Lead translocation and localization in Allium cepa roots

Based on autoradiographic (210Pb) and ultrastructural studies, it was determined that lead is taken up from solution with the same intensity along the length of Allium cepa L. roots. As expected, the rate of uptake is dependent on the lead concentration and exposure time of the roots to lead salts. Lead takes about 80 min to penetrate radially through consecutive root tip layers in the meristematic zone. Lead is first deposited on the root surface, and then is translocated radially through the middle lamella (apoplast) of root cap cells. The lead reaches protoderm cells and meristematic cells of the hypodermis, where it penetrates into the symplast. Lead passes through consecutive ground meristem tissue layers at the rate of 1 cell layer/5 min. Its deposition in the ground meristem tissue is surprisingly nonuniform. It is deposited uniformly in the first six layers, but in the deeper (7th to 10th) layers, lead deposits are encountered in intercellular spaces and the middle lamellae, then in the cell walls near the plasmalemma, and finally in vacuoles.