| In order to study the effects of veterinary drug residues, roxarsone (ROX) and CUSO4, on the leafy vegetables, we selected seedlings of I. aquatica and A. mangostanus as materials. The effects of ROX on the growth, metabolism of I, aquatica seedlings and the distribution, accumulation, and transformation of ROX in matrix and in I. aquatica seedlings were anlysized, and the potential harmness of ROX residues to leafy vegetables were speculated. Then, the effects of CuSO4application on A. mangostanus seedlings from the growth, metabolism, nutrition and food safety etc. were researched. Results are as follows:1. The accumulation and transportation of ROX in I. aquatica seedlings1.1I. aquatica being planted in solid matrixTreatments with certain concentration of ROX on I. aquatica seedlings could reduce its growth speed and root activity, and eventually decreased its biomass accumulation. No obvious impact on its PSII function were found at1mM ROX treatment, while treatment of8mM ROX could hinder the structure and function of its PSII.Solid matrix, peat/vermiculite=5:1, could adsorpt and accumulate arsenic. The arsenic content in the solid matrix being planted with I. aquatica seedlings was only18.85%that of the control. This demonstrated that I. aquatica seedlings had absorbed a large amount of arsenic from the matrix. In the solid matrix, ROX can be transformed into more poisonous As (Ⅲ) and As (Ⅴ), and other non-detected arsenic forms. Among them, As (V) content was higher than that of As (Ⅲ), DMA and MMA was not detected. The contents of inorganic arsenic and ROX in the matrix of1mM ROX group were far lower than those of the control, while the ratio of inorganic/total arsenic was far higher than that of the control; The inorganic arsenic contents and the ratio of inorganic/total arsenic in8mM ROX group were greatly reduced being compared to the control and the1mM group. These suggested that ROX application affected the arsenic selective absorption ability of I. aquatica seedlings, and the seedlings’ root physiological activity also affected the ROX metabolism.The fact that As (Ⅲ), As (Ⅴ) and ROX were detected in each organ of I. aquatica seedlings implied that ROX could be absorbed directly by plants. As to As (Ⅲ) and As (Ⅴ), whether they were absorbed directly from matrix, or/and derived from the absorbed ROX, these were not clear. The contents of total As, As(Ⅲ), As(Ⅴ) and ROX were all the highest in the roots, next came the leaves, the lowest was in the stems, and As (Ⅲ) was the main form. Both the tatal As content and As(Ⅲ) content were exceed the maximum allowable amount in "The National Hygienic Standard for Vegetables". These demonstrated that the veterinary drug residue of ROX may cause a serious threat to food security of leafy vegetables.I. aquatica seedlings could enrich inorganic As and ROX under1mM ROX treatment, and the strongest enrichment effect was on As(Ⅲ). At8mM ROX treatment, the enrichment ability on As(Ⅲ) become more stronger, but as to other forms of As, it decreased. The roots could absorb and accumulate arsenic from the solid matrix, and restrict arsenic transfered to the stems, while the stems mainly acted as the channel for arsenic transportation from the roots to the leaves. These implied that although microbial transformation in soil can improve the arsenic toxicity in the groundwater, leafy vegetables, which can enrich As(Ⅲ) and transform other forms of arsenic into As(Ⅲ), can increase its potential toxicity more greatly. On the other hand, the arsenic contaminated soil can be repaired by planting highly arsenic accunulating plants.1.2I. aquatica being planted in sterile solutionAfter being sterilized, and being replaced in a short period of time (2d), the inorganic arsenic content in the solution only accounted for0.01%～0.27%of the total arsenic, which was far lower than that in solid matrix (10.95～12.52%); and the ROX proportion was74.4%～81.8%, which also was higher than that of19.78%～43.52%in solid matrix. This indicated that the sterilization and prompt replacement of solution could reduce the the degradation of ROX greatly.By planting I. aquatica seedlings in the sterilized ROX solution and replacing the solution every2days, and detecting the arsenic forms and their contents in each organ, we can found that I. aquatica seedlings can directly absorb ROX, and the absorption amount was related to the the content and proportion of ROX in the rhizosphere solution. The higher the ROX concentration, the higher the plants absorbed. The absorbed ROX can be transformed to other arsenic forms vigorously in plants. About half of the ROX were transformed to more toxic As (Ⅲ) within3d (8mM treatment). The absorbed and transformed ROX were mainly accumulated in the roots. Meanwhile, part of them was transferred to the shoots. The arsenic transportation ability from roots to stems was weak, but it is relatively strong from stems to leaves.The above results proved furtherly that I. aquatica seedlings can absorb ROX directly; and can transform the relatively low-toxic ROX to the highly toxic As (Ⅲ).Although the ecological toxicity of ROX to humans and animals etc. can be improved by soil microorganisms, it might can be improved more greatly by leafy vegetables.2. The effects of CuSO4on the growth and food safety of A. mangostanusCuSO4treatment can inhibit the growth of A mangostanus seedlings. The growth rates of stem height, leaf area and and biomass were inhibited. The root activity, number of lateral roots and root hairs etc.were decreased.The photosynthetic pigment contents, which including chlorophyll a, b, total chlorophyll and carotenoids, decresed with the increase of CuSO4concentration. Chlorophyll fluorescence parameters Fo values increased, and values of Fm,(Fv/Fo) and (Fv/Fm) increased with the increase of CuSO4concentration. These indicated that CuSO4treatment would affect the light energy absorption, energy transfer of antenna pigment, photochemical conversion and electron transport activity of PSII, which would eventually reduce the photosynthetic efficiency in A. mangostanus seedlings.CuSO4treatment caused the accumulation increase of O2-· and H2O2, which furtherly promoted the antioxidant enzymes’ activities of SOD, POD, CAT and APX and the antioxidants’ contents of AsA and GSH improving. The increases range of antioxidants’ contents were lower than that of the antioxidant enzymes’ activity. When CuSO4concentration was not over9mM, with the cooperation of antioxidant enzymes and antioxidants, the cell oxidation damage degree, which was represented by MDA content and electrical conductivity, were controlled within a certain range. When CuSO4concentration reached15mM, APX activity droped rapidly, ROS accumulated highly, membrane peroxidation and membrane permeability increased sharply. These all meant that cell damage was serious in A. mangostanus seedlings.CUSO4treatment would increase the nitrate content as well as the nitrate reductase activity in A. mangostanus seedlings. Although the application of CUSO4would improve the nitrate content in A. mangostanus seedlings, according to the Nitrate Standards of Vegetables in China, all the A. mangostanus seedlings, which were applied with CuSO4within the concentration range used in this paper, are edible, but are only suitable for cooked food. The improvement of nitrate reductase activity would improve the nitrite content, which would increase the potential hazards of CuSO4application. When considered from the Cu accumulation, both the stems and leaves of A. mangostanus seedlings were edible when CuSO4≤9mM, and both stems and leaves were no longer suitable for consumption when CUSO4reached15mM. Nowdays, Cu content in normal soils is within1mM CuSO4treatment group and Cu content in heavily polluted soils is within6mM CUSO4treatment group. Therefore, considering from the Cu content, all the A. mangostanus are edible. |
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