Diuron-induced cytological and ultrastructural alterations in the root meristem cells of Allium cepa

EC50 concentration of diuron, a substituted urea herbicide, was determined as 78.9 ppm against the root growth of Allium cepa. Tested concentrations o f diuron exerted inhibition of cell division in a dose-dependent manner and induced various mitotic abnormalities at significant levels in 6- and 24-h exposures. Autoradiographic analysis revealed significant inhibition of ra diolabeled thymidine and uridine at 24 h and that of leucine at 48 h of exp osure to diuron at 80 ppm. Root tips exposed to diuron (40-160 ppm) for 24 h and examined under scanning electron microscope exhibited severe morpholo gical changes such as swelling of the mitotic zone and blunting of the root cap at 48 h. At higher magnifications the epidermal surface of these root tips appeared to be folded or invaginated. Tumor-like growths were also not ed when the treatment time was extended to 96 h. Diuron- (40-160 ppm) treat ed cells, examined under transmission electron microscope, revealed disrupt ion of the cell wall and plasma membrane, stretching of proplastids, and lo bing of nuclei at 24 h. The number of cortical microtubules along side of p lasmalemma appeared to be reduced in interphase and preprophase cells at 24 h of exposure. A parallel array of spindle microtubules was observed to be disturbed in diuron-treated metaphase cells. Telophase cells were observed to contain incomplete cross walls accompanied with very few microtubules p resent at the site of wall formation. These observations indicate the toxic ity potential of diuron at cellular and subcellular levels, particularly wi th regard to nonphotosynthetic tissues, (C) 1998 Academic Press.