Experimental Studies on the Mitotic Activity of the Shoot Apical Meristem and Its Relation to Floral Evocation and Morphogenesis in Sinapis alba

Results of a previous investigation indicate that there are two successive increases of mitotic activity in the shoot apical meristem of Sinapis alba plants immediately following a single inductive long day. The evidence presented in this paper indicates that the critical daylength for the first or "early" rise in the mitotic index is 1 h shorter than the critical daylength for floral induction. This early mitotic activation is induced by a stimulus generated in the leaves. In a variety of experimental conditions, induction of flowering was always associated with the early mitotic wave. Available evidence thus indicates that this mitotic wave is an essential component of floral evocation. However, this early cell division process always occurs a long time (usually 36 h) before the initiation of the first flower buds and apparently has no functional relationship to floral morphogenesis. Since this early mitotic rise occurs in plants exposed to a short day given at an unusual time during the 24-h cycle, and in plants subjected to a long day but deprived of CO2 during the second half of this long day, that is, without an extended period of photosynthetic CO2 fixation, the mitotic rise noticed is not due to an increased supply of assimilates. In all cases examined, the second or "late" mitotic rise occurs simultaneously with the initiation of the first flower buds by the meristem, and it appears to be an essential event of floral morphogenesis. Except in plants deprived of CO2 during the second half of an inductive long day, the early mitotic rise always occurs 26-30 h after the start of the effective light period and the late rise 36 h later.