Botany - Photosynthesis in Higher Plants Question with Solution | TestHub

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'