MERISTEMATIC ACTIVITY AND LEAF INITIATION IN THE SHOOT APEX OF NICOTIANA-TABACUM DURING FLORAL TRANSITION

Meristematic activity in shoot apices of NCTG-22, a short-day (SD) sensitive cultivar of Nicotiana tabacum, was monitored throughout the 17-d period of transition from vegetative to reproductive growth. The analysis included changes in (1) size of the apical meristem, (2) rate of leaf initiation, (3) amount of nuclear DNA, and (4) percentage of cells in division. Plants grown in controlled environment chambers set at 26/22 C day/night temperatures were exposed either to an inductive short-day photoperiod or a noninductive long-day (LD) photoperiod after a 6-wk juvenile period. The first morphological change associated with floral onset was an acceleration in the rate of leaf initiation following three inductive cycles, which continued and resulted in the eventual production of three more leaves by SD plants than by vegetative LD plants. Apices were identical in size until seven inductive cycles were completed, after which SD apices began to enlarge, finally becoming more than twice as wide as LD apices and assuming a domed shape. At the cellular level the percentage of cells in division increased in SD apices after one inductive cycle and generally remained significantly higher (1%-2.5%) than in LD apices during the entire transition period. A majority of the population of cells in LD apices favored the G1 condition. After exposure to one and three SD cycles, however, 12%-14% more cells were in the G1 phase in SD apices than in LD. During SDs 8-15, concurrent with dome enlargement, 10%-26% more cells again were in G1 than in apices. These results and previous observations on the floral transition in Nicotiana suggest that there may be two different processes controlling meristematic activity during two different phases of the transition period.