THE INFLUENCE OF ZINC ON THE CELL-CYCLE IN THE ROOT-MERISTEM OF A ZINC-TOLERANT AND A NONTOLERANT CULTIVAR OF FESTUCA-RUBRA L

Seedlings of a zinc tolerant cultivar (Merlin) and a non-tolerant cultivar (S59) of Festuca rubra L. were grown in aerated solution culture at various concentrations of zinc. The elongation of the longest root was much more inhibited by increasing Zn concentration in S59 than in Merlin. The mitotic index in the root meristem was reduced to a greater extent by increasing Zn concentration in S59 than in Merlin, whilst the cell doubling time (estimated by colchicine arrest) was increased by Zn much more in S59 than in Merlin. The kinetics of the cell cycle in the root meristem were determined by a pulse labelling experiment with low specific activity tritiated thymidine following a 4-d exposure to three concentrations of Zn (0, 0.1, and 0.2 .mu.g Zn cm-3). Treatment with 0.1 and 0.2 .mu.g Zn cm-3 increased the length of the cell cycle by 40 and 132% respectively (compared with the zero Zn control treatment) in S59 but only by 6 and 16% respectively in Merlin. The increase in cell cycle length was due mainly to an increase in the duration of G1 in both cultivars; Zn had little effect on the duration of the other phases of the cell cycle. The growth fraction (the proportion of cycling cells in the meristem) was progressively reduced by increasing Zn concentration in S59 but was increased at 0.1 .mu.g Zn cm-3 in Merlin and then reduced to the control level at the higher Zn concentration.