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A single application of cytokinin benzyladenine causes a threefold increase in the frequency of plasmodesmata in the vegetative shoot apical meristem (SAM) of Sinapis alba plants. This increase is observed 20 h after application within all cell layers (L1, L2, L3) as well as at the interfaces between these layers. Evidence is presented indicating that cytokinin promotes mainly the formation of new secondary plasmodesmata. A similar increase in the frequency of secondary plasmodesmata was observed in the Sinapis SAM during the floral transition induced by a single long day, suggesting that this effect of the long day is mediated by cytokinin.

Entire plants of Sinapis alba exposed to a single long day were induced to flower. However, if only the shoot tip was exposed to the long day, no flowering ensued. In the apical meristem of plants with only the shoot tip exposed to the long day, none of the ultrastructural changes normally observed in the meristem of induced plants were detected, except for a marked increase in the number of mitochondria per cell. We conclude that the great majority of ultrastructural changes normally occurring in the shoot meristem during floral transition are not direct effects of daylength on the tip but are caused by signal(s) generated in induced leaves. [ABSTRACT FROM AUTHOR]

A single application of benzyladenine (BA) at 4.3 × 10-5 M to the shoot apex of vegetative plants of Sinapis alba L. causes an increase of the import of [14C] assimilates in the apical bud and of the starch content. BA also produces the breakdown of the vacuoles into smaller vacuolar units and an increase of the mitotic index leading to near- synchronization of the meristem cell population in G1. All these changes normally occur in the meristem of Sinapis plants during the transition to flowering (full evocation). Other events normally associated with full evocation are not caused by BA, which thus produces only partial evocation. The part of evocation produced by BA is different from that caused by high irradiance. [ABSTRACT FROM AUTHOR]