Twilight Effect: Initiating Dark Measurement in Photoperiodism of Xanthium1

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Six experiments studied the effects of low levels of red and far-red light upon the initiation of measurement of the dark period in the photoperiodic induction of flowering in Xanthium strumarium L. (cocklebur), a short-day plant, and compared effects with those of comparable light treatments applied for 2 hours during the middle of a 16-hour inductive dark period. Red light, or red plus far-red, at levels that inhibit flowering when applied during the middle of the inductive dark period, either had no effect on the initiation of dark measurement (i.e., were perceived as darkness), or they delayed the initiation of dark measurement by various times up to the full interval of exposure (2 hours). Far-red light alone had virtually no effect either at the beginning or in the middle of the dark period. These results confirm that time measurement in the photoperiodic response of short-day Xanthium plants is not simply the time required for metabolic dark conversion of phytochrome. Results also suggest that the pigment system (phytochrome?) and/or responses to it may be significantly different as they function during twilight (initiation of dark measurement), and as they function during a light break several hours later. Possible mechanisms by which cocklebur plants detect the change from light to darkness are discussed.

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