**1. Introduction**

Heavy metals occur naturally in the ecosystem, most of them in trace quantities [1]. Depending on their concentration, some of them like Zn, Fe, Mn and Cu are essential plant nutrients [2, 3, 4], others like Pb, Hg and Cr do not have any known use to plants.

Deposition of heavy metals in soils from anthropogenic activities have been implicated for an increase in heavy metal concentration above background and recommended levels [5, 6, 7]. Heavy metals are important components of agro-allied products such as pesticides, herbicides, fertilizers; manufacturing and other synthetic products such as paints and batteries [8]. Mining activities, industrial, municipal and domestic wastes have been reported to be important sources of heavy metal pollution to the environment [9].

Combustion of fuel from petroleum, abrasion of tyres, brake lining, corrosion of the body work of vehicles and engine wear have been associated with elevated concentrations of Cd, Cu, Mn, Ni, V and Zn [10, 11, 12, 13].

Excessive concentration of heavy metals in the environment is of great concern because of their non-biodegradability. Therefore, their persistence in the environment portends health hazard to plants and animals and consequently trigger ecological imbalance in the ecosystem [14]. Another concern that high concentrations of heavy metals raise is their ability to bioaccumulate across the food chain, with members that are high up the food chain having concentration of such metals several times higher than what is obtainable in the environment [15, 16, 17].

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The effect of metals in living organisms could be chronic, due to exposures over a long period of time as a result of food chain transfer or acute poisoning due to ingestion or dermal contact [18]. The concentration of heavy metals in the shoot of plants may vary with season as a result of inherent growth dynamics of the plant; metal concentration and its bio-availability in the environment. Data on the response of plants to anthropogenic modification of the environ‐ ment, particularly in relation to soil and air pollution by heavy metals in northern Nigeria is limited and includes earlier assessments [3, 6, 15, 16] and more recently [19], in which an attempt was made to document the visual symptoms expressed by some ruderal plant species in relation to air pollution as a step towards developing a reference for field identification of pollution events [19].

This study was carried out to determine the spatial and seasonal variations of heavy metal deposition in soils and plants in Nigeria's Sudan Savanna in order to assess the extent of pollution and to identify indigenous plant species that may be pollution tolerant and thus have potential for use in phytorememediation of heavy metal polluted sites.
