For some reason, a topic that really gets people in the cannabis cultivation community up in arms is whether or not autoflowering Cannabis plants benefit from being subjected to a 24/0 light schedule in flower.

So, The Canna Dogg has investigated the claim, as the first entry in the Canna Myth or Canna Fact series, where I’ll be investigating some of the many myths and tales found in the online Cannabis growing community.

Let’s start out by trying to figure out what an autoflower actually is. 

Agree to disagree?

The species Cannabis Ruderalis was first classified in 1924 by the Russian botanist Janischevsky [1]. One sentence in, and we’ve already hit a controversial point, by using the word species. There’s actually a lot of (heated) debate in the scientific community[29] on whether or not Sativa, Indica, and Ruderalis are different species or just variations or subspecies of Cannabis Sativa L. [2,][3],[11]. 

And it’s not just like one or two fringe taxonomists can’t agree on what it is – it’s actually one of the most controversial plants amongst plant taxonomists, and even Janischevsky himself wasn’t sure if it should be classified as a new species [4] – and he discovered the darn thing. We’ll just be using the word variety here, as The Canna Dogg has no interest in getting on the bad side of any plant taxonomist (you’re awesome, dear plant taxonomists, keep doing what you do!).

This is relevant, as it means that we’re going to have a hard time figuring out if there’s anything applicable specifically to autoflowers, if we can’t even agree on whether or not it’s a variation or a species. Some suggest that this could be due to the inbreeding ability of Cannabis, and the resulting ongoing variability of quantitative traits [2]. So, there’s a bit of a disagreement here, and we haven’t even gone near whether or not strains should be referred to as chemovars instead of strains.

On the plus side, though, the Cannabis varieties seem to be very closely related and have been reported to be genetically hard to tell apart [4], [5]. This means, that for our purposes, it’s relevant to look at the literature for all varieties, and not just Ruderalis.

And so what?

Now, the reason why The Canna Dogg even dares to risk offending taxonomists in the first place is that the cool thing about Ruderalis is that they are auto-flowering, which simply means that they naturally go into flowering, without the need to adjust their light cycle[3],[4].

So, these autoflowers automatically switch from their vegetative state to flowering state, irrespective of night length. But, it’s the least studied variation of cannabis, as it’s not normally used for commercial or medical cultivation, due to its low cannabinoid yields [6].  However, they can be crossbred with Indica and/or Sativa to produce a hybrid plant, that flowers on its own, and still gets you the cannabinoid content you’re after[7],[8],[10].

Ruderalis naturally grows in places where days can be as long as 20 hours, meaning that from an evolutionary perspective, it makes intuitive sense for this trait to be necessary for the plant’s survival [4],[9].

So, essentially, the Ruderalis part is responsible for the autoflowering aspect. But the Sativa /Indica part is primarily responsible for the total cannabinoid yield [11]. As such (despite what the interwebs might have you believe), it makes little sense to disregard studies on Sativa/Indica, as without them, there wouldn’t be much cannabinoid content to even study in the first place. 

That’s all well and good but what about light?

Fair point. Let’s look at some research! For our purposes, we’ll be operationally defining yield as the total cannabinoid yield. Sometimes, studies will report dry floral yield weight, but not look at cannabinoid content. Sometimes it’s the other way around. So we’ll be looking at is as one combined factor – cannabinoid yield – which we’ll call yield. We’re not going to concern ourselves with light spectrum, nutrient uptake, water uptake, or any other factors. We’ll only be looking at the idea that autos benefit from giving them light 24 hours a day in bloom.

Light saturation point

The light saturation point simply means the point where the plant’s photosynthetic rate reaches its upper bound – meaning that giving the plant more light at this point will not increase the rate of photosynthesis any further. We know the light saturation point for all kinds of crops like kale, spinach, Swiss chard, and many more [13],[14],[15],[16], but not for cannabis.

Scientists are working on it, so don’t worry, but we need more studies. But, if you’re a grad student looking for a thesis topic, you know what to do!

However, just because we’re not entirely sure, doesn’t mean that scientists haven’t investigated it. We would just like some more studies to be sure. We do know, however, that the net photosynthetic rate of cannabis is impacted by temperature and light intensity[17],[18],[19].

We actually also have somewhat of a consensus, in that the optimal temperature seems to be around 25-30C, and the optimal light intensity seems to be around 1500-ish PPFD in μmol m−2s−1[19],[20][21]. PPFD stands for photosynthetic photon flux density, and is a hot contender for the most sciency-sounding term ever.

So… Darkness?

Alright, so, the consensus is that more light is good, up until a point, and from there it doesn’t seem to make much of a difference [28]. This is simplified. There are many other factors that play a role, but we are looking at this from a home grower’s perspective, and we’re trying to look specifically at the effects of light. We’ll not be going into details about PAR here either.

PPFD alone is not enough, however, as it’s more relevant for us to look at the DLI – the daily light integral, which, simply put, is a way of measuring the total light over a 24-hour period, instead of just per second. The current consensus is that Cannabis can benefit from between 22-65 DLI in mol/m2/day [18],[23],[24],[25],[26],[27], before you need to start supplementing with CO2. I go into greater detail about the current science on light and cannabis in my book for those of you who want the latest and greatest on the topic.

Now, the thing is, when we’re up here in these high PPFD/DLI numbers, there is a point of diminishing returns, where there’s no longer a linear relationship between light and yield, and instead, the curve flattens, meaning that for every 1% increase in light, you would get less than a 1% increase in yield. This in turn means that you’ll risk adding more light, with a smaller or even no effect on total yield[18],[27].

A recent study looked at 11, 12, and 13 hours of light per day, and found that increasing the hours of light from 12 to 13 hours did not show any significant increase in botanical raw material yield, but it did sometimes lead to an “unwelcome” increase in the height of the plants [31]. In terms of the actual cannabinoid yield in line with our operational definition of yield, the results were inconclusive, in that some plants showed a large decrease in cannabinoid production when exposed to the longer period of light, but others didn’t. However, this study needs to get replicated, and ideally with some Ruderalis hybrids, to actually answer our question.

Conclusion

Unfortunately, this leaves us without a clear cut yes or no answer. The summary is that we don’t yet know whether or not it’s worth the cost of electricity to keep autos under 24/0 light in flower. There even appears to be a lot of variability relative to the individual chemovars/strains, and their responses to many of the environmental factors which impact yield, including light [30].

Summary

  • There is a positive correlation between light and yield.
  • There is a point of diminishing returns for light relative to total yield.
  • It is unclear whether or not autos benefit from receiving 24 hours of light in bloom, in an indoor home-growing setup, without supplementing with CO2, using high-intensity lights, relative to the cost of 24 hours of light per day. 
  • On the other hand, there does not appear to be enough evidence to conclusively say that running them under 24 hours of light hurts them, and the extra light might increase your yield, dependent on the chemovar/strain and your equipment.
  • There is a lot of individual variation between different chemovars/strains affecting the relationship between the environment and total yield.

So, if you want to give your autos 24 hours of light per day, go for it, but I’ll be sticking with 20/4 for the time being.

Make sure to join the friendliest online cannabis cultivation community online over here at The Canna Dogg subreddit! We have a strict “There’s no such thing as a stupid question policy”, and we’re a very friendly bunch, ready to help with whatever question you might have!


References

[1] Small, E., & Cronquist, A. (1976). A practical and natural taxonomy for Cannabis. Taxon, 405-435.

[2] Koren, A., Sikora, V., Kiprovski, B., Brdar-Jokanović, M., Aćimović, M., Konstantinović, B., & Latković, D. (2020). Controversial taxonomy of hemp. Genetika, 52(1), 1-13

[3] Gloss, D. (2015). An overview of products and bias in research. Neurotherapeutics, 12(4), 731-734.

[4] McPartland, J.M., 2018. Cannabis systematics at the levels of family, genus, and species. Cannabis and cannabinoid research3(1), pp.203-212.

[5] De Meijer, E. P. M., & Keizer, L. C. P. (1996). Patterns of diversity in Cannabis. Genetic resources and crop evolution, 43(1), 41-52.

[6] Fischedick, J. T., Hazekamp, A., Erkelens, T., Choi, Y. H., & Verpoorte, R. (2010). Metabolic fingerprinting of Cannabis sativa L., cannabinoids and terpenoids for chemotaxonomic and drug standardization purposes. Phytochemistry, 71(17-18), 2058-2073.

[7] Pollio, A. (2016). The name of Cannabis: a short guide for nonbotanists. Cannabis and cannabinoid research, 1(1), 234-238.

[8] Hillig, K. W. (2005). Genetic evidence for speciation in Cannabis (Cannabaceae). Genetic Resources and Crop Evolution, 52(2), 161-180.

[9] Small, E. (2015). Evolution and classification of Cannabis sativa (marijuana, hemp) in relation to human utilization. The botanical review, 81(3), 189-294.

[10] Beutler, J. A., & Marderosian, A. H. (1978). Chemotaxonomy of Cannabis I. Crossbreeding between Cannabis sativa and C. ruderalis, with analysis of cannabinoid content. Economic botany, 32(4), 387.

[11] McPartland, J. M., & Guy, G. W. (2017). Models of cannabis taxonomy, cultural bias, and conflicts between scientific and vernacular names. The botanical review, 83(4), 327-381.

[12] Vassilevska-Ivanova, R. (2019). Biology and ecology of genus Cannabis: genetic origin and biodiversity. Genetics and Plant Physiology, 9(1–2), pp. 75–98

[13] Boese, S. R., and Huner, N. P. (1990). Effect of growth temperature and temperature shifts on spinach leaf morphology and photosynthesis. Plant Physiol. 94, 1830–1836. doi: 10.1104/pp.94.4.1830

[14] Yamori, W., Noguchi, K., and Terashima, I. (2005). Temperature acclimation of photosynthesis in spinach leaves: analyses of photosynthetic components and temperature dependencies of photosynthetic partial reactions. Plant Cell Environ. 28, 536–547. doi: 10.1111/j.1365-3040.2004.01299.x

[15] Dahal, K., Kane, K., Gadapati, W., Webb, E., Savitch, L. V., Singh, J., et al. (2012). The effects of phenotypic plasticity on photosynthetic performance in winter rye, winter wheat and Brassica napus. Physiol. Plant. 144, 169–188. doi: 10.1111/j.1399-3054.2011.01513.x

[16] Ruhil, K., Ahmad, A., Iqbal, M., and Tripathy, B. C. (2015). Photosynthesis and growth responses of mustard (Brassica juncea L. cv Pusa Bold) plants to free air carbon dioxide enrichment (FACE). Protoplasma 252, 935–946. doi: 10.1007/s00709-014-0723-z

[17] Chandra, S., Lata, H., Khan, I. A., & ElSohly, M. A. (2011). Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L. Physiology and Molecular Biology of Plants, 17(3), 297.

[18] Chandra, S., Lata, H., Khan, I. A., & Elsohly, M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO 2 conditions. Physiology and Molecular Biology of Plants, 14(4), 299-306.

[19] ElSohly, M. A., Radwan, M. M., Gul, W., Chandra, S., & Galal, A. (2017). Phytochemistry of Cannabis sativa L. In Phytocannabinoids (pp. 1-36). Springer, Cham.

[20] Eaves, J. & Eaves, S. & Morphy, C. & Murray, C. (2020). The relationship between light intensity, cannabis yields, and profitability. Agronomy Journal. 112. 10.1002/agj2.20008. 

[21] Backer, R. G., Rosenbaum, P., McCarty, V., Eichhorn Bilodeau, S., Lyu, D., Ahmed, M. B., … & Smith, D. L. (2019). Closing the yield gap for cannabis: a meta-analysis of factors determining cannabis yield. Frontiers in plant science, 10, 495.

[22] Eaves, J., Eaves, S., Morphy, C., & Murray, C. (2019). The Profitability of Growing Cannabis Under High Intensity Light. Available at SSRN 3310456.

[23] Faust, J. E., Holcombe, V., Rajapakse, N. C., & Layne, D. R. (2005). The effect of daily light integral on bedding plant growth and flowering. HortScience, 40(3), 645-649.

[24] Walters, K. J., Hurt, A. A., & Lopez, R. G. (2019). Flowering, Stem Extension Growth, and Cutting Yield of Foliage Annuals in Response to Photoperiod. HortScience, 54(4), 661-666.

[25] Bishoff, H.,  & Burkett, H. (2018) The Importance of Daily Light Integral (DLI) for Indoor Cannabis Cultivation. In SMART GROW SYSTEMS, December 2018. NOTE: Not peer-reviewed.

[26] Weissman, E. M., & Yelton, M. (2020). Cannabis growing with horticultural SSL is a numbers game. In LED Magazine, February 2020. NOTE: Not peer-reviewed.

[27] Justice, A., & Gerovac, J. (2017) Economic impact of light intensity on yield and secondary metabolites. Presented at the 3rd annual PhotoX Summit, San Diego, CA.

[28] Lefsrud, M., Eichhorn Bilodeau, S., Wu, B. S., Rufyikiri, A. S., & MacPherson, S. (2019). An update on plant photobiology and implications for cannabis production. Frontiers in plant science, 10, 296.

[29] Schultes, R. E., Klein, W. M., Plowman, T., & Lockwood, T. E. (1974). Cannabis: an example of taxonomic neglect. Botanical Museum Leaflets, Harvard University, 23(9), 337-367.

[30] Backer, R. G., Rosenbaum, P., McCarty, V., Eichhorn Bilodeau, S., Lyu, D., Ahmed, M. B., … & Smith, D. L. (2019). Closing the yield gap for cannabis: a meta-analysis of factors determining cannabis yield. Frontiers in plant science, 10, 495.[31] Potter, D. (2009). The propagation, characterisation and optimisation of Cannabis sativa L as a phytopharmaceutical (Doctoral dissertation, King’s College London).

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