Far Red Light Modifies Flowering in Some LDPs

Circadian rhythms have also been found in LDPs. A circa-dian rhythm in the promotion of flowering by far-red light has been observed in barley (Hordeum vulgare) and Ara-bidopsis (Deitzer 1984), as well as in darnel ryegrass (Lolium temulentum) (Figure 24.24). The response is proportional to the irradiance and duration of far-red light and is therefore a high-irradiance response (HIR). Like other HIRs, PHYA is the phytochrome that mediates the response to far-red light (see Chapter 17). In both cases, when the plant is exposed to far-red light for 4 to 6 hours, flowering is promoted compared with plants maintained under continuous white or red light—a response mediated by PHYB. The rhythm continues to run in the light.

In SDPs, on the other hand, a characteristic feature of the timing mechanism is that the rhythm of the response to far-red light damps out after a few hours in continuous light and is restarted upon transfer to darkness.

The response to far-red light is not the only rhythmic feature in LDPs. Although relatively insensitive to a night break of only a few minutes, many LDPs can be induced to flower with a longer night break, usually of at least 1 hour. A circadian oscillation in the flowering response to such a long night break has been observed in LDPs, showing that a rhythm of responsiveness to light continues to run in darkness.

Thus, circadian rhythms that modify the flowering response in LDPs have been shown to run both in the light (promotion by far-red light) and in the dark (promotion by red or white light). However, we do not yet know how the circadian rhythm is coupled to the photoperiodic response.

12 24 36 48 60 Time (h) at which far-red light was given

FIGURE 24.24 Effect of far-red light on floral induction in Arabidopsis. Four hours of far-red light was added at the indicated times during a continuous 72-hour daylight period. Data points in the graph are plotted at the centers of the 6-hour treatments. The data show a circadian rhythm of sensitivity to the far-red promotion of flowering (red line). This supports a model in which flowering in LDPs is promoted when the light treatment (in this case far-red light) coincides with the peak of light sensitivity. (After Deitzer 1984.)

12 24 36 48 60 Time (h) at which far-red light was given

FIGURE 24.24 Effect of far-red light on floral induction in Arabidopsis. Four hours of far-red light was added at the indicated times during a continuous 72-hour daylight period. Data points in the graph are plotted at the centers of the 6-hour treatments. The data show a circadian rhythm of sensitivity to the far-red promotion of flowering (red line). This supports a model in which flowering in LDPs is promoted when the light treatment (in this case far-red light) coincides with the peak of light sensitivity. (After Deitzer 1984.)

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