Uptake and localization of lead in corn (Zea mays L.) seedlings, a study by histochemical and electron microscopy

A detailed microscopic analyses of the mode of lead uptake and localization in root, stem and leaf initials of corn seedlings grown in cultural solution is reported. Lead, as other minerals, was passively absorbed into the root tip region and transported by the active apical growth force. The entrance of lead was mainly from the thin epidermal cell walls at the meristematic region. Only limited amounts of lead entered into protoplasts, primarily during the early stage of cell development. However, accumulation of lead within cell walls increased as cells matured. Both local and long distance transport of lead were apoplastic. Once the vascular stele differentiated from the ground meristem, lead entered into the central conducting systems. Lead was also absorbed from the water absorbing zone but this lead mainly remained in the cortex. The Casparin strip was an effective barrier limiting the movement of lead. However, transport through passage cells was observed. The direction of lead movement was also mainly towards high growth rate areas as seen at meristematic root initials, the leaf primordia and the transaction zone at the stem apical meristem. Damaged plasmalemmae allowed lead to enter cells from cell walls. Lead deposition in the cytoplast was observed.