The action spectra for these two antagonistic effects of light on Arabidopsis seed germination are shown in Figure 16.6A. (...) When the absorption spectra of each of the two forms of phytochrome (Pr and Pfr) are measured separately in a spectrophotometer designed to study photoreversible molecules, they correspond closely to the action spectra for the stimulation and inhibition of seed germination, respectively (Figure 16.6B). (...) Thus, when Pr molecules are exposed to red light, most of them absorb the photons and are converted to Pfr, but since of the Pfr made also absorbs the red light and is converted back to Pr (see Figure 16.6B).
As we saw in Figure 16.6, the LFR action spectrum for Arabidopsis seed germination includes a main peak for stimulation in the red region (660 nm) and a major peak for inhibition in the far-red region (720 nm). (...) Wild-type Arabidopsis seed require light for germination, and the response shows red/far-red reversibility in the low-fluence range (see Figure 16.6A). (...) As shading increases, the R:FR ratio decreases (see Figure 16.6B).
"Plant Physiology and Development" int'l 6e - Taiz, L., Zeiger, E., Møller, I.M., Murphy, A.