| Cover plant is a basic component in the urban oranamental plant ecosystem, which plays an important role of linkages among tree layer, shrub layer and lawn in urban vegetation, and environmental conservation, landscape beautification and biodiversity nursing. Theoretically, the growth and reproduction of cover plant might be suffered from increasingly soil lead (Pb) contamination since the cover plant is directly exposed to contaminated-Pb soil. However, more attentions have been given to the responses of crop and garden tree species to soil Pb stress, and focused on the physiological responses of the mature leaves. In fact, the physiological responses to soil Pb contamination might be different in leaves with different physiological stages. Moreover, whether or not does soil Pb contamination influence on plant nutrition? As yet, there is lack of the related reports.Ageratum conyzoides and Ophiopogon bodinieri are two representative cover plants, which are widely used in urban landscape construction. In order to understand the ecological responses of these two cover plants to soil Pb contamination, therefore, a pot experiment was conducted to to investigate the effects of different soil Pb concentration treatments (0,250,500,750,1000,1250, and 1500 mg-kg-1 dry soil) on A. conyzoides and O. bodinieri. The objective was to provide scientific basis for choosing cover plant in urban landscape construction.(1) Soil Pb stress decreased the contents of total chlorophyll, chlorophyll a and b in A. conyzoides leaves, indicating that soil Pb contamination might lower the phytosynthesis of A. conyzoides leaves.(2) uperoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities, plasmalemma permeability, and malondialdehyde (MDA) content in A. conyzoides leaves at the same period increased with the increase of soil Pb content at the lower stress level (≤750 mg/kg or 1000 mg/kg), and thereafter decreased with increasing soil Pb content. POD activity in A. conyzoides leaves gradually increased as soil Pb stress proceeded, and the POD activity treated after Pb-exposed 20,40, and 60 days was significantly higher than that after Pb-exposed 7 days. POD activity decreased sharply in A. conyzoides exposed to soil Pb stress after 20 days, while CAT activity sharply decreased till after Pb-exposed 60 days. The results imply that A conyzoides can adopt the Pb-contaminated soil environment to a certain extent, but higher soil Pb stress (> 750 mg/kg) would influence the physiological metalism of A. conyzoides.(3) The freshly leaf (FL), mature leaf (ML) and old leaf (OL) of O. bodinieri were sampled based on the different physiological periods. The activities of SOD, POD and CAT in O. bodinieri leaves increased with the increase of soil Pb content, and thereafter decreased with the increase of soil Pb stress level, depending on the protective enzyme types. Meanwhile, the contents of free proline (Pro) and MDA in O. bodinieri leaves increased with the increase of soil Pb content. However, different responses of different leaves to soil Pb content were observed in O. bodinieri. The FL was the most sensitive to soil Pb stress, secondary ML, and the last OL. The result also suggested that O. bodinieri could adapt the Pb-contaminated soil environment to a certain extent, but different leaves had different response and adaptation mechanisms.(4) The treatments with lower Pb contents increased the biomass growth, and the contents and nutrient accumulations of nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg) both in A. conyzoides and O. bodinieri tissues in evidence. However, higher Pb stress level clearly inhibited the growth, and the contents and nutrient accumulations of P and Mg in A. conyzoides and O. bodinieri tissues, and altered the nutrient allocation pattern in the trial plants. The result suggested that A. conyzoides and O. bodinieri could adapt the Pb-contaminated soil environment to a certain extent, could be used in landscaping in slightly Pb-contaminated soil area. |
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