Conclusions

Two different explanations have been given for the observation that some FR light is required for optimal flowering in light-dominant plants. These are an HIR operating through phytochrome A (Deitzer, 1984; Thomas, 1991; Johnson et al., 1994) or an inhibitory reaction of Pfr which is prevented (Vince-Prue and Takimoto, 1987). At present, it is not possible to say with certainty which of these explanations is correct. Nor can we rule out the possibility that different mechanisms may operate in different plants or that, even in the same plant, FR may enhance the flowering response in more than one way. The recent availability of mutants in Arabidopsis offers a real opportunity for evaluating the different possibilities but there is need for detailed physiological studies which are so far lacking in this species. In addition, more information is required about phytochrome levels and the kinetics of different phytochrome pools in a range of species before the FR enhancement of flowering in LDP can be fully understood. The availability of conventional or transgenic mutants in other LDP would aid these investigations.

Timekeeping in light-dominant LDP is much less well understood than in SDP. Although circadian rhythms in sensitivity to FR and R light have been observed, it is not known how they operate in relation to photoperiodic timekeeping in these plants. A critical nightlength is measured, but it does not appear to be the overriding factor as it is in dark-dominant SDP since the spectral conditions during the photoperiod may prevent flowering even when the nightlength is less than the critical duration when measured under natural daylight conditions. Any model for the control of photoperiodic flowering in such LDP needs to account for the following observations:

• circadian rhythms in the responses to cycle lengths, night-breaks during the dark period, and FR supplements during the photoperiod

• the changing sensitivity to R and FR light during the daily cycle

• the requirement for long exposures to light for photoperiodic induction.

B (inning's hypothesis proposed that light promotes flowering in LDP when it falls in the skotophile phase. Although this is consistent with the promotion of flowering by a night-break, it has been difficult to reconcile with the apparent requirement for FR (low Pfr ) during a long photoperiod, especially where this can apparently be replaced by a dark interruption, as in Lolium. Vince-Prue and Takimoto (1987) suggested that flowering in LDP may be controlled by two rhythms, one in which Pfr promotes flowering and the other in which Pfr is inhibitory. Sunlight (or any mixture of R and FR which establishes an intermediate photoequilibrium) would generate sufficient Pfr to interact with the promoting response but insufficient to cause inhibition and thus is the most effective for flowering. This proposal would account for the responses of Lolium but does not predict the loss of response to FR of phytochrome A deficient mutants in Arabidopsis. Based on studies with mutants and transgenic plants, responses to R are now ascribed to phytochrome B and maybe other type II phyto-chromes, while the FR-HIR is attributed to phytochrome A. If this holds true for flowering, the changing sensitivity to R and FR in LDP could be considered as changing sensitivity to phytochromes A and B respectively. Thus phytochrome B would be required and effective during the early part of the photoperiod, whereas phytochrome A activation would be required in the latter part of the photoperiod. Red light, acting through phytochrome B, would set the phase of a rhythm in sensitivity to a HIR acting through phytochrome A (Fig. 5.17). This model has much in common with the SDP model proposed in Chapter 2 (see Fig. 2.12). In both cases light-on sets the phase of a rhythm in light sensitivity. In LDP, this continues to run in light whereas, in SDP, the rhythm is suspended. The rhythm (in LDP) establishes a phase of sensitivity to phytochrome A which requires extended periods of light containing FR for a significant photoactivation. In this type of model, extended night-breaks could either interact with the rhythm or rephase the endogenous rhythm so as to potentiate its interaction with the subsequent photoperiod. In practice it would probably do both, thus leading to the quite complex patterns of response described in the literature.

Overall, the photoperiodic control of flowering in LDP has proved more difficult to understand than that in SDP. In part, this is due to the requirement for long exposures

Rhythm phased at light on (phytochrome B)

Rhythm in Far red sensitivity (phytochrome A)

Rhythm phased at light on (phytochrome B)

Rhythm in Far red sensitivity (phytochrome A)

Light Darkness

Peak now coincides with light period

Light Darkness

Red night break re phases (Phytochrome B)

Peak now coincides with light period

Red night break re phases (Phytochrome B)

FIG. 5.17. Model for the mechanism of daylength sensing in LDP.

FIG. 5.17. Model for the mechanism of daylength sensing in LDP.

to light, which are less easy to work with than the brief exposures that have been found to be effective in many SDP. However, whatever the mechanism (and it may not be the same in all species), it is clear that LDP are well adapted to flowering under long days at the R/FR ratio present in natural daylight.

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