Putrescine export from leaves in relation to floral transition in Sinapis alba

Sinapis alba L. was induced to flower by either a long day or a displaced short day. Following collection of leaf (phloem) and root (xylem) exudates from induced and non-induced plants, polyamines in the exudates were extracted, separated and analyzed quantitatively. The titers of free and conjugated putrescine, the major polyamine fractions in all samples, increased early and markedly in leaf exudates during the floral transition, coinciding closely with movement of the floral stimulus out of the induced leaf. By contrast, putrescine titer in the root exudate did not increase. A spray of difluoromethylornithine (DFMO), an irreversible inhibitor of the putrescine-biosynthetic enzyme ornithine decarboxylase, at hour 8 of the long day considerably reduced the titer of free and conjugated putrescine in leaf exudates, and at the same time, markedly decreased the flowering response of induced plants. This effect of DFMO on flowering was substantially reversed by a simultaneous application of putrescine to the roots. DFMO sprayed on induced plants also suppressed early activation of indices of both mitosis and DNA synthesis in the shoot apical meristem. These results support the view that the extra putrescine synthesized in induced leaves is a necessary component of the floral stimulus in Sinapis.