Effects of light on the cell cycle in the shoot apex ofSilene coeli-rosa L. on the first day of floral induction

Abstract

28-day-old plants ofSilene coeli-rosa were exposed, at 1,700 hours, to 5 minutes far-red light, 5 minutes red, 5 minutes far-red followed by 5 minutes red light, or maintained in darkness (short day controls). All plants were exposed to tritiated (methyl-3H-)thymidine for 2 hours (1645–1845) and subsequently sampled at 2-hour intervals for 24 hours. The length of the cell cycle (pulse-label mitoses (PLM) method) and changes in cell number were measured in the shoot apical meristems. The cell cycle in the short day controls was 16–17 hours compared with a mean cell generation time of 18 hours. Exposing plants to far-red light resulted in a shortening of the cell cycle to 11 hours, red light resulted in a shortened cycle of 12 hours whilst far-red, red gave a value of 9 hours. Mean cell generation times following each light treatment were approximetely 2–5 hours longer than the corresponding cell cycle times, suggesting that the shortened cell cycles reverted to longer cycles over the experimental period. Measurements of the proportions of cells with the 2C and 4C amounts of DNA in the apical meristems of unlabeled plants indicated that G1 shortened but G2 lengthened in response to far-red light. A measurement of the labeling index also indicated that S-phase shortened in response to far-red. These data also suggested that red light caused G1 to shorten and G2 to lengthen although the corresponding PLM curve was consistent with a dramatic shortening of G2. Far-red followed by red resulted in decreases in the durations of both G2 and G1.