To us, we can barely see it, where it hangs out on the edge of the visible light spectrum, but to our humble plants, far-red is a game-changer, manipulating both their morphology and ability to photosynthesize. Like UV light, far-red light is called by some the “forgotten” spectrum. Both spectrums have little to no purpose for the everyday person due to their poor illumination. In fact, when defining Photosynthetically Active Radiation (PAR) and studying the effects of the different spectrums on plants, far-red went unobserved by Dr. Keith McCree due to his use of prisms and filters. Thankfully, today we have advanced technology that proves both far-red and UV are incredible spectrums for plant growth.
Despite limited technology that also forced McCree into a methodology of originally only studying detached leaves, PAR became gospel for many. In McCree’s defense — who was aware that spectrums outside PAR were photosynthetically effective — plants grown under just PAR see excellent results. They are often healthy, robust, and deliver near-optimal yields.
However, many of us, both home gardeners and especially those that operate on a commercial scale, aren’t in it for only near-optimal results. To fix this, we need to tackle a few issues, and one of the biggest to fix is ensuring our crops have the optimal amount of far-red photons along with deep red photons in flowering.
Defining The Red Light Spectrum
Photons with wavelengths falling between 600 and 700 nm generally define the red light spectrum. This includes deep red-which occurs around 660nm. Deep red is a powerful driver of photosynthesis at all times in a plant’s life, as it’s the most efficiently absorbed wavelength by chlorophyll.
If we include far-red radiation, we can extend the red spectrum potentially all the way to 750nm or even higher to 850 nm. Though, it can be argued that we should continue to categorize red and far-red as separate spectrums.
All wavelengths of red light have a positive effect on plants thanks to chlorophyll’s affinity for red light over all others. However, you can always have too much of a good thing, and red light is no exception. Too much red light is associated with stem stretching and larger but thinner leaves in the growth cycles. This is often considered an undesirable promotion, but not always.
Now, it’s a common misconception that red photons of any wavelength cause stem elongation, but this isn’t true. More likely, rapid stem elongation and thin leaves occur because the grow light is failing to provide a minimum amount of blue light. These photons directly inhibit stem elongation in plants, making high amounts of them desirable for the vegetative stage. However, there are red photons that can directly cause stem elongation, which reintroduces our far-red photons.
Large amounts of far-red photons will trigger a plant’s shade-avoidance response, causing it to stretch to find light. This happens because as photons in the red light spectrum move down through a plant’s canopy, the red photons with shorter wavelengths are more easily absorbed. Far-red photons, on the other hand, penetrate much deeper into a plant’s canopy as well as into the leaves (Brodersen & Vogelmann). This “shading” effect causes a low phytochrome stationary state (PPS) in plants, thus causing changes in both growth and architecture. Green light may do the same, but there is debate over how effectively. This leaves the deep red photons in the middle where they actually tell plants they are receiving direct light.
Pitfalls of Far-Red
During the germination stage, too much far-red can prevent seeds from properly sprouting, often resulting in their death (Piskurewicz, Urszula, et al.).
In the vegetative cycle, besides causing undesirable stretching, resulting in weak and easily broken steams, too much far-red can hurt secondary metabolite production. Plants may lose their normal healthy color due to decreased levels of anthocyanins and chlorophyll, stemming largely from excessive leaf expansion (Alokam S, Chinnappa CC, et al.). Perhaps worse, the levels of antioxidants, flavonoids, and other compounds beneficial to our health in the plant can decrease.
None of these should discourage either the commercial or home gardener away from supplementing additional far-red in flower. In fact, our lack of it is depriving our crops of what they need, resulting in less than optimal yields.
Far-Red/Deep Red Benefits In Flower
Far-red, often in conjunction with other light spectrums (notably deep red), has shown to significantly increase yield, and it does this potentially in about five areas: cell expansion, phytochrome manipulation, photosynthetic rate, higher total leaf area, and assimilate partitioning. It’s important to note that not all will apply to every plant, making far-red/deep red more critical for some plants over others.
We know far-red causes cell expansion in both the vegetative and flowering cycles. For shade-tolerate plants, this can always be a good thing. Case in point, lettuce and other leafy greens, which experience bigger leaves with minimum stem elongation regardless of the growth cycle.
For shade-intolerant plants that grow upwards, not outwards, to find light when shaded, far-red photons should be kept mostly away during the vegetative phase — deep red is fine, just have the minimum amount of blue light appropriate for your plants. Now, this can all change during the flowering stage, where other advantages of far-red apply without causing the dreaded stretch. Let’s look at that next.
|Lettuce and Basil||“Our results show that supplemental far
red at a moderate intensity is a viable
tool to manipulate extension growth.
When added to red and blue, far-red can
increase leaf size, and thus, fresh weight,
but at the expense of pigmentation.”
|Meng, Q and Runkle, E. “Far-red is the New Red”. Michigan State University.|
|Geranium (Pelargonium × hortorum), petunia (Petunia × hybrida), snapdragon (Antirrhinum majus), and impatiens (Impatiens walleriana||“We conclude that FR radiation increases plant growth indirectly through leaf expansion and directly through whole-plant net assimilation and in at least some species, promotes subsequent flowering.”||Park, Yu Jin and E. Runkle. “Far-red radiation promotes the growth of seedlings by increasing leaf expansion and whole-plant net assimilation.” Environmental and Experimental Botany 136 (2017): 41-49.|
Want to give your crops a shorter flowering cycle along with more hours of light? With far-red/deep red that’s possible!
High amounts of far-red and a lack of other spectrums (such as when lights are off) causes a plant’s active Prf phytochromes to return to their inactive Pr state. Deep red directly does the opposite, driving conversion of inactive phytochromes to become active. This gives plants the ability to tell time as far-red photons are more present at dusk, while deep red photons dominate during dawn. Not only do phytochromes let many plants tell time, but it also lets them know when to flower.
In short-day plants, when a plant is no longer able to able to convert enough Pfr phytochromes from Pf phytochromes, it starts to flower. In long-day plants, when a plant can covert enough PFr phytochromes from Pf phytochromes, they flower. This means we can use different ratios of r:fr to change how fast a plant enters into the flowering cycle. It should be noted that while the phytochromes proteins play the biggest factor in initiating flowering, availability of nutrients and temperature play a role as well.
Crops such as tomatoes and mouse-ear cress have all be shown to transition to flowering faster when given periods of far-red/deep red without suffering a decrease in yield. By shortening the flowering cycle by potentially a week or more, far-red is extremely beneficial in perpetual gardens.
For short-day plants, traditionally around 12 hours of darkness is required to flower these plants under sole-source lighting. However, short individual bursts of deep red, along with far-red, immediately before and after the main lights are turned on/off can reduce this requirement, allowing for more hours of light in a 24hr day.
|Tomatoes||“FR increased fruit yield, which correlated well with the accelerated flowering and overall increase in plant source strength under FR light.”||Kalaitzoglou P, van Ieperen W, et al. (2019) Effects of Continuous or End-of-Day Far-Red Light on Tomato Plant Growth, Morphology, Light Absorption, and Fruit Production. Front. Plant Sci. 10:322.|
|Xanthiulm||“Far-red light given at the
beginning of the dark period promotes flowering and
shortens the critical dark period by some 2 hours”
|Takimoto, A, and K C Hamner. “Effect of Far-Red Light and its Interaction with Red Light in the Photoperiodic Response of Pharbitis nil.” Plant physiology vol. 40,5 (1965): 859-64.|
Rate of Photosynthesis (Emerson Effect)
When comparing plants separated into two grow chambers, with one receiving 300PPFD and the other 350PPFD, we’d expect the plants getting the higher PPFD number to have a greater yield. In most cases, this would be true, but what if the 300PPFD gas exchange chamber is given enough far-red photons to make up for the difference in intensity? Many would expect near equal yields, but research shows this simply isn’t true.
This phenomenon is called the Emerson effect, and it describes how plants experience a greater photosynthesis rate when they receive both deep red and far-red simultaneously rather than them individually. Essentially, this occurs because both photosystems are on and working together, where only one would be triggered if just given either deep red or far-red.
|Spinach, Basil, Kale, Red Leaf Lettuce, Lettuce, Tomato, Bean, Soybean, Apogee Wheat, Tybalt Wheat, Corn, Sunflower||“Adding far‐red photons (up to 40%) to a background of shorter wavelength photons caused an increase in canopy photosynthesis equal to adding 400–700 nm photons. Far‐red alone minimally increased photosynthesis. This indicates that far‐red photons are equally efficient at driving canopy photosynthesis when acting synergistically with traditionally defined photosynthetic photons.”||Zhen S and Bugbee B “Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO2 Fixation and Increased Photon Capture During Long-Term Studies: Implications for Re-Defining PAR.” Front. Plant Sci. 11:581156. (2020).|
|“The increase in ΦPSII by far-red light was associated with an increase in net photosynthesis (Pn). The stimulatory effect of far-red light increased asymptotically with increasing amounts of far-red. Overall, our results show that far-red light can increase the photosynthetic efficiency of shorter wavelength light that over-excites PSII.”||Zhen, Shuyang, and Marc W van Iersel. “Far-red light is needed for efficient photochemistry and photosynthesis.” Journal of plant physiology vol. 209 (2017).|
Higher Total Leaf Area
Even when we take away the three reasons above as to how far-red can increase yield, there are still other ways it can do it. Now, we’re moving into the more unknown and shakier territory.
Since far-red photons are able to penetrate deeper into a plant’s canopy, it’s thought this could help keep lower leaves photosynthetically active. Lack of light is a significant contributor to bottom leaf death; less total leaf area equates to a worse photosynthetic rate.
|Tomatoes||“Simulations with a 3D-model for light absorption revealed that the increase in dry mass was mainly related to an increase in light absorption due to a higher total leaf area when comparing plants grown with and without far-red.”||Kalaitzoglou P, et al. “Effects of Continuous or End-of-Day Far-Red Light on Tomato Plant Growth, Morphology, Light Absorption, and Fruit Production.” Front. Plant Sci. 10:322. (2019).|
Our understanding of the effect of far-red radiation on assimilate partitioning is still in its infancy, but there are some fascinating findings that show it can have major implications.
What we do know is that far-red light promotes fruit growth through dry mass partitioning. But we have to be careful because it does this at the expense of the leaf development. (Ji, Yongran, et al.). However, by the looks of the research results, as long as this is only happening in the flowering cycle, there shouldn’t be much of an issue.
One explanation as to how this happens is that FR light increases fruit sink strength in crops like tomatoes. Unfortunately, measuring sink strength in vegetables is not easy to do, making it difficult to confirm the significance of this effect.
|Tomatoes||“R radiation significantly increased the fraction of dry mass partitioned to fruits and stems at the expense of that partitioned to leaves (Fig. 2). Also, FR radiation increased the dry mass of individual ripe fruits (Table 2). ”
|Ji, Yongran et al. “Far-red radiation stimulates dry mass partitioning to fruits by increasing fruit sink strength in tomato.” The New phytologist, 10.1111/nph.16805. 11 Jul. (2020).|
When it comes to supplementing far-red in flowering, there is little doubt about the wealth of benefits it provides. In fact, the closer we can mimic the varying spectrum of light plants receive throughout the day, the better they appear to grow under sole-source lighting. This means we want additional deep red in the flowering cycle as well, so we can wake our plants up faster for a longer photoperiod.
Both the commercial and home gardener will need to weigh the additional cost of far-red lighting, predominately delivered through individual lighting pucks at the moment. If you grow shade-tolerate plants, run perpetual gardens, want increase yields, or are looking to give your plants an indoor environment that is as close to the outdoors as possible, upgrading your lights with far-red should be a priority. If you prefer quality over quantity, right now, there is no need to rush, and you may wish to look into supplementing in additional blue light before.
All-in-all, the grow lamps of tomorrow will do well by providing the gardener the ability to individually control how much far-red/deep red their plants are receiving. This is especially true for the flowering cycle, though, manipulation of plants in the vegetative cycle with far-red can offer advantages as well.
Don’t forget to check out our article on the importance of blue light during the vegetative growth cycle.
McCree, K. J. (1971-01-01). “The action spectrum, absorptance and quantum yield of photosynthesis in crop plants”. Agricultural Meteorology. 9: 191–216. doi:10.1016/0002-1571(71)90022-7
CR Brodersen and TC Vogelmann (2010) Do changes in light direction affect absorption profiles in leaves? Funct Plant Biol 37: 403–412
Piskurewicz, Urszula et al. “Far-red light inhibits germination through DELLA-dependent stimulation of ABA synthesis and ABI3 activity.” The EMBO journal vol. 28,15 (2009): 2259-71. doi:10.1038/emboj.2009.170
Alokam S, Chinnappa CC, and Reid DM. “Red/far-red light mediated stem elongation and anthocyanin accumulation in Stellaria longipes: differential response of alpine and prairie ecotypes” Can J Bot 80: 72-81. (202).
Meng, Q and Runkle, E. “Far-red is the New Red.” Michigan State University. https://www.canr.msu.edu/floriculture/uploads/files/far-red-on-lettuce.pdf
Park, Yu Jin and E. Runkle. “Far-red radiation promotes growth of seedlings by increasing leaf expansion and whole-plant net assimilation.” Environmental and Experimental Botany 136 (2017): 41-49. – https://www.sciencedirect.com/science/article/abs/pii/S0098847216302738
Takimoto, A, and K C Hamner. “Effect of Far-Red Light and its Interaction with Red Light in the Photoperiodic Response of Pharbitis nil.” Plant physiology vol. 40,5 (1965): 859-64. doi:10.1104/pp.40.5.859 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC550395/
Zhen S and Bugbee B (2020) Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO2 Fixation and Increased Photon Capture During Long-Term Studies: Implications for Re-Defining PAR. Front. Plant Sci. 11:581156. doi: 10.3389/fpls.2020.581156 – https://www.frontiersin.org/articles/10.3389/fpls.2020.581156/full
Zhen, Shuyang, and Marc W van Iersel. “Far-red light is needed for efficient photochemistry and photosynthesis.” Journal of plant physiology vol. 209 (2017): 115-122. doi:10.1016/j.jplph.2016.12.004
Kalaitzoglou P, van Ieperen W, Harbinson J, van der Meer M, Martinakos S, Weerheim K, Nicole CCS and Marcelis LFM (2019) Effects of Continuous or End-of-Day Far-Red Light on Tomato Plant Growth, Morphology, Light Absorption, and Fruit Production. Front. Plant Sci. 10:322. doi: 10.3389/fpls.2019.00322 – https://pubmed.ncbi.nlm.nih.gov/30984211/
Ji, Yongran et al. “Far-red radiation stimulates dry mass partitioning to fruits by increasing fruit sink strength in tomato.” The New phytologist, 10.1111/nph.16805. 11 Jul. 2020, doi:10.1111/nph.16805 – https://pubmed.ncbi.nlm.nih.gov/32654143/