The root cap and control of root elongation in Zea mays L. seedlings exposed to white light.

Light-induced inhibition of the elongation of primary roots of Zea mays seedlings is dependent upon the perception of light by the root cap. Separate exposure of detached root caps and roots from which root caps have been removed (i.e. decapped roots) to white light or darkness has shown that the elongation of a dark-exposed root to which a light-exposed root cap is attached is inhibited significantly, whilst a light-exposed decapped root to which a dark-exposed root cap is attached is not inhibited. Thus the decapped root is not capable of perceiving the growth-inhibitory stimulus of white light and its rate of elongation is greater than that of light-exposed roots with root caps still intact. However, the capacity to perceive light and, therefore, to be inhibited by it, is restored to the decapped root approximately 5 h after removal of the root cap.Abscisic acid and two unidentified growth-inhibiting compounds are present in the root caps of light-grown but not of dark-grown Zea mays seedlings and a period of 2.5-3.0 h is required for transport of the compounds, in levels effective in growth inhibition, from the root cap into the apex of the root.