Content and internal distribution of heavy metals in roots of plants grownat alkaline pH on slag from municipal solid waste incineration

In both, field and greenhouse experiments, the heavy metal (HM) contents of roots of plants grown on heavy metal contaminated slag from the incinerati on of municipal solid waste (MSW) (alkaline soil pH) were analysed as well as the internal distribution of the heavy metals within the root. Generally , roots of plants grown on slag exhibit higher heavy metal contents as root s of plants grown on control sites. This indicates that the alkaline soil s olution of slag is not an absolute barrier against the transfer of HMs from the slag matrix with its largely immobile heavy metals to the root. Transf er coefficients of the roots for heavy metals (HMroot/HMsoil) of slag grown plants are always lower than 1 (no enrichment). In general, they are highe r for essential than for non-essential heavy metals and higher for roots of control plants than for roots of plants grown on slag. The relative high H M content of the hypodermal/rhizodermal fraction of the roots of Zea mays g rown on either slags or control soil indicates its role as a major barrier against uptake of heavy metals into the symplast of the root. HM contents o f the endodermal fractions were generally lower than those of the hypoderma l/rhizodermal fraction, and those of the xylem fraction even lower than tho se of the endodermis. The HM content of the root cortex, respectively the r oot symplast in the roots of the control plants was lower than all other fr actions, whereas in slag grown plants it fits into the topographic gradient between the hypodermal/rhizodermal fraction and the xylem fraction. Result s suggest that in roots of control plants heavy metals are preferentially l ocalised in the root apoplast, and the symplast is largely bypassed. In sla g grown plants higher amounts of heavy metals reach the apoplastic space an d may saturate existing binding sites for divalent cations. Subsequently a higher proportion of heavy metals may enter the symplast of the root. In re sponse to increased HM concentrations in the root cortex of slag grown plan ts the synthesis of non-proteinous SH groups is stimulated as detoxifying m echanism. Criteria for HM toxicity in plants growing on polymetallic soils of alkaline pH are discussed.