Bot. Dep. Women's College Ain Shams Univ., Heliopolis, Cairo, Egypt Pollution Unit, Desert Research Center (DRC), Cairo, Egypt
Studies were conducted to present evidence that nodulation, biomass, chlorophyll and metal content of a forage legume (clover) may serve for ecotoxicological evaluation of contaminated areas. Substances affecting the macro- and/or microsymbionts, such as certain heavy metals were examined. The experiments were carried out as pot experiments using loamy sand soils. Unpolluted soils were first mixed with four levels of soluble salts of four tested heavy metals (Pb 50-450 mg/kg, Cd 25-200 mg/kg, Zn and Cu 250-1000 mg/kg), appropriate controls were run in parallel. Clover (Trifolium alexandrinum cv. Compound sids) was planted in each treated soil and subsequently inoculated with Rhizobium leguminosarum bv. trifolii ARC300. Plants were collected at 48, 80 and 140 days of growth and analyzed for nodulation, dry matter, leaf area, chlorophyll content and metal uptake.
Applying trace element resulted in a significant (P<0.05) decrease in number of nodules and nitrogen content of plants. Nodule formation was inhibited by 60, 56, 48 and 21% at higher concentrations of Pb, Cd, Cu and Zn, respectively, in plants grown for 140 days. A closer look at the nodule tissue of heavy metal treated clover was less than that of the control nodule. In the central zone, most of the infected cells contain less numbers of fully developed bacteroids. The reduction in the number of infected cells within nodules suggested that these nodules fixed little nitrogen. (McGrath, Brookes 1988). This effect of nodulation was reflected in plant nitrogen content. The comparative toxicity of the tested metals to symbiotic N2-fixation indicates that the overall order of decreasing toxicity is Cd > Cu > Zn > Pb. Heavy metal accumulated in various parts of plants resulted in retardation of growth and reduction of chlorophyll content, which expressed itself as the visual symptoms of chlorosis. The reduction in chlorophyll content in turn, at least partly, would lead to a decrease in shoot length and biomass, this could explain the positive correlation between chlorophyll content and biomass as well as metal toxicity. The mechanisms responsible for heavy metal phytotoxicity might be related to membrane impairment (Kabata-Pendias, Pendias 1984). Trends in metal concentration in clover as a function of metal content in soil was linear; uptake of Cu, Pb and Cd by tops does not occur in a linear concentrations of the metal in soil; Zn is mobile inside the plant and can be highly translocated from root to shoot parts, causing a significant increase in Zn concentrations in clover above ground part. This means that clover is especially useful where there is a need for a sensitive indicator of differences between sites or sampling occasions. In conclusion, clover plants represent a wide array of behavior toward metal toxicity symptoms.
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