# Phytochrome

> A family of plant photoreceptors that sense red and far-red light, measuring night length for flowering and shade for stem elongation.

- Canonical: https://genetixbay.com/knowledge/glossary/phytochrome.html
- Also written: phytochromes, Pr, Pfr, red/far-red receptor, phyB
- Glossary category: Plant biology · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Phytochromes are light-sensing proteins that switch between two forms. Red light (about 660 nm) converts the inactive form (Pr) to the active form (Pfr); [far-red light](https://genetixbay.com/knowledge/glossary/far-red.md) (about 730 nm) and darkness convert it back. The balance tells the plant about [day length](https://genetixbay.com/knowledge/glossary/photoperiod.md) and shading.

## In practice

Phytochrome is how cannabis measures the night. During a long dark period, Pfr drops; if the dark lasts long enough, flowering is triggered in photoperiod cultivars. A [light leak](https://genetixbay.com/knowledge/glossary/light-leak.md) or a brief light break during the night can reset the clock. That is why light-tight flower rooms matter, and why green work lights are used: phytochrome responds weakly to green.

Far-red affects shape: a high far-red-to-red ratio (as under a canopy) signals shade and drives stretching. Some [LED](https://genetixbay.com/knowledge/glossary/led.md) fixtures add far-red to speed growth and canopy expansion; brief far-red at the end of the day is used by some growers to nudge flowering. Results vary by cultivar.

## Don't confuse with

- **Chlorophyll**: captures light energy; phytochrome only [senses](https://genetixbay.com/knowledge/glossary/sinsemilla.md) it.
- **Cryptochrome**: blue-light receptor shaping stem length and leaf form.
- **Photoperiod**: the day-length response phytochrome helps measure.

## Related terms

[Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md), [Short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md), [Internode](https://genetixbay.com/knowledge/glossary/internode.md), [Hypocotyl](https://genetixbay.com/knowledge/glossary/hypocotyl.md)

## Read more

- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)

## Sources

1. Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th edition. Sinauer Associates.
2. Zhang M, Anderson SL, Brym ZT, Pearson BJ. 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153