Flowering Responses of Xanthium pensylvanicum to Long Dark Periods.

The flowering of Xanthium pensylvanicum Wallr. was investigated using long dark periods. Attempts were made to ascertain evidence for the involvement of a flowering rhythm in Xanthium by use of variable-dark-length and light-interruption experiments.It was found that factors such as plant height (age), partial defoliation, and various pretreatments had little effect on the general nature of the flowering response. Maximum sensitivity to red light occurred at the eighth hour of 24-, 48-, and 72-hour dark periods. Temperature had little influence on this timing. The time of maximum sensitivity was delayed to the tenth hour by a pretreatment with 8 hours of darkness followed by 6 hours of light. These properties are similar to those of Pharbitis, which showed a clear rhythmic sensitivity to red light interruptions. The possible involvement of a rapidly damping rhythm of sensitivity to red light is discussed on the basis of this similarity. A distinct flowering rhythm similar to that of soybean and Chenopodium was not found. Although the results are inconclusive with respect to a rhythm, they do indicate similarities and differences to the responses of other short-day plants in which rhythms have been demonstrated.