The duration of the cell cycle of Daucus carota L. in vivo and in vitro

Abstract

The duration of the cell cycle in Daucus carota seedling root-tips was measured by scoring the fraction of labelled metaphases after pulse labelling with ( 3H-TdR). This gave a total cycle duration of 7.5 h almost equally divided between the component stages, G 1, S, G 2 and M. The duration of the cell cycle in established diploid and tetraploid suspension cultures grown in the presence of 2,4-D was measured by continuous labelling with 3H-TdR. This gave mean cell cycle times almost identical with the mean cell number doubling times (specific growth rates) obtained from cell number growth curves. Specific growth rates were then used as estimates of the mean cell cycle duration for diploid and tetraploid lines grown in the presence and absence of 2,4-D. As the continuous labelling experiments showed that 93% of cells could incorporate 3H-TdR, cell cycle phase durations were calculated from measurements of the proportions of cells in G 1, S, G 2 and M. These proportions were determined by a combination of autoradiography, microdensitometry and mitotic index determination on culture samples incubated for short periods with 3H-TdR. In the presence of 2,4-D, both diploid and tetraploid cultures had cell cycle times of 45–58 h. In the absence of 2,4-D the diploid culture actively produced embryo plantlets with a mean cell cycle time of 33 h, whereas the tetraploid culture showed a cell cycle time of 82 h and remained undifferentiated. The extension and variation of the cell cycle in culture relative to the root-tip meristem was due almost entirely to changes in the duration of G 1. However, except in the root-tip and the differentiating diploid culture, G 1 and G 2 occupied constant proportions of the cell cycle. S phase duration under all cultural conditions was similar to that found in the root-tips but mitotic duration was variably increased. Despite this variation in mitotic duration, mitotic phase proportions in culture were mostly similar to those of the root-tip.