Zn concentration of S. nigrum was limited, but significant interaction was found in the S. melongena seedlings. From the view of total heavy metal contents, shoots Zn contents of S. nigrum were 7~17-fold of roots, but shoots Zn contents were slightly higher than roots in the S. melongena seedlings.Moreover, Cd application led to a disturbance of macro- and micro-nutrient elements uptake. Cd increased K uptake in the root and shoot of S. nigrum, but had no significant effect on Na. And, Cd increased root Mg uptake, but decreased the transport of Mg from root to shoot. Moreover, low concentration of Cd increased Ca uptake in the leaves of S. nigrum. Low Cd increased Zn concentrations in the roots, stems, and leaves, but high Cd decreased them. Cu concentration in the root increased, but leaf Cu concentration increased firstly, then decreased. Furthermore, Fe concentrations in the root, stem, and leaf decreased with increasing Cd concentration in the soils; however, Cd decreased Mn concentration in the root of S. nigrum significantly.⑥Effect of Chitosan on the available contents and vertical distribution of Cd2+ in different textural soilsIn this study, an environment-friendly biopolymer- chitosan, has been adopted to remedy contaminated soils by heavy metals of Cd2+. Experimental results demonstrated that, within the first 7 d, available Cd2+ contents in three textural soils (clay, loam, and sandy soil) decreased significantly after chitosan application. Moreover, the available Cd2+ contents in soil layers of 14-16 cm and 24-26 cm were significantly reduced than that in 4-6 cm after 7 d of chitosan application. Our investigation suggested that application of 0.9 g chitosan kg-1 DW soil for 7 d could be perfect for the remediation of the soil contaminated by Cd2+.
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