The Influence of Light Intensity and Quality on Secondary Metabolism
Plant growth and development, such as the production of leaves, stems, roots, and floral organs, is the result of primary metabolic processes. The byproducts of photosynthesis are shuttled throughout the plant and utilized in developing tissues. However, these are not the only metabolic processes. There are several other processes in plants such as coloration, warding off predators and infection, promoting pollination and symbiotic relationships, and defense against environmental conditions such as light and temperature. This is what we refer to as secondary metabolism. When secondary metabolism comes into play, resources are diverted away from the primary metabolism (growth) and are used to generate the various attributes seen as crucial to crop quality for human consumption.
When we as humans consume plants as food or medicine, many of the compounds produced via secondary metabolism can have powerful effects on basic bodily functions. We can also experience the alleviation of chronic disease symptoms, the prevention and reduction of cancer symptoms, and insulation against psychological issues like anxiety and stress.
In a controlled growing environment, we can directly influence natural defense mechanisms in plants (aka secondary metabolism) by manipulating light intensity and light quality. In this article, we plan to focus on the secondary metabolites (flavonoids, terpenes, cannabinoids, and others) that plants produce in response to environmental cues, how they affect crop quality, and what you can do to take advantage of these mechanisms.
Editor’s Note: As any good grower should be aware, life is full of tradeoffs. Emphasizing secondary metabolism to the exclusion of primary metabolism can have negative consequences for plants, so a balanced approach is crucial!
In nature, flavonoids primarily serve to interact with the outside environment in some form. They can act to attract pollinators, signal soil microbes for mutually beneficial relationships, protect against oxidation, and defend the plant against harmful wavelengths or excessive amounts of light. Anthocyanins are a class of flavonoids visible as a red to purple coloration of leaf tissue. Red-leaf lettuce and herbs often contain high amounts of these flavonoids, which essentially function as sunblock for plants. When a leaf surface is exposed to blue light (400-500nm) or ultraviolet light (300-400nm) of a high enough intensity (differs between species), the plant’s secondary metabolism is triggered. Blue and ultraviolet (UV) light carry a tremendous amount of energy that can potentially harm various cellular functions within a plant. In order to both protect their tissues from excessive energy and clean up any “free radicals” produced in their cells, plants will produce anthocyanins.
Editor’s Note: Think of deep blue light and UV light like you would x-rays. There is enough energy in these forms of light to do tissue damage. Plants form anthocyanins to protect themselves from this damage.
Accumulation of anthocyanins in response to blue or UV light scales up with increasing light intensity. Plants will always attempt to balance the needs of photoprotection via anthocyanins with the needs of photosynthesis via chlorophyll and carotenoids. It’s important to emphasize that photoprotection is a secondary process of the plant, which diverts energy away from growth. If the light intensity is high enough, a visible change in crop coloration can be observed and plants will show more reds and fewer greens.
Green light (500-550 nm), however, can reverse many defensive functions in plants that are otherwise stimulated by exposure to blue light. Enough green light relative to blue can completely reverse the photoprotection response. For example, red-leaf lettuce grown under a high proportion of green light may not turn red at all. Red-leaf lettuce growers growing under HPS lamps (which have significantly less blue light than green) may struggle to stimulate anthocyanin production. Even adding a fixture that provides additional blue light isn’t likely to stimulate crop coloration unless there is significantly more blue than the portion of green supplied by the HPS, as the proportion of light is what’s important. In such a case, it may be useful to alter crop lighting prior to harvest after the desired amount of growth has occurred. This is referred to as an “end-of-production” light treatment or EOP for short. Recent EOP work by Dr. Garrett Owen and Dr. Roberto Lopez demonstrated an increase in coloration (from green to red/dark red) of four lettuce varieties when provided with 100 µmol/m2s by supplemental LED lighting (red, blue, and a 1:1 ratio) for 5 to 7 days prior to harvest. Their research also demonstrated that incremental increases in supplemental light intensity from 0 to 100 µmol/m2s resulted in increasing amounts of pigmentation. These varieties were also grown under HPS fixtures (providing an additional 70 µmol/m2s). However, this EOP treatment was of insufficient light quality to reach the desired level of crop coloration for market.
The takeaway message for implementing EOP treatments to increase crop quality is that crop coloration is more responsive to blue light so long as it is provided at a sufficient intensity. Additionally, this type of supplemental lighting is more efficient when used as an EOP treatment.
Unlike flavonoids, which are mostly perceived as bitter, terpenes have distinct aromas and flavors. These are very volatile oils that impart the wonderful attributes of flowers, herbs, and medicinal plants. They also greatly increase product quality if produced in sufficient quantities. For example, limonene is the major terpene present in the essential oil of lemons and myrcene for mangoes. The same terpenes can be produced within cannabis. The largest concentrations of terpenes in cannabis are typically found within non-glandular trichomes (IE on cannabis leaves) and glandular trichomes (such as the calyces of cannabis).
Limited research has been conducted on the impact that various wavelengths have on terpene biosynthesis, but numerous plant physiologists believe that specific wavelengths are required for the activation of the metabolism necessary to produce them. What we do know is that increasing light intensity signals certain plants (including cannabis) to produce more glandular trichomes. There is some evidence that these additional trichomes are generated as a localized site for defensive flavonoid secretion (IE photoprotection). The increased synthesis of glandular trichomes also creates new sites for terpene biosynthesis and storage, which can influence overall terpene concentrations in several plant species. Overall, terpene synthesis is a hot topic for researchers who are studying the effects of light intensity and light quality.
Cannabinoids are a unique class of compounds found primarily in cannabis, hence the name. These sticky resinous oils are produced within trichomes during the flowering period and are thought to both protect the developing flowers from insects as sticky traps, as well as protect against excess heat under shifting solar conditions. There are over a hundred different cannabinoids including Δ9-tetrahyrdocannabinol (THC), cannabidiol (CBD), cannabigerol (CBG), cannabinol (CBN) and many others. CBG-A (the carboxylated acidic form of CBG) is the precursor substrate for production of THC and CBD. It does not produce the typical marijuana “high,” but certain researchers are evaluating specific medicinal effects, such as alleviating symptoms from neuropathy, degenerative brain disorders, glaucoma, certain cancers, and anxiety. Most varieties of Cannabis have been bred to produce high amounts of THC and/or CBD, leaving behind relatively low concentrations of other cannabinoids that may carry medicinal qualities. Due to the current legal status of cannabis with federal governments around the world, there is very little scholarly research that investigates the effects of various wavelengths and intensity on production of cannabinoids. The work that has been done was performed on cultivars of inferior quality, often several decades ago. However, there is more peer-reviewed research being conducted on cannabis today, and as laws change quality research will be published in this area.
We do know that UV light and possibly even short wavelength (~400-420nm) blue light can stimulate the production of cannabinoids, although production of these secondary metabolites will occur regardless and this effect is only a “boost” in production as opposed to being a requirement (Editor’s Note: As they say, a rising tide raises all the boats). We cannot say much about which wavelengths of light result in increased content of specific cannabinoids, however we are continually investigating the effects of different wavelengths on secondary metabolites.
So long as carbon dioxide, water, and nutrients are not limiting growth of the plant and it is a fast-growing species or cultivar, higher light intensities will result in faster growth and increased production of secondary metabolites. However, light intensities that are too high can damage cells, especially in sensitive species, and result in the production of free radicals such as hydrogen peroxide within cells. On the surface, you might notice this effect as photobleaching if the plant is not photoacclimated to that light intensity. Many growers notice this issue when they are transferring plants from a rooting phase into a highly productive phase under much higher light intensity. As a part of the photoacclimation process, highly productive or fast-growing species will likely accumulate more chlorophyll to harvest more light. If intensity is too high, production of various carotenoids is increased to protect the photosynthetic reaction centers and dissipate some light energy.
This is why increasing light intensity can have diminishing returns since more light is dissipated in response to higher light intensity. To photoacclimate your plants productively with little to no photobleaching, it is best to incrementally increase light intensity or use a shade cloth for a week or two. Slowly acclimating plants to higher light intensities can be achieved using dimmable lights after determining what your desired PPFD will be and creating a series of incremental increases in intensity over time. A less sophisticated way to achieve the same outcome is to gradually decrease the plant-lamp separation over time, thanks to the Inverse-Square Law.
We know that the proportion of wavelengths supplied to plants, as well as intensity, completely changes the photomorphogenic outcomes as well as the phytochemical concentrations. Increasing light intensity induces production of various secondary metabolites in plants as a form of photoprotection. Blue and UV light have the most powerful influence on secondary metabolism relative to other wavelengths and this scales with intensity. From a production standpoint, secondary metabolites often improve product quality both due to their medicinal value for humans as well as their crop coloring effects (shelf appeal). This differs depending on the species and cultivar. Some species and cultivars are more tolerant to this response and require higher light intensities to show any response while others do not.
One proven method to “get the best of both worlds” is to utilize an EOP treatment in which plants grow and develop under optimal conditions for primary metabolism (broad spectrum), and are then transferred beneath a secondary metabolism promoting light treatment (higher intensity or specific wavelengths) prior to harvest after major crop growth has occurred. Overall, the most important aspect to remember is that secondary metabolism diverts resources away from plant growth. When selecting or making any changes to your lighting system, consider these innate plant responses to ensure your system is optimal for your intended species and market.
For more articles regarding lighting and plant growth and development, visit Fluence Bioengineering.
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About the Author
Josh Gerovac is a horticulture scientist at Fluence Bioengineering. He has spent the last decade working in controlled environment agriculture, ranging from growth chambers to greenhouses. His research and practice is focused on the influence of light intensity and spectral light quality on growth, morphology, and nutrient content of edible, ornamental, and medicinal crop production. He has a Bachelor of Science in Horticulture Production and Marketing, and a Master of Science in Horticulture, both from Purdue University.