Cytokinins in Phloem and Xylem Saps of Sinapis-alba During Floral Induction

Abstract

In Sinapis alba, a long-day plant (LDP) which can be induced by a single long day (LD), it has been suggested that cytokinins may be part of a multicomponent floral stimulus. In order to determine cytokinin fluxes during floral transition, we developed a technique to collect phloem sap reaching the apical part of the shoot, close to the target bud. Exudates collected from roots, leaves, and the apical part of the shoot were analysed by radioimmunoassay for cytokinins. Such analyses confirm previous observations, obtained using the Amaranthus bioassay, indicating that cytokinin export from the roots and mature leaves is enhanced 2-5 fold during floral transition. The flux of cytokinins directed to the upper part of the shoot through the phloem is also rapidly increased (ca 1.5-2 fold) by the inductive treatment, between 9 and 25 h after start of the LD. We suggested that the shoot apical meristem of 2-month-old Sinapis plants probably has a low cytokinin level. Induced leaves rapidly produce a signal which is transported to the roots where it alters cytokinin production and/or export. In addition, or as a consequence, leaf-cytokinins are exported via the phloem to the apical meristem where they induce a mitotic peak and some other events normally associated with the floral transition