Fixed action pattern - A predictable series of actions triggered by a cue, sometimes called the key stimulus.

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There is two kind of photo response

1. Red/Far-red 

2. Blue Light

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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).