The wavelength of light absorbed by $\mathrm{P}_r$ form of phytochrome:
The wavelength of light absorbed by $\mathrm{P}_r$ form of phytochrome:
$680 \mathrm{~nm}$
$720 \mathrm{~nm}$
$620 \mathrm{~nm}$
$640 \mathrm{~nm}$
Solution
Light acts as a signal to initiate and regulate photoperiodism and photomorphogenesis. There are two light-sensing systems involved in these responses, the blue light-sensitive system and the red light-sensitive or phytochrome system. The action spectrum of the light needed for \(\mathrm{P}_{\mathrm{r}}\) responses is at about 680 nm.
So, the correct answer is '680 nm'