Formation and Partial Characterization of Growth Inhibitors from Cultured and Intact Root Caps

Light induces the formation of growth inhibitors in root caps. Two classes of inhibitors are formed, acid and neutral inhibitors. The acid inhibitor forms only in intact caps (attached to the root), whereas the neutral inhibitor forms in caps as well as in more basal regions of the root tip. The acid inhibitor appears to move from the cap basipetally in the root and forms first, usually within 30 min of beginning illumination. For its induction, the neutral inhibitor requires a longer period of illumination, generally 30–60 min. The acid inhibitor is comprised of one compound with chromatographic properties indistinguishable from abscisic acid, whereas the neutral inhibitor is composed of at least two distinct compounds for one of which 14C-mevalonic acid is a precursor. We were not able to demonstrate unequivocally the incorporation of 14C-mevalonic acid into the acid inhibitor.