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The Light Cycle Debate

Dr Photon
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Here i will be discussing how different light cycles are beneficial or harmful for cannabis cultivation. Providing current research and understanding on the topic.

 

Alternate Light Cycles:

Because photosynthesis is the main contributor to energy production, any potential from alternate light cycles is dictated by the total amount of light provided. Meaning no matter how complex the light program, the maximum potential is governed by the duration of light provided by a light source. If the total light cycle is lowered then the potential energy production by the plant is lowered. As such if the total light cycle is increased, the potential energy production is also increased. There is no way to work around the system to get more out from what you put in. If you were able to get the same amount of photosynthesis from less quanta, that would be growing more efficiently. Photosynthesis is a quantum process and is limited by the laws of physics.

 

The Light Cycle:

The light cycle is all about photosynthesis, which is responsible for all metabolic functions in plants. Plants use energy from light, to power the light dependant reactions. During the light dependant reactions, water molecules (h20) are split, producing electrons, hydrogen ions and oxygen (o2) as a byproduct. Electrons and hydrogen ions are used to manufacture intermediates, ATP and NADPH. The intermediates from the light dependant reactions are used to power the chemical stages of the light independant reactions. Or commonly called the calvin cycle. ATP and NADPH power the carbon fixation (Co2) to create the ultimate product of photosynthesis, carbohydrates.

Whats important here is that the light independant reactions occur only during the day, meaning that C3 plants such as cannabis do not need a dark cycle to survive. There is an exception however for some types of plants such as CAM which require a dark cycle in order to complete the light independant reactions, as they only take in co2 at night. However this is stored and released during the day, when light independant reactions occur. 

 

The Dark Cycle:

The issue of whether cannabis needs a dark cycle has been debated for quite some time. Usually with people stating how cannabis is a C3 plant and does not require a dark cycle. However what the question really should be, is whether plants benefit or suffer from a dark cycle. Plants naturally have a circardian rhythm that mediates certain physiological functions during a regular day. Research has shown the connection between the circardian rhythm and many of these genetic expressions. It has also been shown that some plants can perform negatively or positively when entrained with continous light cycles. Plants that respond negatively produce symptoms of foliar chlorosis, limited or reduced plant growth and productivity. Its not clear whether the symptoms of leaf chlorosis are from starch accumilation, photooxidative damage or other processes. What is clear however are the possible advantages and disadvantages of extended light cycles.

 

Vegetative Growth (Photoperiod & Autoflower):

The efficiency of a plants ability to use light, is unchanged with plants that respond well to increased light cycles during vegetation. The only aspect of change between light cycles, is the speed of which a plant can grow. Once a desired growth point is reached, the same amount of energy has been used. Only the time taken to reach the desired growth point has changed. Although increased light cycles do not change the efficiency of how light is used, it does decrease the overall grow time. Which improves the turnover efficiency.

 

Flowering Photoperiod (Short Day Plants):

Varying light cycles will have an effect on yield in photoperiods during flowering. Because short day plants require a specific amount of dark for the initiation and continuance of flowering, there is a limit on how much light can be provided in a day. Flowering time in short day plants is influenced by its light cycle. As such reducing the light cycle, speeds up the flowering time but reduces yield. Increasing the light cycle slows down the flowering time but also increases yield. Not all cannabis strains require 12 hours of dark to flower. Some can flower at 11 or even 10 hours of dark, but we use 12 hours because this satisfies practically all strains and ensures that a cannabis will flower regardless of genetic variances.

 

Flowering Autoflower (Day Neutral Plants):

Autoflower plants do not flower through the measurement of light or dark cycles. They typically flower at a specifc stage of growth or age. As such longer light cycles can be provided during flowering and further yield can be obtained. This is because of more light from the extended cycle provides a higher total level of energy for floral growth. Although yield can be increased due to more energy from longer light cycles, the overall photosynthetic efficiency is unchanged. What this means, is that the electricity used will determine the final yield. In other words, the same amount of energy is used to aquire a certain level of yield return. Regardless of the light cycle.

 

Daily Light Integral:

The Daily Light Integral has been assumed by some as the metric for how much light plants can use. However this is not the case. DLI is used for calculating how much light has been provided, not how much can be used. This is a common metric for greenhouse growers for ascertaining whether enough light has been provided and whether any further additional supplementation is required. For example when light has been limited by overcast days. Cannabis will continue photosynthesizing for as long as light and other essential materials are provided. There is no physiological time or photosynthetic limit that prevents the continuation of photosynthesis in c3 plants (excluding the select that respond negatively to continous light cycles).

 

Conclusion:

As i have repeatidly stated, different light cycles do not change the efficiency of how light is used by plants. Growing of plants under longer light cycles may increase growth, but it also increases light energy input per unit of biomass produced. Meaning the amount of electricity used and cost will determine the growth or yield. One aspect that increased light cycles can improve on, is the turnover efficiency. Meaning the faster vegetation growth from increased light cycle allows for the faster finish of the overall grow. And with autoflowers, higher yield potential for the same duration of time. If your priority is about getting the most produce over time, this will be beneficial. However using continous light cycles could have negative impacts if plant processes respond negatively. As such i argue that the potential small benefits of continous light cycle is heavily outweighed by the possible large negative effects. With that i would recommend having some level of a dark cycle (18/6 or 20/4) as the only downside is a slightly longer grow.

As always please comment your thoughts or any questions on the subject.

Being wrong, is an opportunity for getting things right.
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Posted : 10/13/2018 3:07 pm CrackBabies, Chefomj, Longster and 3 people liked
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Longster
Trusted Member
Cannabis Fan

Have u noticed a significant decrease in yield from 12/12 to 11/13?

 

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Posted : 12/31/2018 10:16 am
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Dr Photon
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Im not sure of the exact questions asked in the last few posts. But more light when all other variables are provided. Will provide more energy and be beneficial. This is how autoflowers can develop as much as twice the yield in the same timeframe. As the duration of light and energy provided is extended beyond that of 12 hours.

Being wrong, is an opportunity for getting things right.
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Posted : 02/12/2020 7:33 pm Chefomj and Sgt.Bone liked
ChrisKristopher
Cannabis Fan

I’m curious on information on a 12/1 light cycle. I’ve used it for a few years now and gotten great results but there’s little expert info out there . In short the cycle goes like this.
veg - 12 on 5.5 off 1 on 5.5 off - flower 8 on 16 off

the idea is the plant only uses light energy for 8 hours after that you’re just wasting electricity. In veg the 1 hour during that dark period keeps plants in vegetative cycle. While I’ve had success using this I’m surprised there’s just little to no info about it. Curious what people here thought. 
-growing with love-

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Posted : 02/13/2020 5:57 am Sgt.Bone liked
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Dr Photon
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The gas lantern routine has never showed any positive results. If it was shown to be beneficial, it would surely be adopted by commercial growers. Where even the slightest potential saving would be large. Someone here ar CFC documented their experience using it and decided it was no good.

The facts are though, that plants will photosynthesize for as long as all other elements are provided to them. Again, for the same reason why autoflowers produce more when light cycles are increased beyond 12 hours. There are massive continous lighting studies which look into this. And found many species are tolerant with continous lighting with linear yield responses. Meaning that the quantum efficiency is unchanged. Some species however, such as tomatoes showed issues with continous lighting. Which has been suspected to be connected with plants circardian rythems and phloem loading and unloading. Suggesting that not all species are tolerant. But still were tolerant over 20 hour light cycles.

Which is why in our light cycle article. We suggest the "potential" negative effects with continuous lighting and recommend 18/6 or 20/4 cycles for vegetation and autoflowers. But its probably fine to use continuous, which is what i use for my veg tent all the time.

Being wrong, is an opportunity for getting things right.
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Posted : 02/14/2020 3:46 pm Sgt.Bone and Chefomj liked
watkaain
Active Member
New Grower

Dumb question but why is the light period restricted to 24hr days? Could you provide a lighting cycle of say 36/6? 

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Posted : 10/06/2020 6:07 am
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Dr Photon
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Posted by: watkaain

Dumb question but why is the light period restricted to 24hr days? Could you provide a lighting cycle of say 36/6? 

You can but it would possibly introduce problems and offer no benefit to typical light cycles such as 18/6 and 12/12

Cannabis like most plants keep their photoperiod time keeping (circardian oscillator) with respect to the rhythm of the natural 24hr earth day and night cycle. And continues oscillating in this photoperiod duration, regardless what cycle its exposed to. This is quite literally embedded into their genetic code and they depend on it to anticipate seasonal changes. Several circardian adaptations such as photoperiodism, senesceing and resource allocation are essential for anticipating and enduring the seasonal shifts on earth.

Using extended photoperiods beyond 24hrs in flowering would result in issues with induction and potentially ruined crops through stress induced hermaphrodites.

The induction of flowering is not just the requirement of a certain length of darkness (critical night length). But also a light requirement during whats called the light sensitive phase. Meaning that a light period is needed after a subsequent dark period for it to be inductive. And if the light signal is not presented during the light sensitive phase (somewhere between 10 - 16 hours of dark). Then the induction process for flowering will likely not occur at all. Hence why you do not see anyone using photoperiod cycles longer than 24hrs for flowering. Because any extended cycles will work against the inductive process preceeding before it. This is because the critical night length required for induction is not about reaching a certain duration of darkness. But infact reaching the peak night break point where induction sensitivity is highest for light to induce flowering. To sum up, both critical night length is required, followed by a light period in order to induce a flowering response. And so any extended photoperiod cycles beyond 24 hours will only counteract the induction process.

As for extended photoperiods for vegetation, we kind of already do this with 24/0 cycles. But as already pointed out by some, there is potential benefits with using cycles that include dark periods which synchronizes with the natural oscillation of the 24hr earth day and night cycle. Which is why its often recommended by us here at CFC. But there are potential benefits with using 24/0 such as tighter internodal growth and faster vegetation that may appeal to some people. Any additional variations of extended photoperiods will work against the natural osscilation of the plants internal clock and will causes issues.

 

 

Being wrong, is an opportunity for getting things right.
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Posted : 10/10/2020 10:39 pm Tropa, watkaain, Chefomj and 1 people liked