plant pigment that is sensitive to certain wavelengths of light. Plays a role in plant responses to light.

Merriam-Webster Online Dictionary
phytochrome (noun)
any of a group of proteins bound to light-absorbing pigments in many plants that play a role in initiating floral and developmental processes when activated by red or near-infrared radiation
phytochrome (Wikipedia)
Crystal Structure of Phytochrome.
Oat phytochrome absorption spectrum (Devlin, 1969)

Phytochromes are a class of photoreceptor in plants, bacteria and fungi use to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-red light, or Type II that are activated by red light. Recent advances have suggested that phytochromes also act as temperature sensors, as warmer temperatures enhance their de-activation. All of these factors contribute to the plants ability to germinate.

Phytochromes control many aspects of plant development. They regulate the germination of seeds (photoblasty), the synthesis of chlorophyll, the elongation of seedlings, the size, shape and number and movement of leaves and the timing of flowering in adult plants. Phytochromes are widely expressed across many tissues and developmental stages.

Other plant photoreceptors include cryptochromes and phototropins, which respond to blue and ultraviolet-A light and UVR8, which is sensitive to ultraviolet-B light.

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