Influence de l'intensité et de la qualité de la lumière

Plant growth and development, such as production of leaves, stems, roots, and floral organs is a result of primary metabolic processes. Byproducts of photosynthesis are shuttled throughout the plant and utilized in developing tissues. However, 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 these processes come into play, resources are diverted away from the primary metabolism (growth) and used to generate various attributes crucial to crop quality from a human consumption standpoint.

When we consume plants as food or medicine, many of these compounds can have powerful effects on basic body functions, alleviation of chronic disease symptoms, prevention of cancers, and provide relief to general psychological issues like anxiety and stress. Within controlled environments, we can influence these natural defense mechanisms in plants by manipulating light intensity and light quality. It’s important to consider that there is a tradeoff between encouraging plant growth (primary metabolism) and boosting production of secondary metabolites that may increase the quality of your crop. In this article, we focus on the secondary metabolites (flavonoids, terpenes, cannabinoids, and others) plants produce in response to environmental cues, how they affect crop quality, and what you can do to take advantage of these mechanisms.

Flavonoïdes

In plants, flavonoids primarily serve as insect attractants (coloring pollen), signals for forming relationships with soil microbes, antioxidants (which scavenge and eliminate compounds that may cause photobleaching and growth inhibition), and photo-protectants (which dissipate harmful wavelengths to protect cells). 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, essentially functioning as sunblock for plants. When a leaf surface is exposed to blue (400-500nm) or ultraviolet (300-400nm) wavelengths of a high enough intensity (differs between species), the plant’s secondary metabolism is triggered for a few reasons. Blue and ultraviolet (UV) light have very high frequencies which means they carry a tremendous amount of energy that can be damaging to various cellular functions within the plant. In order to both protect these tissues from incoming energy as well as clean up any “free radicals” produced in the cell, plants will produce anthocyanins.

Accumulation of anthocyanins in response to blue or UV light scales up with increasing light intensity. Eventually, an equilibrium of photoprotection by anthocyanins and light capture by terpenoids (chlorophyll and carotenoids) is established. Its important to reinforce that this is a secondary process of the plant, which diverts energy away from growth. If the intensity is high enough, a visible change in crop coloration can be observed. Green light (500-550 nm) can reverse many functions in plants that are otherwise stimulated by exposure to blue light. Too much green light relative to blue can completely reverse this response. A red-leaf lettuce grown under a high proportion of green may not turn red at all. Those growing under HPS lamps (which have a very small proportion of blue light relative to green, especially compared to broad-spectrum LED fixtures) may struggle to stimulate anthocyanin production in salad crops. Incorporating a fixture providing 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. In this case, it may be useful to alter crop lighting just before harvest after the desired amount of growth has occurred. This is referred to as an “end-of-production” light treatment or EOP.

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/m2/s 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/m2/s resulted in increasing amounts of pigmentation. These varieties were also grown under HPS fixtures (providing an additional 70 µmol/m2/s), however, this EOP treatment was of insufficient light quality to reach the desired level of crop coloration for market. The take away 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. Furthermore, to prevent growth inhibition, this type of supplemental lighting is more efficient when used as an EOP treatment.

Terpènes

Contrairement aux flavonoïdes, qui sont principalement perçus comme amers, les terpènes ont des parfums et des saveurs distincts. Ce sont des huiles très fluides qui confèrent de merveilleux attributs aux fleurs, aux herbes et aux plantes médicinales et qui augmentent considérablement la qualité des produits s'ils sont produits en quantités suffisantes. Par exemple, le limonène est le principal terpène présent dans l'huile essentielle de citron et le myrcène dans celle de mangue. Ces mêmes terpènes peuvent être produits par d'autres espèces telles que diverses herbes et même le cannabis. La plus grande concentration se trouve généralement dans les trichomes non glandulaires (tels que les poils des feuilles de tomate ou de cannabis) et les trichomes glandulaires (tels que les têtes bulbeuses observées sur les feuilles de sucre et les calices de cannabis). L'impact des différentes longueurs d'onde sur la biosynthèse des terpènes a fait l'objet de recherches limitées, mais plusieurs physiologistes végétaux pensent que des longueurs d'onde spécifiques sont nécessaires à l'activation des composants métaboliques requis. En outre, l'augmentation de l'intensité lumineuse incite certaines plantes à produire davantage de trichomes glandulaires. Il existe des preuves que ces trichomes supplémentaires sont générés en tant que site de sécrétion de flavonoïdes défensifs (comme mentionné précédemment, ces composés protègent contre l'excès d'intensité lumineuse et la lumière UV). La synthèse accrue des trichomes glandulaires crée de nouveaux sites pour la biosynthèse et le stockage des terpènes, ce qui peut influencer les concentrations globales de terpènes chez plusieurs espèces végétales. D'une manière générale, la synthèse des terpènes est un sujet d'actualité pour les chercheurs qui étudient les effets de l'intensité et de la qualité de la lumière.

Cannabinoïdes

Cannabinoids are a unique class of compounds found only in Cannabis. These sticky resinous oils are produced within trichomes during the flowering period and are thought to both protect the developing flowers from insects (sticky trap) as well as 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 low concentrations of most other cannabinoids that carry many medicinal qualities. Due to the current legal status of Cannabis with most federal governments, 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 with cultivars of inferior quality compared to anything on the market today. However, there is more and more peer-reviewed research being conducted on Cannabis, 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 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. 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.

Photo-acclimatation

So long as carbon dioxide, water, and nutrients are not limiting growth of the plant and it is a fast-growing species, higher light intensities will result in faster growth and increased production of secondary metabolites. However, too high of a light intensity can be damaging to cells, especially in sensitive species, producing free radicals such as hydrogen peroxide within cells. On the surface, you might notice this effect as photobleaching (tan to white patches on leaves) if the plant is not photoacclimated to that intensity. Many growers notice this issue when they are transferring plants from a seedling propagation or rooting phase in which light intensity is low, into a highly productive phase under high 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 (for more info, refer to […pigments & photoreceptors]) is increased to protect the photosynthetic reaction centers and dissipate some light.

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 (which inhibits growth), 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 (depending on your fixture capability and species) and creating a series of incremental increases in intensity (beginning slightly above the propagation intensity) over time. A less sophisticated way to achieve this same outcome (if your lights are not capable of dimming) would require you to start with the plant-lamp separation distance much larger than desired and then slowly move the lamp closer to the plants (or vice versa) over the same duration.

As previously mentioned, anthocyanins can accumulate in leaves of many species in response to blue or UV light of sufficient intensity. A similar mechanism protects the fruit of some crops such as tomatoes and peppers. When growing green peppers, you may notice that some fruit surfaces exposed to more light have patches of yellow to orange coloration. This accumulation of photoprotective carotenoids prevents damage to the fruit. Lycopene, an orange to red carotenoid, plays a similar role in tomato fruits. Just like most other light-induced secondary metabolites, production of these carotenoids occurs at a faster rate as light intensity increases.

Conclusion

Nous savons que la proportion de longueurs d'onde fournies aux plantes ainsi que l'intensité modifient complètement les résultats photomorphogéniques ainsi que les concentrations phytochimiques (métabolites secondaires). L'augmentation de l'intensité lumineuse induit la production de divers métabolites secondaires dans les plantes comme forme de protection. La lumière bleue et la lumière UV ont l'influence la plus puissante sur le métabolisme secondaire par rapport aux autres longueurs d'onde, et cette influence varie en fonction de l'intensité. Du point de vue de la production, ces métabolites améliorent souvent la qualité des produits en raison de leurs vertus médicinales pour l'homme et de leurs effets sur la coloration des cultures. Ces effets varient en fonction de la génétique en jeu et sont très variables d'une espèce à l'autre. Certaines espèces sont plus tolérantes à cette réaction et ont besoin d'intensités lumineuses plus élevées pour réagir, tandis que d'autres ne le sont pas. Une méthode éprouvée pour "obtenir le meilleur des deux mondes" consiste à utiliser un traitement EOP dans lequel les plantes poussent et se développent dans des conditions optimales pour le métabolisme primaire (large spectre), puis sont transférées sous un traitement lumineux favorisant le métabolisme secondaire (intensité plus élevée ou longueurs d'onde spécifiques) avant la récolte, une fois que la culture s'est fortement développée. Dans l'ensemble, l'aspect le plus important à retenir est que le métabolisme secondaire détourne les ressources de la croissance de la plante. Lorsque vous choisissez ou modifiez votre système d'éclairage, tenez compte de ces réactions innées des plantes pour vous assurer que votre système est optimal pour l'espèce et le marché que vous visez.