What Is Phytochrome In Cannabis?

Photo by Vortex Farmacy

If you’re asking, “What is phytochrome?,” chances are high that you cultivate cannabis.  Understanding phytochrome and how this biological receptor truly functions, allows the grower to delve into the technical world of plant science and also to discover how to use this knowledge to their advantage in the grow room.

Understanding The Basics

Phytochrome is a pigment that is found within the leaves of all plants, as well as cannabis. The role of this pigment is to detect light to then allow the plant to grow accordingly based on the season or time to induce flower production. When light hits the surface of a leaf, there will be two types of light sensors at work: one which has the job of detecting blue light and one for detecting red light.

The one designed to sense red light is the phytochrome system.

Ultimately, the sensors will know when there are darkness periods or periods of shading that the plant can recognize as biological signals.

When a plant photosynthesizes, there is a chain reaction of events that occur naturally within the atmosphere. The plant absorbs carbon dioxide and light and in return produces sugars to feed itself. Light is the biological signal to activate photosynthesis, and then how the light is used can determine seed germination, stem elongation, chlorophyll synthesis and the induction of flowering.

The phytochrome system consists of two parts and they are defined as Pr and Pfr. Understanding plant wavelength requirements is something lighting companies have been working on for years, as well as understanding how nature is programmed with a 24 hour body clock known as circadian rhythm.

Circadian Rhythm In Cannabis Explained

In the same way that humans, animals and fungi have a naturally body clock, plants do too, and throughout the year they depend on specific lighting conditions. When growing cannabis, the plants will respond chemically to a  light reduction or darkness period that can trigger hormones and induce pre-flowering, blooming and finally ripening of the cannabis buds.

There can be consequences that can lead to genetic problems when changing the circadian rhythm and this is why cannabis plants use two types of phytochrome which are explained below.

Light cycles used during the vegetative cycle will usually range from 16 – 18  hours of light and the plants at this time will respond to darkness or shading periods of 6 – 8 hours. Naturally, a cannabis plant growing outdoors will grow with little internodal spacing and can be trained to grow more wide than tall. When flowering is induced in a cannabis plant, this will signify that hormonal changes have occurred within the plant and can be the time when growing cannabis indoors gets challenging. Receiving only 12 hours or less of light, the plant will react in accordance to what it believe is the change in season.

Phytochrome In Cannabis – Pr

The Pr represents the light absorption at a peak of 666 nm, whilst Pfr represents the light absorption from the range of  705 – 740 nm. When Pr absorbs red light, it will then be converted to Pfr.

Technically this wavelength of light is classed as red light, and during this time, red light changes phytochrome to its biologically active form. Plants that are dark green and usually with short internodal spaces are an expression of Pr range being utilized.

Phytochrome In Cannabis – Pfr

The ‘fr’ stands for far red, and this determines wavelengths or red light that are not Pr but form in the range of 705 – 740 nm, which absorbs at a peak of 730 nm. In the same way that Pr is converted to Pfr, the same occurs when Pfr absorbs far red light and is then converted back to the Pr form.

It sounds complicated and what the relevance of each one does can be hard to understand, however the Pfr phytochrome is responsible for inducing flowering as it corresponds to the amount of available light or shading that is present. There is a role for phytochrome in controlling gene expression, and these can cover the factors of stem elongation, stomatal function and light sensitivity in terms of floral production and plant metabolism.

Manipulating Phytochrome As A Cannabis Grower

Studies have proven that the stretching period that growers experience when inducing flowering can be controlled and manipulated to allow the cannabis plant to stay shorter rather than grow taller. Research has shown that plants treated with far red light focused their energy on root development and growth.

By eliminating the need for the plant to stretch to the highest point, the plant can focus on vigor and developing new shoots, stems and flower sets. The production of chlorophyll will be higher, and as result, the darker leaves produced will attract more sunlight as opposed to lighter green plants.

It is commercially known that blue light at a spectrum of 6400 k is what plants need to grow well. This is classed as a soft blue light and found in most types of fluorescent lighting.

In the same way that the plants require a level of red spectrum in order to perform the next stage of their life cycle and flower. The phytochrome process can ruin your plants by sending the wrong biological signals at the wrong time. These can be accidental errors but can cause enough stress and trauma to the plant, that it can cause the plant to hermaphrodite or experience serious problems with metabolism and function, especially if the circadian rhythm is disrupted.


When understanding the red light spectrum and the plants’ requirements, then you will understand how the phytochrome system works to start the cannabis plants genetic clock and keep it in check.

There is a fine balancing between the plant absorption to lighting and shading. The outcome and variables associated with this natural balancing act can lead to the start and end performance of the cannabis plant.

High levels of 705 – 740 nm allow plants to detect that they are being shaded, which as a result will cause stem elongation. When there is more of  705 – 740 nm, then the leaf development will be smaller than normal and the branching will be significantly less.

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