Phytochrome Is the Primary Photoreceptor in Photoperiodism

Night-break experiments are well suited for studying the nature of the photoreceptors involved in the reception of light signals during the photoperiodic response. The inhibition of flowering in SDPs by night breaks was one of the first physiological processes shown to be under the control of phytochrome (Figure 24.22).

In many SDPs, a night break becomes effective only when the supplied dose of light is sufficient to saturate the photoconversion of Pr (phytochrome that absorbs red light) to Pfr (phytochrome that absorbs far-red light) (see Chapter 17). A subsequent exposure to far-red light, which photoconverts the pigment back to the physiologically inactive Pr form, restores the flowering response.

In some LDPs, red and far-red reversibility has also been demonstrated. In these plants, a night break of red light promoted flowering, and a subsequent exposure to far-red light prevented this response.

Action spectra for the inhibition and restoration of the flowering response in SDPs are shown in Figure 24.23. A peak at 660 nm, the absorption maximum of Pr (see Chapter 17), is obtained when dark-grown Pharbitis seedlings are

Short-day (long-night) plant

Short-day (long-night) plant

Photoperiodism Figure

FIGURE 24.22 Phytochrome control of flowering by red (R) and far-red (FR) light. A flash of red light during the dark period induces flowering in an LDP, and the effect is reversed by a flash of far-red light. This response indicates the involvement of phytochrome. In SDPs, a flash of red light prevents flowering, and the effect is reversed by a flash of far-red light.

Long-day (short-night) plant

Long-day (short-night) plant

FIGURE 24.22 Phytochrome control of flowering by red (R) and far-red (FR) light. A flash of red light during the dark period induces flowering in an LDP, and the effect is reversed by a flash of far-red light. This response indicates the involvement of phytochrome. In SDPs, a flash of red light prevents flowering, and the effect is reversed by a flash of far-red light.

How Does Phytochrome Control Flowering

600 700

Wavelength (nm)

FIGURE 24.23 Action spectra for the control of flowering by night breaks implicates phytochrome. Flowering in SDPs is inhibited by a short light treatment (night break) given in an otherwise inductive period. In the SDP Xanthium stru-marium, red-light night breaks of 620 to 640 nm are the most effective. Reversal of the red-light effect is maximal at 725 nm. In the dark-grown SDP Pharbitis nil, which is devoid of chlorophyll and its interference with light absorption, night breaks of 660 nm are the most effective. This 660 nm maximum coincides with the absorption maximum of phytochrome. (Data for Xanthium from Hendricks and Siegelman 1967; data for Pharbitis from Saji et al. 1983.)

600 700

Wavelength (nm)

FIGURE 24.23 Action spectra for the control of flowering by night breaks implicates phytochrome. Flowering in SDPs is inhibited by a short light treatment (night break) given in an otherwise inductive period. In the SDP Xanthium stru-marium, red-light night breaks of 620 to 640 nm are the most effective. Reversal of the red-light effect is maximal at 725 nm. In the dark-grown SDP Pharbitis nil, which is devoid of chlorophyll and its interference with light absorption, night breaks of 660 nm are the most effective. This 660 nm maximum coincides with the absorption maximum of phytochrome. (Data for Xanthium from Hendricks and Siegelman 1967; data for Pharbitis from Saji et al. 1983.)

used to avoid interference from chlorophyll. In contrast, the spectra for Xanthium provide an example of the response in green plants, in which the presence of chlorophyll can cause some discrepancy between the action spectrum and the absorption spectrum of Pr. These action spectra and the reversibility between red light and far-red light confirm the role of phytochrome as the photoreceptor that is involved in photoperiod measurement in SDPs.

In LDPs the role of phytochrome is more complex, and a blue-light photoreceptor (which will be discussed shortly) also plays a role in controlling flowering.

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Responses

  • SEBASTIAN
    What is the evidence that phytochrome is the primary photoreceptor in photoperiodism?
    9 months ago
  • Debora Pisano
    What is effect of red light and Pr phytochrome on flowering in LDPAnd SDP?
    8 months ago

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