Microscopic analysis of lead accumulation in tobacco (Nicotiana tabacum var. Turkish) roots and leaves

A hydroponic experiment was carried out to localize lead accumulation in tobacco roots and leaves. Plants were grown for seven days in Hoagland's solution supplemented with different concentrations of lead nitrate [Pb(NO3)2]: 0.0 (control), 10 μM, 100 μM, and 500 μM. Growth was inhibited in plants treated with 100 μM, and 500 μM. Light and electron microscopic studies showed that lead accumulated mainly in cell walls and vascular tissues of roots of plants exposed to 100 μM, and 500 μM lead nitrate. In contrast, roots exposed to 10 μM lead nitrate did not show any detectable accumulation of lead in the roots. The TEM images confirmed the presence of lead outside the epidermis of the roots in the form of electron dense clusters of fine needles. No lead was detected in the leaves. However, plants exposed to 100 μM lead nitrate were characterized by the presence of ferritin clusters in the chloroplasts. This suggests a protective mechanism by the plant to prevent oxidative damage caused by Pb. Plants exposed to 500 μM lead nitrate showed mesophyll cells containing altered chloroplasts with disrupted thylakoid systems.