[Editor’s Note: Are PGRs carcinogenic or safe? Why do some countries ban the use of PGRs? Should you use synthetic or natural PGRs?]

Why is there such an interest in the PGRs for cannabis?

For those of us who have held dense buds of cannabis, many have likely thought that aesthetically: “This is a high-grade flower”. The dense compact appearance is especially apparent when compared to buds that are lighter and fluffier in appearance. Perhaps also you have noticed a potent fuel/gas aroma when presented with imported or visually impressive “high grade”. These are usually telltale signs that the cannabis may have been grown using plant growth regulators (PGRs).

Increased profit margins

However, there is also another factor which plays a major role in the growing demand for PGRs: increased profit margins. With cannabis rapidly becoming a big business, increased yields per harvest and shorter growth cycles, have sparked an interest in PGRs from commercial perspectives. With lucrative returns for growers, it is interesting to note that the global PGR market is to surge from $3.5 Billion observed in 2014, to $6.4 Billion by 2020.

Courtesy of Humboldt Seeds

What are Plant Growth Regulators?

Discovered in the late 1920s and 1930s, PGRs have been used in agriculture for decades to increase the commercial viability of crops. In more recent years, fears about the safety of some synthetic PGRs came to light. Due to their apparent toxic nature, many have subsequently been banned for use on consumable food crops since the 1970s and are regarded as pesticides in many countries. Some may be familiar with the “Alar scare” which cost the US apple industry over a $100 million, after the controversial PGR daminozide was deemed to be a “probable human carcinogen” by the US government. This particular type of synthetic PGR, that interferes with hormonal pathways is often regarded as a “Plant Growth Retardant”.

To begin defining exactly what plant growth regulators are we need to understand plant hormones, also known as phytohormones. It is these hormones that PGRs influence and act upon. Plant hormones are natural to the plant kingdom and similarly to animal hormones, play major roles in a plants growth and development. It is traditionally accepted that there are 5 major classes of natural plant hormones (endogenous) that play key roles in a plants life cycle.

  • Auxins
  • Gibberellins
  • Cytokinins
  • Abscisic acid
  • Ethylene

They all have regulatory functions and can either inhibit or increase cellular growth and activity. They most often work in tandem with one another in varying ratios throughout a plants life cycle. The table below shows the timing and overlap of the 5 major classes and their significance throughout the developmental stages of a Cannabis plants life.

How do PGRs affect plants

So how do PGRs actually manipulate a plants growth cycle and increase it’s yields? To understand this we need to take a look at signal transduction pathways, hang in there!

Manipulating Growth Cycles

Plants make extensive use of signal transduction pathways throughout their life. They are feedback mechanisms that allow plants to respond to environmental and chemical changes. Signal transduction pathways work in sequences of biochemical reactions. From this a cell generates a response to a stimulus. Cell signaling in plant development usually involves a receptor (i.e. for a hormone or light molecule) and a signal transduction pathway, which concludes with a cellular response that is relevant to the plants development.

A common mechanism for plant hormone action is the breakdown or activation of DNA transcription proteins. These proteins work as activators or repressors of growth-stimulating genes. Essentially, activators and repressors act like the stop and start pedals on a car. When a repressor is present, it stops the formation of growth-stimulating genes, in parallel activators start the transcription of growth-stimulating genes. In response to the signal transduction pathway (initiated by the presence of plant hormones or PGRs), repressor proteins for example are then broken down and the “stop breaks” are removed. This allows the “car” to drive ahead and create growth-stimulating genes.

Increased Yields

PGRs increase yields by cellular expansion through signal transduction pathways. When growth stimulating genes are activated, cells begin to grow and increase in size. Cell expansion is primarily driven by water uptake into the cells cytoplasm, which accumulates in the cells central vacuole. Here the central vacuoles volume expands as water enters the plant cell. As a result of this the cell wall also expands through turgid pressure. This expanding outward pressure from the extra water inside the plants cell is one cause for the added weight observed in crops grown with PGRs. This results in the plant cells having increased water retention. Auxins also play roles in the enlargement of plant cell walls for growth, this is known as the “Acid growth hypothesis”.

Here auxins essentially acidify the cell wall, with the help of Expansion proteins to loosen and expand it. A plant’s cell wall is mostly made of cellulose which is a material that increases as auxins work to activate cell wall growth. From this, we can assume that cellulose material also contributes to the added weight gained with the use of PGRs. Relating this back to your fruits and flowers, this means that when total harvest weight increases, the over-all quality decreases.

Evidence for PGRs’ being dangerous

The verdict on whether PGRs are dangerous to human health continues to remain open for discussion. With differences in testing, regulations and laws across the world, PGRs are deemed as both safe in some countries and toxic in others. In the USA and Europe they are largely regarded as pesticides, though still widely used in agriculture. The PGRs that interfere with hormonal pathways (in particular Gibberellin) and their biosynthesis are seen to be the most dangerous.

Potential threat to human health

When considering the use of Cannabis from a medical perspective it would be best to avoid Cannabis grown with synthetic PGRs where possible. Although there are many PGRs out there, here is an outline of some popular synthetic PGR Gibberellin inhibitors:

Environment pollutants

As well as posing a potential threat to human health PGRs being pesticides have been found to be environmental pollutants. Residual PGRs in the soil and water are shown to have toxic effects on the digestive organs of fish and their embryos. Microorganism diversity also changes with PGR applied soils. This environmental impact is likely to be causing more damage than realized, due to soil run off and infiltration of aquatic systems.

Plant Growth Regulator (PGRs): Weed on Steroids?

Courtesy of Humboldt Seeds

What PGR natural alternatives are there?

So far we have largely discussed synthetic PGRs because at present the vast majority of the market are using these. But are there any natural alternatives out there? When it comes to growing Cannabis and most crops for that matter we have to start with the basics: How can one best mimic mother nature ?. The use of PGRs should be to enhance the genetic potential of a Cannabis plant, so a good start, rather than using synthetic man-made plant hormones (which appear to have the more dangerous effects) is to look at where we may find similar organic compounds naturally.


Derived from chitin this organic molecule is found in the exoskeletons of crustaceans, from Mantis shrimps to Beatles. Chitosan provides structural support in the hard shells of these animals. It is a vastly abundant biodegradable material with a low molecular weight. When chitosan is applied as a foliar or soil drench to plants it has been found to exhibit PGR qualities. Targeting a plant cells plasma membrane and nucleus, chitosan regulates gene expression and other cellular processes. NASA have also taken interest, experimenting with chitosan to aid plant growth in space!


This plant growth stimulant can be found in a variety of natural sources such as alfalfa meal, sugarcane and bees wax. Triacontanol is a “fatty alcohol” and is sometimes referred to as Melissyl or Myrcicyl alcohol. It is non-toxic, environmentally friendly and safe to consume. Research has shown Triacontanol to be a powerful growth stimulant, effecting basic metabolic processes such as photosynthesis, enzyme activity, nutrient uptake CO2 assimilation and much more. In the correct doses Triacontanol significantly increases the amount of chlorophyll in leaves, improving the rate of photosynthesis.

Final Thoughts

With the use of PGRs continuing to grow, it will be interesting to see how they pursue to play a role in the future of agriculture and Cannabis. There is plenty of room for research in this area, but there are evident benefits and costs to using PGRs. Plant growth regulators are essentially a human attempt at “bio-hacking” a plants biological system. With that in mind we have a “you get back what you put in” scenario. It appears that if sourced naturally, PGRs can have positive up-regulating effects on the growth and development of plants, with fewer negative consequences and health concerns. However when created synthetically PGRs that manipulate growth cycles and yields come at the cost of poorer quality and potential health hazards.

For more information: https://www.humboldtseeds.net/en/blog/plant-growth-regulators-weed/

Plant Growth Regulator (PGRs): Weed on Steroids? was posted on High Times.

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