Rates of Growth and Cell Division in the Shoot Apex ofSileneDuring the Transition to Flowering

Abstract

The growth rates of the shoot apex during and after floral induction were measured in Silene, a long-day plant. Plants were induced to flower with 4 or more long days (LD) but not with 3 long days or with short days (SD). The rate of increase of cell number in the apical dome, above the youngest leaf pair, was exponential and in plants given 3 LD remained the same as in plants in SD. In plants induced to flower with 7 LD, until the end of the inductive period the rate of increase of cell number in the apical dome remained the same as in plants in SD. Only when the apex began to enlarge as the first stage in the formation of the flower did the growth rate of the apical dome increase. The rates of increase of cell numbers in the apex corresponded to mean cell generation times of 20 to 33 h for plants in SD, for plants given 3 LD, and during the 7 days of induction for plants given 7 LD, and 6 to 8 h for induced plants when flower formation was beginning. The distribution of cell division in the apex was examined by treating plants with colchicine and noting in sections the positions of the resulting metaphases. In vegetative apices and also in apices undergoing transition to flowering the whole of the apical dome appeared to consist of cells dividing at a similar rate. The rate of leaf initiation during induction was the same as in vegetative, non-induced plants.