Flowering of Xanthium under Long-Day Conditions

Abstract

EFFORTS to induce flowering in the short-day plant Xanthium pensylvanicum under long-day conditions have consistently failed up to now. From numerous reports it has been concluded that for flower induction a short photoperiod must be followed by a long night. The role of the short photoperiod seems to be the photosynthetic production of energy substrates, since it can be substituted for it by sugar treatment¹. An uninterrupted long dark period of 9 hr. or more is considered to be necessary to build up a minimum amount of ‘flower stimulus’. However, in Perilla, also a short-day plant, the long-night requirement was alleviated by exposing the plants to a low light-intensity during the day². Furthermore, a low-temperature treatment during part of the long photoperiod was found to promote flowering (Wellensiek, private correspondence). Thus it appeared worth while to study the effect of low light-intensity and of low temperature during a long photoperiod on the flowering response in Xanthium.