| Heavy metals are potentially harmful pollutants in the soil environment and cannot beeasily leached with water, and cannot be decomposed by soil microorganisms. A factor thatmakes them difficult is that they tend to accumulate over time and reach toxic levels. Someheavy metal elements in the soil can also be transferred into more toxic methyl compounds.Therefore, the soil which is polluted by heavy metals will affect plant growth anddevelopment. Furthermore, when the pollution is serious enough, it will threaten the safety ofagriculture, animal husbandry and other industries. By contrast, the salts of heavy metals mayprovide biological enrichment which can benefit the soil environment. Because some turfplants can grow normally in the polluted soil, the heavy metal ion content in the soil could begradually reduced through frequent mowing and disposal of the clippings. By this meansrecovery the polluted soil could be achieved.To evaluate both the effects of heavy metals on various turf species and the prospects forremediation of polluted soil, the toxic effects of heavy metals on turf plants and their responsemechanism to the stress were studied. The objective was to improve the turf plants andprovide a theoretical basis to breed resistant turf plant varieties to play an important role inphytoremediation. In this experiment, four turf plant species, including Kentucky bluegrass(Poa pratensis cv. Barrister), perennial ryegrass (Lolium perenne cv. Premier, tall fescue(Festuca elata cv. Reebok) and white clover (Trrifolium repens cv. Haifa), were selected asthe research material to study the effects of four heavy metals (cadmium, Cd; copper, Cu; lead,Pb; zinc, Zn). The effect on seed germination, physiological characteristics, shoot and rootsystem, photosynthetic physiology, cell membrane permeability, activities of antioxidantenzymes were studied. The ability of metal ion to accumulate within tissues and theirdistribution sites as well as cell ultrastructure of four turf plants was also examined. The turfplants’ ability to accumulate heavy metal ions was assessed via screening process. Some turfspecies showed an ability to grow well in the presence of some heavy metals, but not all. Themain results are as follows:1. Effects of heavy metal stress on Germination of turf plants’ seedsHeavy metal ions had different effects on the germination of four kinds of turf plants.Kentucky bluegrass showed the lowest resistance to Cu2+stress; Perennial ryegrass and tallfescue seeds had a certain resistance to Cu2+; Tolerance to Cu2+of White clover was at anintermediate level. Under the stress of Zn2+, tall fescue and perennial ryegrass showed strongtolerance. The germination of tall fescue was more than100%, when concentration of Zn2+reached1100mg/kg showing the promoting effect. Pb2+had significant stress on thegermination rate of four kinds of turf plants, and the influence was more pronounced in whiteclover and Kentucky bluegrass than in tall fescue and perennial ryegrass; The influence ofCd2+at low concentration on germination of perennial ryegrass, tall fescue and white cloverrelative germination was minor but at high concentration germination was poor. Lowconcentration of Cd2+promoted germination of Kentucky bluegrass.The development of the radicle and plumule under the stress of Cd2+was assessed on allfour species. There was a pattern promoted under low concentration and suppression underhigh concentration. At the same time, inhibition of root growth in all four species over arange of heavy metal ion concentrations was higher than inhibition of the radical and plumule. Germination index and vigor index of4kinds of turf species decreased as heavy metalion concentration increased, but the two indexes had different trajectories, the decline of thegermination index was smaller than that of the vigor index as concentration of metal ionsincreased.2. Effects of heavy metal stress on growth of turf plants.Except at the lowest concentrations, the total root length, root surface area, total branchnumber and total number of root tips of tall fescue were significantly lower than the control,and decreased gradually as ion concentration increased; Perennial ryegrass, Kentuckybluegrass and white clover showed stronger resistance to low concentration of heavy metalions but resistance to high concentration was poor, and response to incremental increases inconcentration was more obvious.The effects of heavy metal ions on aboveground biomass of four turf species weredifferent. Low concentrations of Cu2+and Zn2+could promote above ground biomass, but asthe ion concentration increased the above ground biomass decreased. Low concentrationtreatment of Cd2+could also promote above ground biomass of perennial ryegrass, whiteclover and Kentucky bluegrass, but with increasing of ion concentration above groundbiomass decreased. Under the stress of Pb2+, the above ground biomass of four kinds of turfplants significantly decreased, irrespective of concentration.Low concentration of Cd2+treatment promoted the height of perennial ryegrass, but hadlittle effect on tall fescue, white clover and Kentucky bluegrass. With the increasingconcentration of Cd2+, the plant height of four kinds of turf plants decreased. Lowconcentration treatment of Cu2+and Zn2+promoted the height of perennial ryegrass, tallfescue and white clover, increasing ions concentration inhibited growth. Both Cu2+and Zn2+,inhibited the height of Kentucky bluegrass (lower than the Control) and increasing of ionsconcentration had a greater impact. The heights of four kinds of turf species were lower thanthe control at all levels of Pb2+and the effect was more severe with increasing ionconcentration.3. Effect of heavy metal stress on Photosynthetic Physiological Characteristics ofturf plantsLow concentration of Cd2+and Zn2+treatment had little effect on chlorophyll content ofperennial ryegrass, but under high concentration, the content of chlorophyll was significantlylower (P<0.05) than the control. Both Cu2+and Pb2+had significant effects on chlorophyllcontent of perennial ryegrass. Tall fescue showed strong resistance to low concentration ofCd2+, but resistance to other ions was poor. The growth of white clover and Kentuckybluegrass was suppressed by Cu2+and Zn2+treatment even at low levels. The resistance toPb2+of these two species was also poor.The effect on photosynthetic physiological characteristics of different concentration, offour kinds of heavy metal ions on the Fv/Fm of the turf species, was minor or evenstimulatory at low concentration, but at the higher concentration, there was a strong inhibitoryeffect; the PSⅡsystem showed a gradually decreasing trend.Under different concentration treatments of four heavy metal ions, qP and ETR values offour turf species were lower than the control, which showed that the photosynthetic activityand electron transfer efficiency were affected but there were differences in the qN valueacross the four species. 4. Effects of heavy metal stress on resilience of physiological characteristics of turfplants.Different heavy metal ions produced different degrees of stress on four turf species. Cd2+and Pb2+promoted the content of Proline significantly, even as the ion concentrationincreased. Effects of Cu2+and Zn2+stress on Proline content of four kinds of turf plantsshowed the phenomenon of Low levels promote and high levels suppress, but the contentincreased significantly as the ion concentration increased. At the same time, when the Pb2+and Cu2+reached maximum concentrations, the content of four kinds of turf speciesdecreased.The stress of Cd2+and Pb2+had more significant effects on membrane of four kinds ofturf species. Under low concentration treatment of Cu2+, the content of MDA had nosignificant difference than control. But as the concentration increased, the damage to plantsstrengthened, and the content of MDA increased. However, when the ions concentrationreached a threshold value, the content of MDA was reduced. The stress of Zn2+on membraneof perennial ryegrass and tall fescue was stronger than that of white clover and Kentuckybluegrass.Influence on activity of antioxidant enzyme activity by four kinds of heavy metal ionswas big. Low concentration treatments of Cd2+, Cu2+and Zn2+on antioxidant enzyme activityof perennial ryegrass, tall fescue and white clover had little influence, but the influence wasbig under high concentration; Treatments of Pb2+had a greater effect on antioxidant enzymeactivity of four kinds of turf plants. The effects of four kinds of heavy metal ions onantioxidant enzyme activity of four kinds of turf species were gradually diminished with theincreasing of ion concentration. But at the highest concentrations of Cu2+and Pb2+, theantioxidant enzyme activity of four kinds of turf species declined in different ways.5. Absorption and distribution of heavy metal ions on turf plants.The ability of white clover to accumulate heavy metals without serious damage was high,in contrast to Kentucky bluegrass where it was low. Perennial ryegrass and tall fescue were atan intermediate level.Tall fescue had high transport ability of Cd2+, Cu2+and Pb2+, white clover had strongtransport capacity of four kinds of heavy metal ions, perennial ryegrass had higher Cu2+andZn2+transport capacity, but the Cd2+and Pb2+transport ability was poor, Kentucky bluegrasshad strong transport capacity of Cd2+, but the transport capacity of the other three kinds ofheavy metal ions was poor.6. Effects of heavy metals on the microscopic structure of turf plants.Under the high concentration of heavy metal ions, leaf ultrastructure of four kinds ofgrasses was damaged seriously, mainly due to plasmolysis of cell wall, change of chloroplastshape, destruction of chloroplast grana lamellae structure, partial dissolve--in some caseseven the grana form cannot be seen. |
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