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Indirect Action of Benzyladenine and Other Chemicals on Flowering of Pharbitis nil Chois

Action by Interference with Assimilate Translocation from Induced Cotyledons

^a CSIRO, Division of Plant Industry, P. O. Box 1600, Canberra City, A.C.T., 2601, Australia

Benzyladenine (BA) brushed on the cotyledons of 4-day-old seedlings of Pharbitis nil Chois. markedly stimulates flowering. Greates response is obtained for concentrations between 44 and 440 micromolar. The action of BA is on processes in the cotyledon as shown by the response to its site of application, to the dosage applied and to the requirement for its application prior to the dark period. There was little or no effect of BA treatment on either the time measurement processes of photoperiodic induction or on the generation of floral stimulus. Transport of photosynthetic assimilate from the cotyledons to the shoot apex was altered.

When only one of the pair of cotyledons was treated with BA it exported less ¹⁴C-labeled assimilate to the shoot apex and there was a compensatory increase in assimilate outflow from the other cotyledon. When BA was applied to a cotyledon exposed to an inductive dark period, flowering was inhibited in association with the reduced export of assimilate. Conversely, when BA was applied to the noninduced cotyledon, flowering was promoted in association with an enhanced export of assimilate from the induced leaf. Clearly, cytokinins can have an indirect effect on photoperiodic induction by altering assimilate and, hence, floral stimulus translocation to the shoot apex.

Two other chemicals which were previously considered as specific inhibitors of processes of floral induction in the cotyledon [Tris(2-diethylaminoethyl)phosphate trihydrochloride (SK&F 7997-A₃) and cycloheximide] acted in the same manner as BA. Inhibitory effects of an illuminated cotyledon on flowering of Pharbitis were also shown to be mediated by interference of assimilate flow with transport of the stimulus for flowering.