Upon absorbing UV-B photons, the tryptophans undergo structural changes that break the salt bridges, leading to the dissociation of the two functionally active monomers. The monomers then interact with COP1–SPA complexes to activate gene expression

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Effect of light is Photomorphogenesis

 A decrease in the rate of stem elongation

The beginning of apical-hook straightening

The initiation of the synthesis of pigments

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Phytochromes are photoreceptors that absorb red and far-red light most strongly (600–750 nm)

they also absorb blue light (350-500 nm) and UV-A radiation (320–400 nm).

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The amount of light is referred to as the fluence, which is defined as the total number of photons impinging on a unit surface area. Total fluence = fluence rate × the length of time