Physiological Response And Tolerance Evaluation On Six Urban Greening Seedlings Under Soil Cu And Pb Contamination

The pot experiment was adopted and biennial seedlings of the urban greening tree species, containing Fraxinus pennsylvanica, Sophora japonica, Salix matsudana, Amorpha fruticosa, Euonyrnus japonicas, Forsythia suspense, were selected as test materials to study their changes of growth, physiological and biochemical indices under heavy metal stress of Cu and Pb. Response of seedlings to the heavy metal stress was also discussed. Indices with larger contribution to resistance and accumulation of greening tree species to heavy metal stress of Cu and Pb were chosen by the principle component analysis. Plant monitoring system of resist to soil pollutions of Cu and Pb was built in order to accumulate the fundamental data for selecting restoration plants for the heavy metal polluted soil and provide basis and references for choosing greening tree species of the polluted area. The results were as follows:On the conditions of the heavy metal stress, leaves faded green firstly and then appeared yellow with the increase of stress concentration. Height growth and basal diameter growth of plants were subjected to restriction of different extent with the increase of the concentration of heavy metal stress and the extension of time. On the same time, specific leaf weight declined significantly (P＜0.01).There was dose-effect relationships between stress concentration and chlorophyll content of the greening tree species under the different concentration of stress. The ratio of chlorophyll a and chlorophyll b having a trend of descending (P＜0.01)showed that the effect of stress of Cu and Pb on chlorophyll a of greening tree species was higher than chlorophyll b. Stress inhibited photosynthesis, the net photosynthesis rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) were all on the decline with the increase of stress concentration. However the intercellular CO2 concentration ( Ci ) was consistent with stress concentration. All showed that restrain factors of photosynthesis were mainly caused by non-stomatal reasons.Stress of heavy metal of Cu and Pb gave rise to the increase of the permeability of leaf membrance.This simulated the production of proline, then membrane peroxidation was enhanced and root activity decreased. That the activity of protective enzyme SOD and CAT increased initially and then declined with the increase of stress concentration showed that the active oxygen scavenging capacity of the protective enzyme system was inhibited. The activity of POD was higher than control group and contents ofmalondialdehyde kept increasing.On the condition of stress of Cu and Pb, the uptake of metals by greening tree species was mostly cumulated in root and absorption of aboveground part also mainly distributed in stem but less in leaves. The amount of heavy metal of all parts of greening tree species raised with the increase of concentration of Cu and Pb. In 2000 mg·kg-1 of Cu concentration, Root of Forsythia suspense had the highest amount of Cu, reaching at 725.86 mg·kg-1.Stem and leaf of Euonymus japonicas had the most ones with 426.56 mg.kg-1 and 88.87 mg·kg-1 respectively. On the condition of stress of Pb, there was the highest amount of Pb in all parts of Euonymusjaponicas. Concentration of Pb in leaf was 33.29 mg·kg-1, stem and root were 60.87 mg·kg-1 and 184.59 mg·kg-1 respectively.Stress of Cu and Pb significantly affected contents of K, Ca and Mg of organs of greening tree species.The amounts of K, Ca and Mg showed a trend of decreasing (P＜0.01) with the increase of concentration of Cu and Pb.Index system for plants' tolerance to heavy metal was established, and indexes with larger contribution to resistance and accumulation of greening tree species to heavy metal stress of Cu and Pb were chosen by the principle component analysis.Applying the method of subordinate function, six green tree species were measured and a comprehensive evaluation was given as follows. Tolerability order of six tree species to Cu pollution is: Euonymus japonicus＞Sophora japonic a＞Fraxinus pennsylvanica＞Salix matsudana＞Forsythia suspensa＞Amorpha fruticosa. Accumulation ability order to Cu is: Euonymus japonicus＞Forsythia suspensa＞Sophora japonica＞Amorpha fruticosa＞Salix matsudana＞Fraxinus pennsylvanica.Tolerability order of six tree species to Pb pollution is: Euonymus japonicus＞Sophora japonica＞Fraxinus pennsylvanica＞Forsythia suspensa＞Salix matsudana＞Amorpha fruticosa.Accumulation ability order to Pb is: Forsythia suspensa＞Salix matsudana＞Sophora japonica＞Euonymus japonicus＞Amorpha fruticosa＞Fraxinus pennsylvanica.