SELECTIVE HYPOXIC DEPRESSION OF LATE REPLICATION IN MERISTEMATIC CELLS OF ALLIUM-CEPA

Under hypoxia (10 and 5% partial oxygen tension) meristematic cells of Allium cepa L. roots acquired Dew cycle kinetics, characterized by reduced but constant rates of root growth. Under these conditions, there was preferential lengthening of G1 and of the last third of the S period, S3. Since hyperoxygenation shortened S3 but not G1 in these cells, the high sensitivity of late replication to environmental oxygen is demonstrated. The preferential depression of the replication rate when those cells replicated the last third of their DNA was not associated with diminished cell size. Rather, the lower the oxygen level the larger the mean size of the cycling cells. Under anoxia (0% oxygen tension) the rate of growth slowed, accompanied by preferential accumulation of cells in G1. However, steady state kinetics of root growth was not achieved under these extreme conditions.