[Action of 5-bromouracil and its nucleosides on the morphogenesis of the cuttings of etiolated tomato seedlings].

1. 5-Bromouracil inhibits the growth of the hypocotyl and the rhizogenesis in the cuttings of etiolated tomato seedlings only at high concentrations. 5-Bromouridine is inactive, but 5-bromodeoxyuridine strongly inhibits both phenomena. 2. The inhibition induced by 5-bromouracil cannot be reversed by thymine, deoxythymidine, uracil, uidine or β-alanine, a catabolic product of uracil. 3. Deoxythymidine is the one pyrimidine nucleoside which reverses the inhibition of growth and rhizogenesis brought about by 5-bromodeoxyuridine. 4. Deoxycytidine enhances the inhibition of rhizogenesis caused by 5-bromodeoxyuridine. Such an effect on the growth of the hypocotyl is not observed. 5. Taking into consideration the data in the literature, it is concluded: a)5-bromouracil can act in several ways which possibly differ according to the plant species: in some instances (as in the case described here), it seems to act by a mechanism unconnected with the metabolism of pyrimidine nucleic bases: b) 5-bromodeoxyuridine is a specific antimetabolite of deoxythymidine, but other metabolic interactions exist between 5-bromodeoxyuridine and deoxycytidine, each of the latter substances probably acting as a modifier of the metabolism of the other.