Posted on by Jason G

 Phytochrome, PPF, PPFD & DLI

Phytochrome (P-r) is a pigment that exists in two interconvertible forms. It absorbs red and far red light as well as influencing several aspects of plant growth and development, including flowering.

Phytochrome is a photoreceptor, a pigment that plants, bacteria and some fungi use to detect light.

PPF is the amount of light leaving the fixture and at different distances dictate the PPFD measurement the plant canopy is subjected to.

PPFD is the measurement of how much light the plants receive at plant canopy per sq./m per second

P-r (r =Red light 620-750nm) when p-r absorbs red light, it rapidly converts to p-fr, the change from its inactive form to an active form.

P-fr (fr =far red light 710-850nm) absorbs far red light it changes from P-fr to P-r so, changing from the active form to the inactive form. During a normal day, there is a difference in the way the two forms of red light are produced, unfiltered sunlight contains an abundant source of red light, whilst lacking in far red light.

PPF, PPFD & DLI can be measured using a quantum meter to assess the amount of photons coming from your lighting fixture to your canopy. Using formulas you can then ascertain your PPFD and DLI from these numbers you collect using your quantum meter.


Short-day Plants

Short day plants in spring/summer have short nights, which means during the night there is less degradation of the P-fr stores back to P-r. Meaning the threshold level of darkness was not long enough to completely decay the P-fr stores that had accumulated during the day.

So, in short-day plants P-fr inhibits flowering meaning that during spring/summer our days are much longer and nights, shorter, because of the day length flowering would not occur.

During winter, the nights are longer and the days are shorter, now the nights are longer the P-fr now has sufficient time to decay stores and initiate flowering.

Now, with all short-day plants there is a limit to the amount of light absorbed each day, this is called the daily light integral and measures the amount of light available to the plant each sq./M each day (DLI/Moles M-2 D-1)

So, during the day there is rapid conversion of P-r to P-fr with minimal conversion of P-fr to P-r meaning that during daylight hours there is an accumulation of P-fr. At night, the P-fr stores slowly degrade back into the inactive P-r form, this conversion of P-fr to P-r allows the plant to determine the length of its day and night­­ cycles.

Indoors, these photoperiods are generally dictated by grower preference, so ‘day’ or a vegetative photoperiod is considered; 18 hours on and night is 6 hours off, when we want to flower our plant/s we want to expose them to uninterrupted darkness so a schedule of 12 hours on 12 hours off is chosen so nights are much longer thus allowing the P-Fr to fully decay allowing flowering to initiate.

With powerful light sources, technically 18 hours of light can be too much light for any short-day plant, the number of moles needed is approx. 23-40 moles per day. There are many theories that manipulate the duration of light given in either veg and/or the flowering stage, one of the most plausible is Gas Lantern Routine (GLR) GLRs principle is as follows:

  • 12 Hours On
  • 5.5 Hours Off
  • 1 Hour On
  • 5.5 Hours Off


The following hours stated, are used to keep the plant in the vegetative phase, the single hour interjection is to prevent the P-fr from fully decaying, technically your plants still receive 17.5 hours of light per day, however this application is in line with the theory that there is only so much light a plant can absorb per day (DLI) thus eliminating any possible morphological stress responses that occur in an environment where the amount of light exceeds the number of moles required. Continue using this method for vegetating your plants, once happy with the size of the plant, we now want to initiate flowering allowing all of the plants P-Fr stores to fully decay, once fully decayed, flowering inhibition is removed and the phytochrome of the plant now shifts into the distribution of flowering hormones, which is a separate topic but still being studied to this day 

Lighting hours for GLR flowering as follows: 

  • 12 Hours On
  • 12 Hours Off


After 2 weeks of this photoperiod and the ‘stretch’ has completed, the idea is to drop the light hours by 30mins each week, so that on the last week of flower your lights are only on for 9 hours.

The majority of GLR growers opt for:

  • 10 Hours On
  • 14 Hours Off


The more experienced GLR Growers:

  • 8 Hours On
  • 16 Hours Off


There are lots of theories surrounding GLR (all of them completely plausible) this isn’t a pitch for any of you to attempt GLR over your usual routine, more-so an explanation behind SDP (short-day plants) and how much light they can ‘technically’ photosynthesise per day before other plant mechanisms are affected.

I hope this tutorial helps you understand more on the topic, I’d love to see a conversation form from this so please comment with your thoughts and questions.