Morphological response of radish (Raphanus sativus, Brassicaceae) roots exposed to toxic concentration of copper (Cu) include many root growth changes. Kováč et al. find that such changes are coupled with the formation of subero-lignified apical deposit (SLAD) very close to the root tip.
This unique structure consists of modified cell walls of central cylinder cells and endodermis. SLAD forms after cessation of root elongation, and it is coupled with xylem differentiation and root branching close to the root apex. Formation of SLAD is probably a part of stress induced responses, which relates to decreased root growth or permeability in heavy metal stress.
Kováč, J., Lux, A., & Vaculík, M. (2018). Formation of a subero-lignified apical deposit in root tip of radish (Raphanus sativus) as a response to copper stress. Annals of Botany. https://doi.org/10.1093/aob/mcy013