LEAD IN THE APOPLAST OF ALLIUM-CEPA L. ROOT-TIPS - ULTRASTRUCTURAL STUDIES

Onion (Allium cepa L.) adventitious roots were treated with lead, whic h was found to accumulate in increasing amounts in the apoplasts of ro ot tips if the metal continued to be present in the roots' external en vironment. Ultrastructural observations made it possible to formulate the hypothesis that there are two protective mechanisms against lead i n 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 roo t tip apoplast, this makes it possible to inactivate the lead that has a toxic effect on the root symplast. (C) 1998 Elsevier Science Irelan d Ltd. All rights reserved.