Effects of 3-Indolylacetic Acid and 3-Indolylacetonitrile on the Growth of Excised Tomato Roots

Abstract

The inhibition by IAA (3-indolylacetic acid) and by IAN (3-indolylacetonitrile) of the growth of excised tomato roots cultured for 7 days at 27° C. in a modified White's medium is described. 5×10−9 g./ml, IAA or 5×10−6 g./ml, IAN cause approx, 50 per cent, inhibition of the linear growth of the main axis. With IAA decrease in number of laterals closely parallels the decrease in linear growth of the main axis; with IAN reduction in linear growth of the main axis occurs at concentrations above 10−8 whereas number of laterals does not decrease until the concentration exceeds 10−6. Study of the course of cell elongation in the exodermal cells showed that in the standard medium and in media containing 5×10−9 IAA or 5×10−6 IAN the process takes about 7 hours; the final cell lengths in IAA and IAN media are lower than in standard medium owing to a slower rate of elongation. The decrease in linear growth of the main axis in presence of IAA could be accounted for by the decrease in cell length; this was not the case with IAN. The implications of this are considered. Determinations of the distance (mm.) between, and of the number of exodermal cells separating, the adjacent laterals in one orthostichy showed that IAN enhances the frequency of laterals whereas this is either unaffected or decreased by IAA. The enhancement of lateral frequency in IAN arises from shortening of the cells of the main axis and decrease in the number of cells separating adjacent laterals. The results are considered to support the view that IAN has effects on root growth different from those of IAA. Study of the degree of inhibition of main axis growth and of alterations in lateral frequency resulting from treatment with mixtures of IAA and IAN provided data which could also be most easily explained on this hypothesis.