Partial Reactions in the Formation of the Floral Stimulus in Xanthium

The formation of the flower-inducing principle in the leaves of Xanthium pensylvanicum Wallr. has been shown by Hamner (1) to consist of at least two partial reactions: a preliminary high intensity light process, followed by a process requiring for its completion a minimum period of continuous darkness. Recently it was reported (3) that the flowering response of Xanthium may be suppressed when the inductive dark period is followed (after a brief interruption by light) by a second dark period of 4 to 6 hours duration. The effectiveness of the second dark period is greatly enhanced by treatment of the plants with indoleacetic acid (IAA). These results suggest that certain processes necessary for the formation of the flowering stimulus must take place in the leaf following an inductive dark period. Experiments were, therefore, conducted in an attempt to further characterize these processes. The flower inducing principle has not been identified as a single chemical compound, therefore we prefer not to refer to it as a hormone. In this paper it will be referred to as an internal stimulus, which is formed in the leaves and moves to the terminal meristem, where it changes the environment of the growing point causing the differentiation of the floral inflorescence. The results reported here will show that a photochemical process stabilizing the floral stimulus takes place after the inductive dark period and before the flowering stimulus is transported from the leaf. This process may be measured by a decrease in the effectiveness of the second dark period resulting from exposure of the leaves to light after termination of the inductive dark period. The photochemical reaction which stabilizes the stimulus requires high intensity light for approximately 5 hours and in the absence of such light treatment the stimulus remains sensitive to destruction by a second dark treatment for at least several hours.