No abstract

(14).Subsequent experiments with daylength extensions, rather than dark interruptions, found that a mixture of R and FR was more effective for promotion than R alone (7, 21, 31). The action spectrum for this response (27) shows a single peak in the R-FR region between 710 and 720 nm and some action in the blue at high irradiances. Such responses have been termed "high irradiance responses" (HIR) (28), to distinguish them from the low energy R/FR reversible responses, and are assumed to be a complex function of the photostationary state between Pr and Pfr (13).Although Hartmann (13)

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.Flowering in the long-day plant Hordeum vulgare cv. Wintex barley has long been known to be quantitatively promoted by R3 when provided as short (2-h) irradiations in the middle of a 12.5-h dark period (4). This response is R/FR reversible, and the action spectra for both the promotion (4) and its reversal (20) clearly indicate the involvement of phytochrome. Flowering in long-day plants is also promoted when the daylength is extended by dim light, but FR or mixtures of R and FR are more effective than R alone (1, 9-11, 18, 21-23, 25, 26 it is considered possible that the photoreceptor pigment is also phytochrome acting through what has come to be known as the 'high irradiance response' (12). The demonstration that increased flowering was inversely related to the phytochrome photoequilibrium (Pfr/Pt,t) (8, 13) supports this conclusion. Such data have led to the suggestion that there are two separate phytochromemediated events involved in the photoperiodic control of flowering, one that responds to a low Pfr/Ptt photoequilibrium that is irradiance-dependent, followed by one that responds to a high Pfr/Pt.t photoequilibrium that is independent of irradiance and readily reversible.Results obtained with barley (6) are consistent with the hypothesis that the effect of FR is mediated by phytochrome. The response to a si...