| With the expansion and further enhancement of industrialization and urbanlization of human society, material civilization has reached its highest peak, thus it can provide human beings with more convenience and affluence that has never seen before. Meanwhile, more and more emergent situations were produced in the ecological environment, such as heavy metal pollution, persistent organic pollutants and desertification. With its ecological toxicology characteristic of strong concealment, long half-life, immense toxicity, it’s hard to be eliminated and easy to be biological signifying and inyade the food chain, heavy metal pollution has become one of the biggest threat to ecological environment in the world. There are many ways to removal heavy metals from the contaminated environment, which mainly based on three kinds of remediation technologies including physical, chemical and bioremediation. Currently, phytoremediation is regarded as the most promising method because of its low cost, environmental friendly, enduring restoration process and its suitability for the large scale of heavy metal pollution in current situation. The key of phytoremediation is hyperaccumulator plants. Taking B. pilosa L. as research object, we studied the responding mechanisms of photosynthetic physiology, plasma membrane peroxidation, organic acid and chemical forms, osmotic regulation substances and antioxidant enzymes, organic acids and nutrient elements in the rhizosphere soil to B. pilosa under the stresses caused by cadmium(Cd)and lead(Pb)in order to estimate the phytoremediation’s potential abilities to restore Cd, Pb pollution. We still explore its restoration to Cd and Pb in polluted water by using the induced hairy roots from leaves of B. pilosa. The followings are the main conclusions of this paper.1. The response of photosynthesis by Cd and Pb. Content of chlorophyll, net photosynthesis, stomatal conductance and water vapor rate of B. pilosa has the trend of increase at the beginning and then decreas with the increase of the density of the stress of Cd or Pb, and the intercellular CO2concentration keeps weak increase. The compound stress of Cd-Pb shows addictive effect, in other words, low compound density of stress of Cd and Pb promotes photosynthesis, and high compound density restrains photosynthesis. Cd, Pb and Cd-Pb stresses destroy the compintegrality of photosynthetic apparatus and activate the self-restoration of photosynthetic apparatus by restraining the light and restraining or promoting the non-radicative energy, the transimiting rate of photosynthetic electricity and the opening of PS Ⅱ. Cd and Pb stress can promote or restrain photosynthesis mainly because of their promoting or restraining the composition of chlorophyll, thus result in the increase or decline of the content of chlorophyll.2. Cd, Pb and Cd-Pb stresses all can lead to the accumulation of superoxide radicals in B. pilosa. By increase the density of single Cd or Pb stress, the content of superoxide radicals increases at the beginning and then decreases; low and middle concentration of Pb stress promotes the accumulation of superoxide radicals in Cd-Pb stressed plants and high concentration of Pb restrains the acuumulation. Cd stress has no influence on the accumulation of superoxide radicals in Cd-Pb stressed plants. The variation of the amounts of malondialdehyde in plants primarily is the same as that of superoxide radicals. The activity of lipoxygenase increases with the increase of heavy metal stress but different among organs of the plants. With the increase of Cd or Pb, unsaturated fatty acids and long chain saturated fatty acids increase and keep in high level with high metal stress. Both Double band index(DBI)and Unsaturated fatty acids to percentage(UFA%)keep in high level with high density stress too. That means in the station of Cd or Pb stress, B. pilosa can enhance the fluidity of membrane by increase the unsaturated degree of fatty acids and maintain the stableness of the membrane structure by increase the content of the long chain saturated fatty acids. So as to improve their ability of endure the oxygen destruction caused by heavy metal stress.3. Cd, Pb and Cd-Pb stresses can lead to the accumulation of soluble sugar and proline in the leaves of the B. pilosa. The degree of the accumulation differs from one to another due to different kinds of heavy metal stress and the plant’s senstineness to the concentration. With the increase of Cd concentration stress, soluble sugar in leave of B. pilosa first increases and then decreases. The amount increase continuingly with the increase of Pb. Soluble proteins in B. pilosa increase at the beginning and then decrease with the increase of Cd or Pb stress. The Cd-Pb compound stress can promote the contents decreased of soluble protein and soluble protein in B. pilosa. With the increase of Cd. Pb or Cd-Pb stress, the activity of main antioxidant enzyme SOD. CAT and POD do not show any uniformity. With the increase of Cd. Pb or Cd-Pb compound stress, the activity of SOD in leaves of B. pilosa gradually increases. With the increase of Cd stress, the activity of CAT in B. pilosa increases first and then decreases. With the increase of Pb stress, the activity of CAT in B. pilosa continuingly increases. Under the condition of Cd-Pb compound stress, high concentration of stress promotes the increase of the activity of CAT in B. pilosa. With the increase of Cd stress, the activity of POD in B. pilosa increases at first and then decreases. And the activity of POD increases gradually in leaves of the paint with increase of Pb stress. Cd-Pb compound stress can slightly promote the activity of POD in leaves of plant and it relies on the concentration of Pb. Under the stress of Cd, Pb or Cd-Pb, B. pilosa can maintain the osmosis balance mainly by adjusting the content of proline, soluble sugar and soluble protein in the leaves and adjust the activity of SOD. CAT and POD to improve their ability in order to endure the antioxidant injury by heavy metal. As a result, they can enhance their ability to endure Cd and Pb stress.4. Cd, Pb stresses promote the accumulation of oxalate in B. pilosa. They also chelate the absorbed heavy metal by adjusting the composition of malic acid, citric acid, acetic acid and amber acid in the roots and leaves of plants to reduce the toxicity of the heavy metal. In plants’ bodies. Cd mainly exists in B. pilosa with the form of Fr3, which combined with protein and pectin. In this situation, low stress is beneficial to the transmission of Cd from roots to other positions and high stress restian the transport of Cd to leaves; Pb mainly exists in form of Fr5, a kind of oxalate, and high stress promotes the accumulation of Pb in the leaves. In compound stress conditions. Pb promotes Fr3turning to Fr4or Fr5to endure the contamination. Compared with Fr3, Fr4or Fr5is inactive. But Cd promotes Fr5to turn to active forms of Fr3, Fr4, Fr2and Fr1resulting to more toxicity effects. Cd and Pb mainly distribute in the subcellulars in form of cytoderm(F1)and dissoluvable parts(F4), but the content of chloroplast or plastid(F2)and other organelles is sparse. Pb can promote the increase of the content and proportion of low density Cd stressed plants’ cytoderm and dissoluvable parts in case of Cd-Pb compound stress Cd can promote the increase of the proportion and content of Pb in chloroplasts or plastids, but reduce the content and proportion of cytoderms and dissoluvable parts. The plants enhance their ability of chelating heavy metals and reduce the toxicity of absorbed Cd and Pb by strategies of increase oxalate, malic acid and citric acid. They also increase the inert chemical forms of heavy metal in the subcellular distribution in order to enhance their ability to endure Cd and Pb stress.5. In the case of Cd, Pb and Cd-Pb stresses, B. pilosa can accumulate large amount of axalate in rhizosphere. With the increase of Cd stress, the content of oxalate increases first and then decreases, while its content continuingly increases with increase of the density of Pb stress. Under the condition of Cd-Pb compound stress. The content of oxalate raises with the increase of Pb in low density Cd-Pb compound stress but increases first and then decreases in high density Cd-Pb compound stress. Cd exists mainly in the form of residual and its content increases with the increase of Cd stress. Pb mainly exists in the form of carbonate combination and Fe-Mn oxygenized combination take second place. Under the condition of Cd-Pb compound stress, increase of Pb can promote the Cd increase in changeable and carbonate combination form. The promotion is most obvious when adding Pb is1000mg/kg. Cd can promote residual Pb increase and M3of Pb decrease. Pb promote Cd transforming from less active form(M5)to active form(M3)but Cd promote Pb transforming from active form(M2)to less active form(M5). Cd or Pb has no obvious influence on the rhizosphere nutrition elements but K in the plant. But Cd-Pb compound stress decrease the accumulation of N, P and K. B. pilosa can enhance secreting oxalic acid to combinate heavy metal in order to affect the bioactivity of heavy metal in roots. As a result, they adjust the activity ratio and mobility ratio of the heavy metal and control the amount and time of the absorbed heavy metal, so that plants can enhance their ability to endure heavy metal.6. Within a life period of B. pilosa, content of heavy metal in upper ground parts and roots gradually increases with the growing of plants, which consistent with biomass. After90days’ stressing, when growth of the plants reaches their climax, the content of heavy metal and biomass in plants achieve the largest scale. The amount of heavy metal maintains in their maximum or slightly declines with life period changes from nutritive growth to reproductive growth. Under the condition of single heavy metal stress, the content of heavy metal in plants increases with the increase of the stress. Generally, in Cd-Pb compound situation, low and moderate density of Pb can promote the absorbing and transporting of low density Cd, but high density Pb seriously restrains the absorbing of Cd. In high density Pb-Cd compound stress, Pb restrains the plants to transmit Cd to upper ground parts but Cd shows no obvious influence on the absorbing and transporting of Pb. Generally, in a test group, when the content of Cd is large, the content of Pb is relatively small and if the content of Cd is small, the content of Pb is large, but the total amount of heavy metal remains invariablenes. Within life period of the plants, they show ability to accumulate and transport heavy metal, which increase at the beginning and then decreasing with the growth of the plants. In generally, the life span of B. pilosa is180days. So the best time to collect plants is90-120days for B. pilosa to phytoextract the heavy metal in polluted soil in excellent performance.7. Low density of Cd or Pb promotes the accumulation of biomass in hairy roots of B. pilosa but high density restrains. In case of Cd-Pb compound stress, low and moderate density of Pb stresses promote accumulation of biomass; while other density Pb compound to high Cd stress restrain the accumulation of biomass. Under the condition of Cd or Pb stress, the hairy roots’ accumulation of Cd and Pb is the additive result of the density and time of stressing. Hairy roots have different maximum absorbing amount in different density of stresses. In short period stresses, high density stress avails the hairy roots to accumulate heavy metals. Under the condition of low and high density stress, the absorbing of heavy metal gradually approaches peak and then maintains to the end of the stress. In case of Cd-Pb compound stress, low and moderate density of Pb promotes the hairy roots to accumulate Cd, while high density of Pb restrains the accumulation; low density of Pb stress has weak promotion merely to the accumulation of low density of Pb but it restrains the accumulation of Pb in the case of other varieties of density. Under the condition of Cd or Pb stress, the maximum contents of Cd or Pb in the hairy roots are2532mg/kg and2011mg/kg respectively. Under the condition of Cd-Pb compound stress, the maximum contents of Cd and Pb are2223mg/kg and1960mg/kg respectively. Pb stress has the effect of promoting the absorbing of Cd while Cd has the effect of restraining the absorbing of Pb. Compartmentalization and fixed holding of detoxification mechanisms of heavy metal by cell wall vacuole are the strategies of the hairy roots’ endurance to Cd and Pb stress. |
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