Effects of Inhibitory Concentrations of 3-Indolylacetic Acid and 3-Indolylacetonitrile on Cell Division and Tissue 2

Abstract

Inhibitory concentrations of 3-indolylacetonitrile (IAN) cause, in cultured excised tomato roots, a marked decrease in the rate of cell division at the apical meristem but only a slight reduction in the lengths of mature exodermal and cortical cells. The reduced rate of cell division is associated with a decrease in the number of meristematic cells at the root apex. By contrast, 3-indolylacetic acid (IAA) causes marked reduction in the lengths of mature cortical cells but does not markedly reduce cell-division rate at the apical meristem. Various lines of evidence indicate that both IAA and IAN cause a relative increase in the number of longitudinal and a decrease in the number of transverse division walls in the meristematic zone of the root apex. Partial inhibition of the linear growth of excised tomato roots by IAA and IAN is accompanied by increases in root and stelar diameters. These increases result from radial enlargement of the cortical cells and increase in the number of stelar cells in the transverse section. The enlarged steles contain an increased number of lignified xylem elements, but only with the most inhibitory concentra tion of IAN (10 4g./ml.) is there evidence of the development of secondary xylem. Both auxins increase significantly the xylem vessel unit length.