Bioassay of Floral Stimulus in Xanthium

Abstract

Maximum floral stimulus (florigen) was generated in donor leaf tissue of X. pensylvani-cum by one 17-hour dark break in otherwise continuous light. Florigen was regulated by cutting the donor leaf to a tab of definite size and was channeled by appropriate trimming to a single receptor shoot in the donor petiole axil. Upon dissection of the receptor buds at 162 hours, floral response was evaluated as the percentage of the receptacle surface bearing florets. Separation of the floral development-rate curves for 7- and 10-cm2 donor tabs clearly registered the differential stimulus from as little as 3 cm2 of leaf tissue. At 24-42 hours from start of induction, noninduced leaves enhanced the response, but beyond this time, depressed it. The inhibition resulted from transmissible factors acting at the bud. The expansion index of donor tab and final weight of receptor shoot helped to detect specific regulants of reactions on the florigenic pathway. 6-Azauracil and 5-fluorouracil acted on the receptor bud and arrested not only floral differentiation but meristematic activity generally. 5-Bromo-3-isopropyl-6-methyluracil, also nonspecific, interfered with leaf processes that supported florigen synthesis and transport.