Light Quality In The Photoperiod

Early studies on the floral responses of a limited range of LDP to light of different spectral quality indicated that orange or red light was the most effective spectral region and that blue and green were relatively ineffective (Rasumov, 1933; Withrow and Benedict, 1936). Most of these studies used fairly rudimentary light sources based on TF lamps in which the likely contamination by far red (near infrared) wavelengths was not assessed. In more rigorous studies by Stolwijk and colleagues in Wageningen

FIG. 5.1. Effects of day extensions with light from different spectral regions on the flowering responses of the LDP Sinapis alba, Brassica rapa and Spinacia oleracea. (From data of Wassink et al., 1950, 1951; Stolwijk, 1952a, b).

Blue Green Yellow Red Far-Red White Dark

FIG. 5.1. Effects of day extensions with light from different spectral regions on the flowering responses of the LDP Sinapis alba, Brassica rapa and Spinacia oleracea. (From data of Wassink et al., 1950, 1951; Stolwijk, 1952a, b).

(Wassink et al., 1950, 1951; Stolwijk, 1952a, b) effects of day extensions on the flowering responses of the LDP Sinapis alba, Brassica rapa and Spinacia oleracea were determined. As shown in Fig. 5.1 the pattern for Spinacia was somewhat different from that of Sinapis and Brassica rapa. In Spinacia, day extensions with green, yellow and red were as effective as with white light but blue and infrared had little effect on flowering. In Sinapis and Brassica, blue and infrared had a strong promoting effect. This observation supported earlier work of Funke (1948) who had noted that members of the Cruciferae appeared to show unusual sensitivity to blue light for floral promotion. This response to blue light is probably mediated by a specific blue photoreceptor. The more puzzling observation was the sensitivity to near infrared (now termed FR) light. The action spectra for night-breaks had indicated that R and hence Pfr promoted flowering. Therefore, it would be expected that FR should inhibit flowering. However, for Sinapis in particular, giving FR as a day-extension was much more effective than R in promoting the floral response. Another feature was the contrast with Spinacia which was virtually insensitive to FR extensions. Did this indicate that a completely different mechanism was being used by these LDP?

In a series of studies in the 1950s (Wassink et al., 1951; Downs et al., 1958, 1959; Piringer and Cathey, 1960; Friend et al., 1961), researchers consistently found that day extensions with mixtures of R plus FR, or with TF light (which contains both R and FR light), were more effective than R light alone or light from white fluorescent lamps (which contains R but not FR). Acceleration of flowering by the addition of FR light to a day extension with R was first examined in detail in Hyoscyamus and was subsequently observed in many other species of LDP, including Petunia, dill, barley, sugar beet, wheat, lettuce, carnation, Lolium and Silene (Takimoto, 1957; Friend, 1963; Vince et al., 1964; Lane et al., 1965; Vince, 1965). The inhibition of flowering in the

SDP strawberry and Portulaca has also been shown to be more effective with a mixture of R plus FR than with R alone, indicating that this pattern is not exclusive to LDP; in contrast, the LDP, Fuchsia cv Lord Byron showed little acceleration of flowering with added FR indicating control more like that of a SDP (Vince-Prue, 1976).

Vince et al. (1964) working on the response to light quality, found that day extensions with R were unable to promote flowering in carnation and lettuce unless FR was added. Vince also carried out experiments with Lolium temulentum (Ba 3081), which required four LD cycles for induction. Initial experiments showed that light sources producing R or FR alone were rather ineffective as day extension treatments but that, in combination, they were strongly promotive. In order to determine the relative importance of the R and FR components, further experiments were carried out in which daylength extensions were given with a) 8 h R

Of these, treatments (b) and (d) strongly promoted flowering but plants in (a) and (c) remained vegetative (Vince, 1965; Fig. 5.2). Thus, even where a FR extension had only a small effect it could be very strongly promotive if followed by a R treatment before the start of the dark period. Evans et al. (1965) also studied the effects of R and FR red light during day extensions using the Ceres strain of Lolium temulentum. They found that extending an 8 h photoperiod to 16 h with either R or FR resulted in induction. However, the strongest response was obtained with TF light, which is a

Weeks

FIG. 5.2. The effect of R and FR on flowering in Lolium temulentum Ba3081. Plants were grown in natural daylight for 8 h and supplementary treatments were given over the next 8 h. Treatments: R, 8 h R; R+FR, 8 h R+FR; FR,R,, 7 h FR followed by 1 h R; R7FR,, 7 h R followed by 1 h R; S.D., no extension. Plants were transferred to LD and dissected at intervals. After Vince (1965).

Weeks

FIG. 5.2. The effect of R and FR on flowering in Lolium temulentum Ba3081. Plants were grown in natural daylight for 8 h and supplementary treatments were given over the next 8 h. Treatments: R, 8 h R; R+FR, 8 h R+FR; FR,R,, 7 h FR followed by 1 h R; R7FR,, 7 h R followed by 1 h R; S.D., no extension. Plants were transferred to LD and dissected at intervals. After Vince (1965).

122 5. DAYLENGTH PERCEPTION IN LONG-DAY PLANTS

TABLE 5.1 Effects of night-breaks on flowering in Fachsia hybrida cv Lord Byron. Night-break Duration of night-break/h

122 5. DAYLENGTH PERCEPTION IN LONG-DAY PLANTS

TABLE 5.1 Effects of night-breaks on flowering in Fachsia hybrida cv Lord Byron. Night-break Duration of night-break/h

0.5

1

2

4

8

16

Red

43.4

43.2

42.7

42.1

41.5

39.2

Red plus far-red

44.5

43.7

42.4

42.0

38.9

37.5

Results are given as the number of days to anthesis.

Results are given as the number of days to anthesis.

mixture of both R and FR. It should be noted that the FR source used by Vince had a longer wavelength cutoff and would have established very low Pfr levels, which could explain the differences in results between the two groups. The marked effect of changing the spectral distribution of the light during the photoperiod, especially altering the FR component, contrasts strongly with the behaviour of most SDP. For this reason it was proposed by Hillman (see Vince-Prue, 1979) that these plants be designated as light dominant to distinguish them from plants in which the main criterion for floral induction is the occurrence of a sufficiently long, uninterrupted dark period. These latter plants were then dark dominant. Dark dominant and light dominant responses correspond largely, but not exactly to the classification of species as SDP and LDP. However, there are a few LDP (e.g. Fuchsia hybrida Lord Byron) where flowering is responsive to a brief night-break, as in SDP, and there is little further effect of increasing the duration beyond 30 min (Table 5.1). There are also a few SDP (e.g. strawberry) which respond poorly, or not at all, to a brief night-break and where the inhibition of flowering requires exposures to long photoperiods containing both R and FR light (Guttridge and Vince-Prue, 1973). Whether these two

Irradiance / W m 2

FIG. 5.3. Interaction of irradiance and duration of a single period of supplementary light on flowering in Brassica campestris cv Ceres. Seedlings were given one 8 h SD extended with 8, 12 or 16 h TF light at a range of intensities. After Friend (1968a).

Irradiance / W m 2

FIG. 5.3. Interaction of irradiance and duration of a single period of supplementary light on flowering in Brassica campestris cv Ceres. Seedlings were given one 8 h SD extended with 8, 12 or 16 h TF light at a range of intensities. After Friend (1968a).

categories really represent different basic mechanisms is unknown, but light-dominant plants show a number of features which appear to be characteristic and which are different from those exhibited by dark-dominant species (Fig. 5.6).

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