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Marijuana: The Best Lighting Intensity for Maximum Yield

Updated: May 29

The Study of Discussion: Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light intensity levels. By, Victoria Rodriguez-Morrison, David Llewellyn and Youbin Zheng. Published in Frontiers of Plant Science, in 2021.

You may have read that cannabis yields will not efficiently increase once the light intensities (photosynthetic photon flux densities, or PPFD) reach 1000 µmolm-2s-1 and beyond under ambient CO2.

I want to talk about a report providing evidence that both the yields and bud quality increase, even as canopy level light intensities are maintained as high as 1500-1800 µmolm-2s-1 corresponding to daily light integrals (DLI) as high as 78 molm−2d−1.

Lighting is Extremely Important in Cannabis Cultivation

Growing cannabis indoors means we have complete control over the environmental conditions. Growers can manipulate crop yields and quality using light.

The costs of electrical lighting amount to around 60% of the total energy used to grow cannabis, making it the primary input cost for indoor cannabis production. The prices of producing photosynthetically active radiation (PAR) include:

  • The cost of the grow light

  • The efficiency of the light

  • Managing the excess humidity and heat

For this discussion, I'm focusing on the canopy level light intensity, which I will just be calling light intensity for now. Crop yield increase as light intensity increases.

There is a thought to be a limit to this principle concerning higher light intensities. There is a limit on how much light a single leaf can tolerate (a light saturation point), and light usage efficiency begins to decrease at light intensities well below the saturation point.

There’s An Issue With The Way We Think Cannabis Uses Light

Some cannabis research shows us how she uses light. If you have done enough reading into this topic, you have probably come across this curve before (fig. 1).

You can find this concept explored a bit more by Dr Bruce Bugbee on YouTube with his popular video titled "Cannabis Grow Lighting Myths and FAQs with Dr Bruce Bugbee".

When we think about some of the pioneers in this field, most sources will reference works by Suman Chandra.

These works recorded the rate of single leaf photosynthesis under various light intensities resulting in a curve shaped like the one you see above.

These studies demonstrate that cannabis leaves have a very high photosynthetic capacity. However, the data from these studies is not an accurate gauge of whole plant photosynthesis or predicting yield. The photosynthetic ability of each leaf on the entire plant is highly variable.

As the plants grow, many factors influence the photosynthetic capacity of a mature leaf, the local environment (think airflow and temperature), the age of the leaf, and the leaf's lighting history (the lighting environment it matured under).

In particular, the leaf's age and lighting history were uncontrolled in earlier studies. Other forensic studies have attempted to develop models to estimate crop yields from indoor cannabis production.

However, the report I'm referencing noted that none of the past literature reported evidence of saturation of flower yields at very high light intensities.

Therefore, there is reason to believe that you can efficiently increase cannabis yields by supplying light intensities going well beyond 1000 PPFD without providing additional carbon dioxide.

Details of the Experiment:

How the Plants Were Grown

The team used hydroponic methods and LED lighting, all clones, and it seems that many plants were grown (I can not precisely tell how much), but they did not analyse them all, only the ones directly under the light fixtures. Regardless, it seems like they studied at least 100 plants.

The team separated the plants into six blocks, each with different target PPFD levels for their flowering stages: 200-1600 µmolm-2s-1 separated evenly by intervals of 200, they vegged each plant at 425 µmolm-2s-1 for 18 hr photoperiods.

The growers adjusted lighting height as the plants grew to maintain the target PPFD at the apex of the plants. The CO2 levels were ambient (not enhanced), and they fed the plants a balanced nutrient feed with an NPK of approximately 7:3:2.

The team maintained target PPFDs at the tops of the plants to the best of their abilities; the target values were not accurate enough to represent the actual PPFD dynamics experienced by each plant throughout the trail.

As a correction, the researchers calculated total light integrals for each plant, accounting for the entire light exposure throughout the grow; average PPFDs were then back-calculated, with values ranging from 120 – 1800 µmolm-2s-1.

Measurements of Leaf Photosynthesis

The analysis of leaf-level gas exchange was carried out on the youngest fully expanded fan leaves on the 1st, 5th, and 9th week after they initiated the flowering period.

Firstly, researchers recorded the light intensity that the chosen leaf had been exposed and accustomed to, the LPPFD (localised PPFD). Next, they proceed with the photosynthesis measurements.

The net carbon exchange rate at the surface of the chosen leaves was recorded at PPFD levels of 2000, 1600, 1400, 1200, 1000, 800, 600, 400, 200, 150, 100, 75, 50, 25 and 0 µmolm-2s-1, starting sequentially from highest to lowest to ensure stomatal opening wasn't limiting photosynthesis.

Reading the results, I can see they threw many parameters about; the three most relevant to the discussion are:

The Light-Saturated net CO2 exchange rates (LSE): I couldn't quite find a definition of this term, but I could interpret some helpful information based on an equation given in the study.

If you had two leaves exposed to the same light intensity, the leaf with the higher LSE would photosynthesise faster.

The localised net CO2 exchange rates (LNCER): The rate of photosynthesis detectable at the leaf surface

The Light Saturation Point (LSP): The PPFD level at which any further increase in light intensity doesn't yield a significant increase in the net CO2 exchange rate.

Discussing the Results

Cannabis Leaves Adapt to the Light Intensities They Mature Under

Cannabis leaves are highly adaptable; the photosynthetic responses of an individual leaf will not correlate to the yield efficiency of the crop. Let's go back to (fig 1.) note that this is just an image I drew up; it's not accurate to any extent other than the general shape of the curve.

I read through various growing forums and other blogs, and similar information is regurgitated regarding the optimum lighting intensity to give cannabis.

Most of the time, you will see a curve similar to the one shown above, and they will tell you not to go too far into the territory where net photosynthesis was no longer increasing linearly. This report insists that the erroneous threshold lies at a PPFD of 1500 µmolm-2s-1, but I more often or not see it restricted to 1000 µmolm-2s-1 or lower when I read online.

I mentioned earlier that the issue is that the data was collected when several important variables were uncontrolled:

  1. We don't know the exact physiological ages of these leaves.

  2. The PPFD levels the leaves matured under was, in some cases, unknown.

  3. They did not investigate the physiological and morphological adaptability of the leaves to their local lighting environment.

As with other plant species, they found that cannabis has the plasticity to rapidly acclimate its morphology and physiology at both leaf- and whole-plant levels, as the intensity of lighting changes within its growing environment.

Importantly, this study provided evidence that the photosynthetic variables I defined prior: LSE, LNCER, and LSP, were higher in the leaves that matured under higher light intensity, showing cannabis leaves adapt to their local light intensity.

These photosynthetic variables increased proportionally to the localised light intensity. Furthermore, this linear relationship showed no signs of a plateau, even as localised PPFDs rose into the range of 1000-1200 µmolm-2s-1. These photosynthetic parameters would also decline gradually with the leaf's age. (The tested leaves were not at the direct top of the plant; therefore, they couldn't test leaves at the highest possible LPPFD levels).

The distinction between the "leaf photosynthesis" and the "whole plant yield" responses to light intensity is their LSPs (light saturation points). The LSP for leaf photosynthesis is substantially lower than the LSP for yield, which remains undefined because cannabis yields didn't show any signs of hitting a plateau at the highest light intensities.

To conclude, you should not use the photosynthetic response of a single leaf to impose limits on the maximum amount of light to provide cannabis to get the highest possible yield.

A Specific Light Intensity for the Optimal Yield of Cannabis Remains Undefined

It was thought that saturating yield responses to increasing light intensity would be seen, which would mean that an optimum range of light intensity would be relatively simple to define.

The lack of a saturating yield response at very high light intensities distinguishes cannabis from other crops. It is a testament to the excellent light sequestering ability that cannabis possesses.

Even under ambient CO2, the results indicated that the light intensity was stalling the maximum rate of photosynthesis (of the whole plant) even when average PPFD levels were as high as 1800 µmolm-2s-1 at the apex of the plants. The team exposed the cannabis plants to DLIs up to 78 molm−2d−1, and yields increased efficiently even at these extreme light intensities.

Therefore, the optimum light intensity is whatever you can afford. So I guess it is up to you how much light you intend to give your plants, keeping in mind the more light you give them, the more feed/water they will demand. Focus more on optimising other environmental conditions to suit the light levels.

To predict yields, the research team suggested that it may be more appropriate to correlate yield to the absorbed photons over the entire production cycle, the Total light integral, which I briefly described earlier. If you want to see a bit more of the mathematics behind yield predictions, I suggest going and reading the report first hand.

There were indications of increases in terpene concentration, resulting in more intense smelling flowers; other than that, the cannabinoid profile remained unchanged. Another essential detail is increasing light intensities increase flower quality by making the buds denser. You gain an increased ratio of marketable biomass; therefore, you'll spend less money or time labouring away, separating the leaves and stems per unit of flower you've grown.

Need More Convincing?

I also came across another account in 2019 by a different group of people who reported similar results, a linear yield response to light intensities at least as high as 1500 µmolm-2s-1. At the top of the page, I referenced the report I discussed, and the supporting report iv just mentioned is below.

The relationship between light intensity, cannabis yields, and profitability. By James Eaves, Stephen Eaves, Chad Morphy, Chris Murray. Published in Agronomy Journal in 2019.

If you enjoyed the content, I encourage you to stick around for more. Thanks for reading, happy growing!

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