Lead in the apoplast of Allium cepa L. root tips -: ultrastructural studies

Onion (Allium cepa L.) adventitious roots were treated with lead, which was found to accumulate in increasing amounts in the apoplasts of root tips if the metal continued to be present in the roots' external environment. Ultrastructural observations made it possible to formulate the hypothesis that there are two protective mechanisms against lead in onion root tip cells: (i) the amount of polysaccharides in the cell wall and the thickness of the wall increases, this allows more lead to be accumulated in a manner that is not harmful to the cell; and (ii) lead is expelled from inside the cell through plasmatubules to the root tip apoplast, this makes it possible to inactivate the lead that has a toxic effect on the root symplast. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.