Studies on growth inhibitors in the root tips of Zea mays L. and their distribution during geotropic stimulation

Abstract

A micro-assay based on the growth inhibition of root segments Zea mays roots has been designed to investigate the root growth inhibiting substances present in the root tips of Zea mays. Two major growth inhibitors are present in the acid fraction of the extracts of root tips of maize. One of them is principally located in the root cap. This inhibitor, from its chromatographic properties as well as antitranspirant activity is suspected to be ABA. This inhibitor cannot move far beyond the meristematic zone. The other major inhibitor is predominantly present in the meristem and absent in the cap. The chemical nature of this inhibitor is unknown. Neither of these substances is IAA, whose presence in meristems is sometimes indicated by small inhibitions (or stimulations) at the Rf of marker IAA. The third inhibitor which is comparatively less active but appears in the acid and neutral fractions of the tip extract is also present in the meristem and absent in the cap. This inhibitor also possesses the anti-transpirant property like the cap inhibitor and is suspected as xanthoxin.

Production of this ABA-like cap inhibitor is dependent on light and its production is maximal in the root tips stimulated in light. In the dark greater concentrations of these two major inhibitors are found in the lower half of a stimulated root. On the other hand, when roots are stimulated in the presence of light the cap inhibitor moves in the upper half whereas the inhibitor of the meristem is restricted in the lower half of the stimulated tips. The implications of these findings for the geotropic responses of roots is briefly discussed.