Experiments were done with the objective of describing floral induction in tobacco. A short-day mutant, grown in controlled-environment cabinets, was used to define the stages of development, and the results were used to interpret the behaviour of both short-day and day-neutral plants grown in the field. The shoot apex passed through an apparent juvenile phase, characterized by a progressive increase in its size. Next, in the absence of floral induction it entered an equilibrium stage during which its size, staining properties, and activity remained constant. After 10 inductive cycles, apices of short-day plants became committed to flower. The rate of leaf inception increased, the apical meristem became domed, and the region of intense pyronin staining (indicative of RNA) spread to the central zone. Differentiation of the inflorescence followed, and the terminal flower was recognizable about 20 days after the start of induction. The apices of field plants remained juvenile for very much longer than did apices of cabinet-grown plants, and floral induction did not take effect until after recovery from transplanting. Apices of day-neutral plants then passed directly to the induced state, whilst those of the mutant remained indefinitely vegetative in the equilibrium stage, natural day lengths at the time being non-inductive. The protracted juvenility of field plants was attributed to the stresses of seed-bed conditions and transplanting damage.
WUSCHEL in the shoot apical meristem: old player, new tricks