Phase Shift in the Circadian Rhythm of Floral Promotion by Far Red Energy in Hordeum vulgare L.

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Abstract
Eight day old barley seedlings (H. vulgare L. cv. Wintex) were pretreated with a single 24-h daylight fluorescent photoperiod that was supplemented with sufficient far-red energy (FR) to produce a relative red (R)/FR ratio of 0.5. These plants undergo floral initiation about 1 wk after they are returned to 12-h daylight fluorescent photoperiods (R/FR ratio, 5.5), but floral development does not begin for an additional 2 wk. Addition of FR light to a subsequent 12-h photoperiod decreases the lag period between initiation and development by 10 days without affecting the rate of development. Extending the photoperiod to 24 h has the same effect on the lag period, but this treatment increases the rate of development. FR present during the 2nd half of this 24-h photoperiod increases the rate of development. The presence of FR during the 1st half of the photoperiod appears to affect the time of onset of floral development, while its presence during the 2nd half of the photoperiod affects the rate of this development. When a 6-h pulse of FR was given at various times during a 72- to 96-h continuous daylight fluorescent period, the response varied rhythmically and was maximal during the 2nd half of each 24-h cycle. When one 6-h FR pulse, given at a point of maximal response, is followed by a second 6-h FR pulse at various times relative to the first, the phase of this rhythm is advanced by .apprx. 12 h. This suggests that FR has 2 separate but concomitant effects. It causes earlier and/or more rapid flowering and it alters the phase of the endogenous circadian rhythm that regulates the ability of the plant to respond.