Purple For many years, the only purple varieties of cannabis were those that had been grown outdoors and subjected to cold conditions. Many strains of cannabis purple to some extent when exposed to cold; now, selective breeding programs have yielded cannabis genetics that are purple even in normal environmental conditions.
Anthocyanins are a group of around 400 water-soluble pigment molecules that due to their structure and biosynthesis are classed as flavonoids, and appear red, purple or blue according to their pH (in acidic pH levels they appear more red, in neutral conditions purple, and in alkaline more blue). Flavonoids are generally yellow in appearance, hence their name, which is etymologically derived from the Latin for “yellow”, flavus, and does not indicate any association with flavour. In fact, flavonoids are usually extremely bitter, and are generally associated with pigmentation.
Cannabis and all other higher plants (those possessing a vascular system consisting of a xylem and phloem, structures that distribute nutrients and water throughout the plant) contain anthocyanins in all parts of the plant: leaves, flowers, fruits, stems, and even roots. Depending on the genotype of the cannabis plant, these anthocyanins may be expressed in the latter part of the flowering period, irrespective of environmental conditions; conversely, they may express in cold conditions, or may never be produced in sufficient quantities to affect the appearance of the plant at all.
How do anthocyanins affect plant appearance?
Anthocyanins are not produced throughout the lifetime of the plant, and it is only during the last few weeks of flowering that they begin to alter its appearance. However, the absence of chlorophyll in the final stages allows the pigments to show through even more distinctly. As the days shorten and hours of darkness increase, photoperiod-dependent plants are given the signal to cease producing chlorophyll (which is essential for photosynthesis and vegetative growth), so that energy can be focused solely on producing flowers and ultimately fruits. As chlorophyll breaks down and dissipates from the structures of the plant, and anthocyanins begin to accumulated, the plant takes on vivid purple, blue and red hues.
Even cannabis plants that contain low levels of anthocyanins often change colour towards the end of the flowering cycle. Most growers will be familiar with the gold, orange and ochre hues that appear in many strains of cannabis in the weeks prior to harvest—and while yellowing in the vegetative period or in early flowering is usually indicative of disease or deficiency, in the latter stages of flowering, it is entirely natural.
In this case the pigments responsible are carotenoids, a group of around 600 molecules that range in appearance from pale yellow to deep orange-red. As carotenoids are produced throughout the life cycle of the plant, their appearance in the final stages of flowering is a result of chlorophyll production ceasing rather than enhanced production of the pigments.
Purpling due to cold
Certain strains of cannabis contain anthocyanins but are unaltered in appearance unless subjected to prolonged periods of cold temperatures. The mechanism behind this phenomenon is not fully understood, but there is a clear link between cold and enhanced anthocyanin production, as blood oranges also require a cold period to fully acquire their red colouration. It is believed that a complex set of genetic circumstances lead to this occurrence: a gene known as Ruby, which is common to all citrus varieties but is not expressed in most, is expressed in blood orange due to the presence of a special section of DNA known as a retrotransposon, which is a mobile genetic element capable of being transcribed as part of several essential genes.
Due to the ability of retrotransposons to become inserted into essential genes and thereby cause mutations and potential non-viability, plants have evolved complex mechanisms to ensure that retrotransposons and similar genetic elements remain inactive. However, these mechanisms may be disrupted in times of stress, such as when exposed to periods of cold. In blood oranges, activation of the retrotransposon triggers expression of the otherwise inactive Ruby gene, and anthocyanin production kicks in. While studies into cold-dependent anthocyanin production in cannabis have not been conducted, it is likely that a similar mechanism is in play.
Are anthocyanins useful, or just attractive?
Anthocyanins are known to be powerful antioxidants, and are also thought to possess analgesic, anti-inflammatory and neuroprotective properties. Recent research has demonstrated that certain anthocyanins have some selective affinity for the cannabinoid receptors, with some types binding to the CB1 and others to the CB2 receptors.
Following a diet rich in vegetables and fruits that contain high levels of anthocyanins is associated with a reduction in chronic lifestyle diseases and increased general health and performance in humans; particularly, consumption of berry fruits such blackberry, raspberry, goji berry, blueberry and cranberry, all of which contain very high anthocyanin levels, is thought to play a significant role in maintaining human health in many communities throughout the world. Consumption of blood oranges too has been variously associated with improved cardiovascular health, and there are indications that they may assist in preventing obesity even in individuals fed a high-fat diet.
Solving the mystery of why cannabis and other plants produce abundant anthocyanin when cold or stressed could have many potential benefits in medicine and research. Furthermore, if anthocyanins are found to genuinely possess the health benefits they appear to have, purple cannabis could find itself elevated far above its current status of simply being aesthetically pleasing, and be a serious candidate for further investigation.