Effect of Desiccation on Dna Synthesis and the Cell Cycle of the Moss Polytrichum Formosum

SUMMARY The effects of a 24‐h dehydration period on the mitotic and DNA‐synthetic activities have been studied in the meristematic cells of the leafy gametophyte of the moss Polytrichum formosum. During the first 2 h of water stress the mitotic index decreased dramatically, and after 6 h none of the cells was observed in division. It is concluded that the stress provokes not only cessation, but also completion of the mitotic phase, so that all the cells of the desiccated plants become interphasic. On rehydration, a period of 24 h was necessary for the first cell to enter division. After 4 h of recovery from water stress, DNA synthesis resumed in the nuclei of a few cells indicating that replication processes were not injured to any marked extent by the desiccation treatment. However, a clear increase in the number of nuclei entering the S phase was only observed 24 h after the start of rehydration. It is suggested that the effect of the dehydration period is to prevent the cells which are situated in G 1 or G 2 phases to enter respectively the phases of S and M of the cell cycle immediately upon rehydration. In any case the result is to delay, for about 24 h from the beginning of rehydration, the return of proliferative activity. These data are compared with those previously obtained on the metabolism of water‐stressed mosses.