Evolution of the apical meristem of Sinapis alba L. in long days, in short days and during the transfer from short days to long days

Bernier, G.

Caryologia 15(2): 303-325


ISSN/ISBN: 0008-7114
Accession: 024645184

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The evolution of the apical meristem of S. alba is described under various photoperiodic conditions in the dark rooms of the Liege phytotron. In long days, the apex remained vegetative 9-12 days after sowing. Its cytohistological structure was typical. Three zones were distinguishable: (a) a peripheral zone in which the cells possessed the most dedifferentiated characters and in which mitoses were numerous; (b) a central zone considerably less meristematic and less active; (c) a pith-rib meristem. The 10-12th day, the mitotic activity increased and was distributed more homogeneously throughout the apex. The vegetative zonation partially faded away. This was the prefloral stage. The next day, the floral phase began. The zonation and the functioning of the inflorescence apex resembled to some extent those described in the vegetative apex. Several profound differences, however, permitted separation of these 2 types of functioning. In short days, the apex indefinitely remained vegetative but progressively acquired a particular structure: the central corpus became more and more stratified as well as the pith-rib meristem. This apical structuration has been named following Lance (1958), the intermediate state. Plants cultivated for 60 days in short days reached this intermediate state. Wnen transferred to long days, the first apical modifications already appeared the next day, and the intermediate state was rapidly transformed into the prefloral state. The apex initiated the 1st floral buds 3 days after the transfer. These observations are discussed in relation to the fact that plants cultivated in short days and which have not yet reached the intermediate state react more slowly to the same transfer. It appears that the aptitude of the plant to react immediately in response to an inductive treatment is thus linked with the complete establishment of the intermediate state. Other essential differences have been found between the plants cultivated in short days and those cultivated in long days: (a) in 8-hour days, the periclinal mitoses of foliar initiation occur in the 3rd cellular layer while, in 16-hour days these divisions are seen in the sub-epidermal layer; (b) the plastochron length is 2.7 days in 8-hour days, while in 16-hour days it is 1.1 day. The literature concerning the apical functioning is discussed.