Responses And Potential Remediation Of Three Three Xerophilous Shrubs To Cd And Pb Stress Under Drought Condition

Cadmium and lead are the most common toxic heavy metals in soil.When heavy metals accumulate in topsoil,the function of soil ecosystem would be destroyed.And when the heavy metals enter into the plant,it would reduce the chlorophyll content of plant leaves,inhibit respiration,have a direct impact on cell metabolism,destroy the plant enzyme system,affect the activities of various enzymes,reduce plant biomass.Heavy metals are enriched through the food chain,threatening human health.The heavy metal usually appears a variety of metals in contaminated soil forming compound pollution.Variety of coexistence of heavy metals is likely to change each other,s biological effectiveness,thus to explore the coexistence of a variety of heavy metals on the influence of each other is necessary,can be more targeted for phytoremediation.In arid and semi-arid area,drought stress has become a major limiting factor for plant growth.Plant diameter,height,biomass,root shoot ratio will change under long-term drought stress.Therefore,drought is one of the most important limiting factors in phytoremediation of heavy metal contaminated soil in the northwest inland area.In this study,seedlings of the biennial Hippophae rhamnoides Linn.,Amygdalus pedunculata Pall.,Buddleja alternifolia Maxim.were used as test materials,and two factors including water and heavy metal stress were tested,comprising two water treatments(60%–65% and 30%–35% soil relative water content)and three kinds of heavy metal stress including different concentration gradient cadmium(Cd),lead(Pb)single and compound pollution.A two factor randomized block design was used to design a total of 18 treatments with 5 replicates per treatment.The seedlings’ growth,biomass,heavy mental content,and photosynthetic parameters were measured.The purpose of this study was to understand the mechanism of heavy metal tolerance of three species and explore the potential of these three species to repair Cd and Pb contaminated soil.The main results were as follows:Under normal water condition,plant growth and biomass of all three were rejected under single and combined Cd and Pb stress,and the inhibitory function was raised with increase of concentration.Plants showed the same law under drought and heavy metal combined stress.Because of the difference of heavy metal toxicity to plants,different heavy metals have different inhibitory effects on plant growth and biomass.From the point of biomass,the inhibitory effect of Cd-Pb combined stress on plants is higher than that of Cd single stress,and the inhibitory effect of Pb stress on plants is relatively light.Under different pretreated conditions,B.alternifolia biomass accumulation was better than the other two species.The damage of heavy metals to plants is a slow process,and high concentration and long time of heavy metal stress can block the process of photosynthesis.Different heavy metal treatments led to decreased in net photosynthetic rate,stomatal conductance,transpiration rate,the intercellular CO2 concentration that Pn,Gs,Tr,Ci decreased more under Cd-Pb compound stress than Cd,Pb single stress.Drought stress could aggravate the effects of heavy metal stress on plants,which further lowers the photosynthetic capacity.Under drought and Cd-Pb stress,the chlorophyll content in leaves of Hippophae rhamnoides Linn.,Amygdalus pedunculata Pall.was higher than that of single Cd-Pb stress due to excessive water loss and chlorophyll concentration,but the ratio of chlorophyll a/ b decreased significantly.In Cd,Pb single and combined stress,with the increase of soil heavy metal concentration,the content of proline,soluble protein,MDA and the activities of SOD,POD,CAT increased in different degree of all three xerophilous shrubs.However,the magnitude of the increase varies with plant species and heavy metals.On the whole,under the combined stress of drought and heavy metals,the six indexes increased more than single heavy metal stress.Based on the determination of Cd and Pb in three xerophilous shrubs,the tolerance and accumulation ability of three plants to Cd and Pb were studied,the results showed that: under different treatments,there was a significant positive correlation between the absorption of two heavy metals in the roots,stems and leaves of the three species and the contents of heavy metals in soil.Under compound stress,the accumulation of Cd and Pb in all three had a greatly declined to single treatments of Cd and Pb.While under drought conditions,the accumulation of Cd and Pb in plants was lower than that of the conventional water conditions.Buddleja alternifolia Maxim.showed stronger enrichment ability of Cd and Pb than Hippophae rhamnoides Linn.,and Amygdalus pedunculata Pall.Based on the calculation of translocation factor for the aboveground tissues,an extremely low TF was found in stem and leaves,indicating that Pb sequestration was mainly restricted in roots.From all the results stated above,we could know that: as the main heavy metal pollutants in the city environment,Cd and Pb had an obviously effect on three xerophilous shrubs,and drought stress could aggravate the effects of heavy metal stress on plants.Compared with the variation of all the characteristics,both of them could affect phytoremediation to complicated heavy metal ground.However,as a good candidate for phytoremediation,B.alternifolia exhibits better growth and biomass accumulation than the other two varieties,thus could be recommended for phytostablization of Cd and Pb in contaminated areas.