Control of Flowering of Xanthium pensylvanicum by Red and Far-red Light

Abstract

A study was made of the effects of various durations, intensities and combinations of red and far-red light interruptions on the flowering responses of Xanthium pensylvanicum Wallr. A dual response to treatments of far-red light was observed. In short dark periods, far-red light alone did not greatly affect flowering but was able to overcome the inhibition of flowering caused by red light. In dark periods longer than 15 hr., far-red inhibited flowering and added to rather than overcame the inhibition by red light. The dark period length required for far-red inhibition remained the same whether far-red was given at the start or at the 8th hr. of darkness. In 48-hour dark periods Xanthium showed 3 responses to additions of red and far-red light breaks: response to red light; response to far-red light; and response to red followed by far-red light. Red light given any time in the first 30 hr. of darkness overcame the inhibitory effect of far-red light given at either the start or the 8th hr. of darkness. Red light given later than the 13th hr. did not overcome the far-red effect. Approximately the same energy of red light was required to overcome the inhibitory effect of far-red at the 2nd hr. of darkness as was required to produce maximum red light inhibition at the 8th hr. Although far-red light was most inhibitory when given early in a long dark period, approximately the same energy of far-red light was required to saturate the far-red response at the 4th, 8th and 16th hours. The results are discussed in relation to other reports of far-red inhibition of flowering in short-day plants.