Inhibition of Photoperiodic Induction in Xanthium by Applied Auxin

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Abstract
When an active auxin as indoleacetic acid or napthaleneacetic acid is sprayed on the foliage of Xanthium. a short-day plant, during the photoinduction period, the initiation of inflorescence primordia is suppressed. If plants whose response to photoperiodic induction has been inhibited by auxin application are subsequently grafted as donors to vegetative receptor plants, the donors do not elicit any flowering response in the receptor plants. This indicates that the effect of applied auxin is to inhibit production by or transport from the leaves of the stimulus to floral induction. When cuttings of Xanthium are placed in soln. containing an active auxin, the flowering response to photoinduction is suppressed. This effect is reversible in that such cuttings may be normally induced after removal of the exogenous auxin supply. 2,4-Dichloranisole, an analogue of 2,4-dichlorophenoxyacetic acid and an antagonist of auxin, suppresses the effect of auxin in inhibition of floral initiation. When dichloranisole alone is supplied to cuttings of Xanthium during photoperiodic induction, floral development is hastened. If the mature photoperiodically active leaves of Xanthium are submerged during the dark periods only in a soln. of an active auxin, the flowering response to photoperiodic induction is suppressed, concentrations of naphthaleneacetic acid is low as 1 mg./l. sufficing to carry out the inhibition.