Growth changes in the shoot apex of Sinapis alba during transition to flowering

The aim of the work was to report morphological changes which occur in the shoot apex during the morphogenetic switch to flowering in the model long day (LD) plant, Sinapis alba. During the floral transition induced by 1 LD the growth rate of all components of the shoot apex is modified profoundly, The earliest changes, detected at 24 h after start of LD, include a decrease in plastochron duration and an increase of growth of leaf primordia, One day later, the meristem dome starts to increase in volume, apical internodes have an increased height and there is a precocious outgrowth of axillary meristems, All these changes precede initiation of flower primordia, which starts at about 60 h after the start of LD. Later changes include meristem doming, a decrease in the plastochron ratio and a shift to a more complex phyllotaxis, All the changes, except the decreased plastochron ratio, are characteristics of an apex with an increased tempo of growth. The stimulation of longitudinal growth (height of apical internodes) is more marked and occurs earlier than the reduction of radial growth (plastochron ratio).