GROWTH CHANGES IN THE SHOOT APEX OF SINAPIS-ALBA DURING TRANSITION TOFLOWERING

The aim of the work was to report morphological changes which occur in the shoot apex during the morphogenetic switch to flowering in the mo del long day (LD) plant, Sinapis alba. During the floral transition in duced by 1 LD the growth rate of all components of the shoot apex is m odified 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 i ncrease in volume, apical internodes have an increased height and ther e is a precocious outgrowth of axillary meristems, All these changes p recede initiation of flower primordia, which starts at about 60 h afte r the start of LD. Later changes include meristem doming, a decrease i n the plastochron ratio and a shift to a more complex phyllotaxis, All the changes, except the decreased plastochron ratio, are characterist ics of an apex with an increased tempo of growth. The stimulation of l ongitudinal growth (height of apical internodes) is more marked and oc curs earlier than the reduction of radial growth (plastochron ratio).