Cellular changes induced by exogenous and endogenous gibberellins in shoot tips of the long-day plant Silene armeria

Stem elongation and flowering are two processes induced by long-day (LD) treatment in Silene armeria L. Whereas photoperiodic control of stem growth is mediated by gibberellins (GAs), the flowering response cannot be obtained by GA applications. Microscopic observations on early cellular changes in the shoot meristem following LD induction or GA treatment in short days (SD) were combined with GA analyses of stem sections at various distances below the shoot apex. The earliest effects of both LD and GA induction on the subapical meristem were an increase in the number of cells per cell file and a reduction of cell length in the meristematic tissue approx. 1.0-3.0 mm below the shoot apex. Within 8 d after the beginning of LD induction or after GA application, the cells in the subapical meristem were oriented in long files. In induced tips, cellulose deposition occurred mostly in longitudinal walls, indicating that many transverse cell divisions had taken place which, in turn, increased the length of the stem. In contrast to LD induction, GA treatments did not promote the transition from the vegetative to the floral stage. Endogenous GAs were analyzed by selected ion monitoring (SIM), using labeled internal standards, in extracts from transverse sections of the tip at various distances below the apical meristem. In control plants, the levels of the six 13-hydroxy GAs studied (GA53, GA44, GA19, GA20, GA1, and GA8) decreased as the distance from the apical meristem increased. Except for GA53, GA levels were higher in tips of LD-induced plants, particularly in the meristematic zone approx. 0.5-1.5 mm below the apical meristem. In comparison with SD, the highest increase observed was for GA1, the content of which increased 30-fold in the zone 0.5-3.5 mm below the shoot apex. These data indicate a spatial correlation between the accumulation of GA1 and its precursors, and the enhanced mitotic activity which occurs in the subapical meristem of elongating Silene apices.