Ornamental Shrub Capacity for Absorption and Accumulation of Heavy Metals from Urban Polluted Soil

Roadside soil is often strongly polluted by heavy metals from vehicular traffic, especially in urban areas. Plants can contribute to reduce this environmental concern of many cities across the world not only by the direct and indirect action of the canopy but also by uptake of heavy metals by roots and translocation to epigeous organs. The subsequent removal of plants is the basic phase of soil-phytoremediation but metal immobilization in tissues is already effective to prevent leaching. The ability of 9 evergreen ornamental shrubs, commonly used in Mediterranean urban landscapes, to absorb and accumulate Cu, Ni, Pb and Zn in the plant organs was evaluated. Young plants of Arbutus unedo, Eleagnus x ebbingei, Ilex aquifolium 'Nelly Stevens', Laurus nobilis, Ligustrum japonicum, Photinia x fraseri 'Red Robin', Pittosporum tobira, Viburnum tinus and Viburnum tinus var. lucidum were potted with two media including 20% sphagnum peat and 80% 'non-contaminated' soil from a rural area (control) or 'polluted soil' obtained by mixing (1: 1 vol.) soil from a petrol station and soil from a drain-line area, next to an urban street with intense vehicular traffic. After 1-year growing in a screenhouse, the polluted soil had no negative effects on canopy height and diameter, shoot number/plant, leaf surface and dry weight of leaves and stems. A significant interaction between cultivar and substrate was detected for the new leaf surface and for the dry weight of roots and 1-year-old shoots. In detail, polluted soil influenced the new leaf surface only in Photinia and Pittosporum (+47.9% and +78.6%, respectively), the root dry weight in Arbutus, Ilex, Photinia and Ligustrum (-37.7%, -23.9%, +55.5% and +33.6%, respectively) and 1 year old shoot dry weight in Pittosporum, Photinia and Ilex (+96.7%, +58.3% and -25.0%, respectively). The heavy metal content in leaves, stems and roots, determined by ICP-MS, demonstrated the different aptitude of species to absorb and translocate the heavy metals. Ligustrum accumulates especially Ni and Pb (0.33 and 0.26 mg/plant) with the highest amounts in the thin root fraction; Viburnum tinus var. lucidum accumulates mainly Zn (5.66 mg/plant) with the highest concentration in thin roots and 1-year-old leaves.