Inhibition of Flowering of Xanthium pensylvanicum Wallr. by Prolonged Irradiation with Far Red

Interrupting each long night with a prolonged period of far red radiant energy resulted in the inhibition of floral initiation in cocklebur. Irradiations inducing different relative levels of P(FR) from 1 to 2% to 80% had about the same effect under 4-hour photoperiods. The lower levels of P(FR) induced by continuous far red irradiation were not as effective as the higher levels induced by red under 8 and 12-hours photoperiods. The critical P(FR) level required to induce inhibition of flowering seems to increase with increasing photoperiods.