Effects of heavy metals and strontium on division of root cap cells and meristem structural organization

In order to define relations between the behavior of quiescent center cells and the condition of root cap cells, effects of various metal salts on the root meristem structure, root growth, and division of root cap cells were investigated. Two-day-old maize (Zea mays L., cv. Diamant) seedlings were incubated on solutions containing 35 μM Ni(NO₃)₂), 10 μM Pb(NO₃)₂, or 3 mM Sr(NO₃)₂ in the absence or in the presence of 3 mM Ca(NO₃)₂. Toxic effects of metals were assessed from inhibition of the primary root length increment following 24-h and 48-h incubations as compared to the roots grown on water or on 3 mM Ca(NO₃)₂ solution. Metal localization in the root apex tissues following 24-h and 48-h incubations was determined using histochemical techniques. Cell lengths in three upper layers of root cap columella were determined, and the mitotic index in these cells was calculated. In the absence of Ca(NO₃)₂, the metals were found both in the meristem and in the root cap. Pb and Sr were revealed primarily in the cell walls, and Ni, in the cell protoplasts. In the presence of Ca(NO₃)₂, metal content in all root tissues was decreased, and their toxic effect on root growth was ameliorated. Pb and Ni inhibited cell division in the root cap. Pb caused an increase in the root cap cell length as early as following 24-h incubation, and Ni, only following 48-h incubation. Pb activated division of quiescent center cells in the direction of root cap. These effects, as well as possible involvement of dermatogen and cortex cells, resulted in a regrowth of a new root cap already after a 24-h incubation period. In this case, the meristem was transformed from a closed structure into the open one. Following 48-h incubation, Ni brought about only few divisions of quiescent center cells in the direction of root cap. It was suggested that inhibition of divisions of the root cap upper layer cells and a decrease in the sloughing off its cells can stimulate the quiescent center cell divisions. A similarity of the quiescent center and animal stem cells is discussed.