Cell Cycle Responses to Red or Far-red Light, or Darkness, in the Shoot Apex of Silene coeli-rosa L. During Floral Induction

Twenty-eight-day-old plants of Silene coeli-rosa L. were maintained in short days (SD) for 9 d (0–8) or exposed to 7 long days (LD), or 7 SD with a 5 min exposure at 1700 h of each day to far-red (FR), red (R) or 5 min FR/5 min R, or 7 dark-interrupted (di = 1700–1720 h) LD. Treatments were followed by two further SD. The mitotic index and G1 and G2 proportions were measured in the shoot apices of plants sampled at 2000 h of each day of each replicated treatment. Exposure to 7 LD (= 100 per cent flowering) resulted in significant increases, relative to the SD controls, in both the G2 proportion and the mitotic index on d 0 to 3, 7 and 8. Five minute FR (= 0 per cent flowering) resulted in cell cycle responses similar to those in LD only from d 0 to 2. R and FR/R (both = 0 per cent flowering) did not result in any increases in the G2 proportion in the apex apart from d 3 of FR/R. However 5 min FR/5 min R, and to a lesser extent 5 min R, did result in significant increases in the mitotic index on d 0, 1, 7, and 8. diLD (= 8–10 per cent flowering) also prevented any significant increases in the G2 proportion on d 0 to 3, and 5 to 8 but the mitotic index was again higher on these days compared with control data. Thus the transition to floral growth for 90 per cent of the plants is associated with changes in the cell cycle in the shoot apex measured as increases in the G2 proportion at 2000 h of LD 0 to 3 and 7 to 8.