In situ localisation of cytokinins in the shoot apical meristem of Sinapis alba at floral transition

In plants of Sinapis alba L. induced to flower by one long day (LD), previous work showed that the phloem sap feeding the shoot apex is enriched in cytokinins of the isopentenyladenine (iP)-type between 9 and 25 h after start of the LID [P. Lejeune et al. (1994) Physiol Plant 90:522-528]. We have checked the hypothesis that the cytokinin content of the shoot apical meristem (SAM) should increase in response to floral induction by one LD using histoimmunolocalisation techniques and rabbit antiserum against isopentenyladenosine or zeatin riboside. The free bases iP and zeatin are present only in apical tissues containing dividing cells. At 30 h after the start of an inductive LID, a markedly increased iP immune reaction is observed in SAM tissues while the level of zeatin is not modified. Our results are in line with the data obtained by analysis of phloem sap.