Phytochrome (Pr)

Definition - What does Phytochrome (Pr) mean?

Phytochrome is a pigment found in plants that allows the plants to detect of light. It is a crucial element to plant survival and is used to regulate flowering and to set the plant’s circadian rhythm, among other things.

MaximumYield explains Phytochrome (Pr)

Plants rely on light for their food and to ensure growth. However, they require a way to detect light, changes to light levels and quality, and more. Phytochrome is a blue-green pigment found in plants that acts as a light detector.

Unlike chlorophyll and other pigments found in plants, phytochrome occurs in small amounts and is not visible unless using a spectrograph. However, its role in plant growth and overall health is significant. It is also found in higher concentrations during certain points of time, such as during leaf expansion and stem growth, and in lower concentration during nutrient mobilization.

In dicots, there are five phytochrome genes, but in monocots, there are only three genes. A is responsible for germination, while B is responsible for shade detection. C is thought to work differently than A or B, but it is not clear how.

While phytochrome is used to detect light, it also regulates many other functions with plants. These include:

  • Circadian rhythm
  • Seed germination
  • Leaf expansion
  • Stem growth
  • Flowering

Phytochrome seems to be closely associated with red light and far-red light, similar to some lights used in indoor growing. When exposed to a brief flash of red light, Pfr phytochrome is formed, and the seeds will begin to germinate. However, a subsequent flash of far-red light, or a single flash of only far-red light, creates Pr phytochrome, and causes germination to stop (or discourages it from beginning in the first place).

Indoor growers can use this action of phytochrome to better control the strength of germinating seeds, as well as control over-blooming and fruiting in many types of plants.

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