Persistence & transport of flowering stimulus in Xanthium

Abstract

Characteristics of the flowering stimulus and critical requirements for its initiation were examined quantitatively in single leaves of a strain of X. pensylvanicum that grows vigorously under continuous light and at constant temperature. Under these conditions, flowering is governed only by the absolute length of a single dark break. Although most of the stimulus results from a dark period extending from 9 to 16 hours, incremental amounts continue to form as the dark period is prolonged. Once formed, the stimulus is stable to either light or darkness. Absence of periodicity in response indicates that endogenous rhythms play no direct role in floral initiation. Since the postinduction reactions and transport of the stimulus were found to occur readily in either darkness or dim light, the so-called second high-intensity light process is not considered a fundamental part of the mechanism of floral initiation.