This insight into the world of Plant Growth Regulators (PGRs) is designed to educate and bring awareness to both the scientific and Cannabis community. First, we highlight why PGRs have become of such interest recently and what defines them. Then we further explore how PGRs work in plants and if they are potentially dangerous for our health. Finally we explore alternative and natural PGRs that could be potential considerations for use as plant growth substances.

Why the interest of 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). Genetically some strains are naturally more dense than others, Cannabis indica for example is known to be more dense than Cannabis sativa. Increased levels of light during flowering periods and optimal hydration , throughout a plants life has also been found to increase bud density.

However with the use of PGRs, growers can make an extra effort to ensure buds are compact, even if genetic and environmental factors are against them. This desire for dense, tightly formed buds, which gives cannabis a desirable look and feel is one factor driving the use of PGRs in the Cannabis world.

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. Even those who are not looking for a commercial benefit, purely medicinally concerned, may be unsuspectingly using PGRs. They are often sold as nutrients, growth boosters, vitamins and hormone enhancers, that can be applied through foliar feeds and root drenches. 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.

The competitive nature of the cannabis market, makes the use of PGRs a tempting offer. With cannabis at times being literally worth its weight in gold, every extra gram yielded makes a big difference. Manipulating a plants morphology to fit into more confined spaces or speeding up growth cycles will help save on costs and maximise output. On top of this Increasing flower weight to maximise profits makes the use of PGRs seem logical, but does this come at the cost of poorer quality and potential health risks?

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. Hormones in the tiniest of doses can turn "off and on" gene expression, cell growth and or cell-death (apoptosis). For relatively simple molecular structures they have huge and diverse effects on a plants growth cycle. Natural regulation of these plant hormones comes via environmental cues and receptors as well as the plants genome. 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.

Since the discovery of the 5 major classes of plant hormones, research has revealed additional classes, such as Brassinosteroids. This hormone has also been found to regulate a wide range of physiological processes including plant growth, development and immunity. Relatively little is known about plant hormones compared to mammalian hormones and so it is likely that many more are yet to be discovered. Synthetic plant hormones and synthetic PGRs aim to either mimic hormones or interfere with pathways involved in the synthesis or breakdown of hormones in the plant. It's the PGRs that interfere with hormonal pathways that are most notorious. Big names include : Paclobutrazol, Chlormequat chloride, Daminozide, Uniconazole etc. (See table below). Since plant growth and developmental processes are mediated by hormones, plants can be successfully manipulated by the application of appropriate PGRs by humans. It is the application of natural and synthetic plant hormones to manipulate growth cycles and plant morphology that then classes these chemicals as "Plant Growth Regulators". Plant growth regulators defines a function for a range of chemicals, but does not refer to a particular chemical nature. Therefore it is not possible to make simple statements which apply to all PGRs as chemicals. However there are clear differences when you look at synthetic and natural PGR applications.

-Table of commercial PGR functions - Professor Frederick M. Fishel

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 signalling 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.

This is the basic nature of how PGRs operate; of course this varies some what for each individual chemical, but they all use signal transduction pathways in this way. PGRs that interfere with the biosynthesis (creation) of these plant hormones can therefor stop or retard growth in the same way other PGRs can stimulate growth. From this it is possible to see how genes can be activated to induce earlier onset of flowering or manipulate flowering periods. In conjunction genes can also be turned off to stop apical growth keeping a plant short and stocky. This is often a desirable trait for growers to maximise grow space potential.

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. This is because the majority of the weight gained from PGRs comes from extra cellulose material and some water retention. Therefore pound for pound, cannabinoid content becomes saturated, which means lower ratios of THC in the final product.

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:

An overview of the evidence out there for the dangers of PGRs has a strong case with studies showing synthetic pathway inhibitor PGRs in particular to be carcinogenic , toxic to the liver and cause infertility to name a few. These effects were observed in mammalian studies. The level of exposure to PGRs in these trials however is likely to be much higher than the levels of residual PGRs found in crops. It may take large unrealistic amounts of PGRs remaining in or on the final product to have significant health hazards. It is also worth noting that for every study supporting the dangers of PGRs there is another showing them to have little negative affects on mammalian health. As a consumer who isn't strict with maintaining a healthy diet in general you could argue that the consumption of PGR grown cannabis is no different to eating a meal from your local takeaway or fast-food joint. Perhaps more worrying for Brits, despite being prohibited in many countries, Paclobutrazol is still licensed for use on apple, cherry, pear and plum trees in the U.K. With that in mind your PGR joint and local supermarket apples are both as potentially toxic to you…

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 realised, due to soil run off and infiltration of aquatic systems. One way to look at PGRs is similar to performance enhancing drugs in sport. The use of anabolic steroids, similarly to PGRs in plants, can have performance enhancing effects in athletes, aiding muscle growth and recovery. This may provide a competitive edge to an extent but there are many side affects and potential health hazards that come with this. A body builder who has used steroid hormones for muscle growth may look physically strong on the outside but may have a struggling heart and be at serious risk of liver failure. Similarly dense "juiced" up buds may look good but at a sacrifice of general quality.

The synthetic PGRs that are biosynthesis inhibitors seem to be the most dangerous but what about synthetic PGRs that mimic plant hormones? There are two lines of synthetic PGRS, one mimics natural hormones, the other interferes with the synthesis of natural plant hormones. These two modes of action have different outcomes. Synthetic auxins for example are applied as rooting hormones. The application of auxins to clones is technically the use of PGRs however here they are working differently. They encourage meristem cells (a plants equivalent to human stem cells) to proliferate into root cells, similar to stem cell therapy. It is not clear whether synthetic PGRs that mimic natural hormones are less dangerous than PGRs that interfere hormone synthesis. From the evidence there is a case supporting this but there is certainly plenty of room for more research. After evaluating the evidence out there, despite concerns for safety it appears that when properly used, PGRs in general seem to have a satisfactory safety record. However, if the wrong type, concentration, equipment, or method of application is not correct, poisoning may well occur in plants, animals, and humans.

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.

Chitosan

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 is because Chitosan has been found to increase photosynthesis, enhance plant growth, stimulate nutrient uptake and aid in seed propagation. Chitosan also has strong anti-pathogenic properties, naturally up-regulating innate defence responses in plants to combat insects, pathogens and soil-borne diseases. Further research has also shown chitosan applications to increase flavonoid and terpenoid production in a variety of resinous plant species.

Triacontanol

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. Similarly in root systems cell growth is enhanced creating stronger root networks allowing for greater nutrient uptake. Due to its up-regulating qualities many studies have shown the application of triaconantonl as a foliar feed, increased yields dramatically compared to control groups. When applied to mint plants in experiments it has also been found to increase the overall yields and interestingly the mints essential oil content, suggesting similar results could be obtained for Cannabis.

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. To further avoid these health hazards, it is essential for those using PGRs to have a thorough understanding of the correct methods and concentrations for PGR application. Whether or not consumers agree with the use of PGRs in their Cannabis one point remains clear, everyone should have the right to know what they are consuming. All agricultural nutrient manufactures should be legally required to provide a full chemical analysis of their products. It is also perhaps a duty of care for growers to state whether PGRs, pesticides and any other chemicals are used in the production of their Cannabis. With this in place consumers can at least make informed decisions about what they choose rather than being misled. If you are interested in learning more about organic PGRS and for a list of scientific sources contact Blunt Science on : blunt.science@humboldtseeds.net