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the following order: roots > rhizomes > non-green leaves > green leaves. Our finding that Commelina and water hyacinth are capable of bio-accumulating metals in their tissues confirms other studies reporting the metal-cleansing potential of water hyacinth due to its rapid growth in wastewater and capacity to absorb metals. Blake et al. (1987) demonstrated the tendency for water hyacinth roots to exhibit a high affinity for Cd. Therefore these nonedible plants are useful for cleaning up wetlands already contaminated by heavy metals. Although the least amount of metals accumulated in plant leaves, accumulation in vegetable leaves is nevertheless of concern. Vegetables are reported to take up metals from contaminated soil as well as from aerial deposits on the aboveground parts exposed to polluted air. In Lagos, Nigeria, Yusuf et al. (2003) observed a higher degree of contamination in soils and higher concentrations of Ni in vegetables in industrial compared to residential areas, with the highest concentrations in Corchorus compared to other vegetable types. We found some vegetables accumulated higher levels of trace metals in the leaves than in the fruits. Pumpkin leaves (a local delicacy) accumulated the highest levels of trace elements from Namuwongo wastewater irrigated site which receives industrial effluents. Leafy vegetables grown in contaminated wetlands could therefore pose health risks to consumers. Fortunately, in the case of the popular Kampala root crop colocassia esculenta (cocoyam), metal concentrations accumulated in the order: Root > leaf > peel > tuber. Since the root is not eaten and the tuber is normally peeled before cooking, this decreases potential health risks to consumers. Traffic Pb contamination of roadside soils and Amaranthus leaves was clearly a function of traffic density and atmospheric deposition, mediated by distance from roads. Total Pb content in roadside soil was within the recommended maximum levels except for the three sites Banda, Najjanankumbi and Bukoto that also ranked highest in traffic density. Wheeler and Rolfe (1979) reported that Pb levels in vegetation increased linearly with traffic density. Exhausts from motor vehicles using leaded gasoline have been identified as one of the sources of Pb in the environment. Daines et al. (1980) reported that Pb in the urban environment is strongly related to traffic density. Studies on quantity and distribution of soil Pb have shown high soil Pb contamination in the inner city decreasing toward the periphery in several cities in the USA, e.g. New Orleans (Mielke 1994), Washington DC, (Elhelu et al. 1995) and New York (Johnson & Bretsh 2002), as well as Oslo in Norway (Tijhuis et al. 2002) and Ibadan, Nigeria (Sridhar et al. 2000). Total Zn content was above the recommended levels in soils only at Banda. Hence most roadside sites were found to contain Pb and Zn at concentrations considered acceptable for agricultural soil. In a similar study, Voutsa et al. (1996) observed low trace element content in agricultural soils in the greater industrial area of Thessaloniki, Greece, despite elevated concentrations of Pb, Cd and Zn in air particulate matter. Metal contents in soil were observed to decrease rapidly with increasing distance from roads. Accumulation of Pb in soils above background levels took place up to a distance of 30 m comparable to the approximately 33 m found by Rodriguez-Flores & Rodriguez-Castellon
HEALTHY CITY HARVESTS: GENERATING EVIDENCE TO GUIDE POLICY ON URBAN AGRICULTURE