Floral transition as a sequence of growth changes in different components of the shoot apical meristem ofChenopodium rubrum

The changes in cell division rate were studied in different components of the shoot apex of Chenopodium rubrum during short-day photoperiodic induction and after the inductive treatments. Induced and vegetative apices were compared. Accumulation of metaphases by colchicine treatment was used to compare the mean cell cycle duration in different components of the apex. A direct method of evaluating the increase in cell number obtained by anticlinal or periclinal divisions was applied if the corresponding components of induced and non-induced apices had to be compared. The short-day treatment prolonged the cell cycle more in the peripheral zone than in the central zone and still more in the leaf primordia. The importance of changing growth relations for floral transition was shown particularly if the induced plants were compared with the vegetative control with interrupted dark periods. Induced plants transferred to continuous light showed further changes in the rates of cell division. The cell cycle was shortened more in the central zone than in the peripheral zone, i.e. there was a further shift in growth relations within the apical dome. The cell cycle in the leaf and bud primordia was also shortened if compared with the vegetative control, the acceleration being stronger in the bud primordia. There was a subsequent retardation in cell division in the leaf primordia formed during and after the inductive treatment if the plants were fully induced. An inhibition of the oldest bud primordia was observed in fully induced apices, as well.