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

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