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Vegetative plants of Sinapis alba L. grown under short days were induced to flower by exposure to one long day or continuous long days. Irrespective of the number of long days, the first flower primordia were initiated by the shoot apical meristem 60 h after the start of the inductive treatment. An indirect histoimmunofluorescence technique was used to search in the apical meristem for three antigenic proteins which had been previously detected by immunodiffusion tests in the whole apical bud (Pierard et al. (1977) Physiol. Plant. 41, 254-258). One protein called protein A, present in the vegetative meristem, increased in concentration during the first 48 h following the start of the inductive treatment. It stayed constant up to 96 h and disappeared completely at a later time. Two other proteins called B and C, absent in the vegetative meristem, appeared in the meristem of induced plants between 30 and 36 h after the start of the inductive treatment and progressively accumulated at later times up to 240 h. These proteins appeared 8 h before the irreversible commitment of the meristem to produce flower primordia (point of no return) was reached and 24 h before start of flower production. These observations support an interpretation of floral evocation as consisting, at least partially, of an early and qualitative change in gene expression.

Vegetative plants of Sinapis alba L. grown in short days were induced to flower by exposure to one or continuous long days. In both inductive conditions, the first flowers were initiated about 60 h after the start of the treatment. Soluble protein extracts were prepared from apical buds and just-expanded leaves of both vegelative and induced plants. Rabbit anlisera were prepared using extracts from vegetative and reproductive buds. Immunodiffusion tests were performed. Analysis of the precipitin bands indicated that: (I) one antigcnic protein was present in the vegetative buds and disappeared from the buds of induced plants between 96 and 240 h after the start of the inductive treatment; (2) the concentration of another antigenic protein increa.sed in buds of induced plants 30 h after ihe start ofthe induclive treatment: (3) the concentration ofa third antigenic protein increased in buds of indueed plants at 96 h.