Horticulture lighting technologies have improved dramatically over the past century, but manipulation of light spectrum is a fairly new concept. Since plants tend to absorb red and blue light most strongly, other wavelengths have been regarded as unnecessary for plant growth and development. As LED technology progressed, the ability to provide individual spectra did as well, and pink/purple light fixtures flooded the horticulture lighting market. Regardless of this influx of products, research on plant lighting continued to explore the many interactions plants have with light and eventually began to discourage the idea that plants only needed two individual spectra for optimal growth. While there is still much discrepancy over the primary function of light in the 550 nm to 600 nm range as well as in the far-red and ultraviolet wavebands, research has shown many functions and even benefits of incorporating these spectra. In order to help you understand how plants perceive light, this article delves into the many known receptors and pigments plants use to sense and respond to their environment.
Figure 1: Photosynthetic light response curves
How Plants Use Light
Human and plant light perception use many of the same molecules, however, our eyes are more easily fooled. Narrow band red, blue, and green light when mixed in the right proportion are perceived as white light to the human eye. However, a plant is quite aware of the fact that it is receiving three individual spectra and its growth habit will demonstrate that perception. Broad spectrum white light can come in many forms depending on the source. To the human eye, we mostly perceive these different spectra as cooler (blue) or warmer (orange/red) depending if it is a metal halide or high-pressure sodium lamp, or in the case of LEDs and fluorescent bulbs, what type of phosphor coating is used. To a plant however, each individual wavelength may promote a different growth habit and photomorphogenic response.
Incoming photons are absorbed by pigments, which absorb light as energy, and photoreceptors which perceive light as a signal. When absorbed by the most known pigment, chlorophyll, photons can be used to drive photosynthesis and growth. However, chlorophyll alone with its wide-ranging absorption spectrum is not enough to efficiently harvest light. The “antenna complex” is a concept that describes how accessory pigments such as carotenoids can assist both in capturing light that chlorophyll does not absorb, or dissipating excess light as heat (non-photochemical quenching) when photosynthetic reaction centers are overloaded with incoming energy.
Accessory pigments are primarily carotenoids such as beta-carotene, lutein, zeaxanthin, antheraxanthin, and violaxanthin. These pigments are yellow to orange in color and absorb most strongly in the range of 450 nm to 550 nm. Some of these pigments change forms based on lighting conditions through processes called epoxidation and de-epoxidation. If fluence is too high, damage can occur to the photosynthetic apparatus, so it is important for a plant to be able to deal with this incoming energy. Under pure sunlight where fluence may fluctuate throughout the day, the antenna complex adjusts to accept or dissipate light. When fluence is low, violaxanthin will capture photons and transfer this energy to chlorophyll, improving efficiency of light absorption. When fluence is high, violaxanthin is de-epoxidated (converted) into zeaxanthin which then dissipates excess photons as heat. Beta-carotene functions similarly to violaxanthin and lutein functions like zeaxanthin but without this interconversion process called the “Xanthophyll Cycle.” This flow of energy between pigments occurs spontaneously as they become “excited” by photons. Interestingly, carotenoids that protect plants from light and improve their ability to capture light can also serve similar functions within the eyes of many animal species. There are several other plant pigments unassociated with the photosynthetic light-harvesting complex including anthocyanin and lycopene. Though these compounds do absorb light, their main function is to protect cells and DNA from damaging UV radiation as well as scavenge “free radicals” such as hydrogen peroxide, preventing further cellular damage.
Photoreceptors in most cases are proteins paired with a “chromophore” that absorbs certain wavelengths of light and then sends a signal to the plant that influences photomorphogenesis. There are several different types of photoreceptors and their light absorption ranges overlap. Cryptochromes use light in the range of 300 nm to 500 nm though it most strongly absorbs at 350 nm (UV-A) and 450 nm (blue). This receptor, when excited by light, prevents elongation of hypocotyls (main stem of seedlings) and even mediates flowering and photoperiod in some species. Phototropins are also blue/UV-A absorbing photoreceptors but with a much stronger absorption peak at 450 nm and are thought to regulate phototropism (process in which plants move in response to light), stomatal aperture (opening and closing), movement of chloroplasts (photosynthetic organelles) within leaf cells, and inhibition of leaf expansion. Phytochromes are some of the more famous photoreceptors as they can strongly influence flowering. It is a little known fact that phytochromes actually absorb light in the range of 300 nm to 800 nm. Most of the known functions however, are a result of the absorption peaks at 660 nm in the Pr form and 730 nm in the Pfr form. Phytochromes are constantly changing form and reach a “photoequilibrium” (more information available in the “Guide to Photomorphogenesis”) that is regulated by spectral ratio and PPFD present in the growing environment. Depending on the photoequilibrium of phytochrome, different signals are sent to metabolic pathways within the plant that regulate many processes including germination, seedling establishment, stem elongation, leaf expansion, and of course flowering and photoperiod. Different ratios of R:FR light received by a plant will dictate how the plant develops in terms of compactness, flower size, flower number, etc. There are several other newly discovered and under-researched photoreceptors (UV-B receptor correlated with anthocyanin accumulation) but these will not be discussed in this article.
Since there is much overlap in the absorption spectra of these photoreceptors, most photomorphogenic responses are co-regulated. Some responses may be turned on and off by one receptor, but the expression of that response can be amplified by another receptor. The enigmatic “circadian clock” that regulates so many functions within plants is a culmination of activity from multiple photoreceptors entraining a rhythm of growth patterns based on photoperiod, light spectrum, and PPFD. This rhythm of growth patterns within a plant strongly influences photomorphogenic outcomes however, just like photosynthesis, there is an action spectrum for all photomorphogenic responses that is dictated by a mixture of signals from these photoreceptors and does not necessarily mirror the absorption spectrum.
Supplementing Specific Wavelengths Vs Broad Spectrum
When considering using narrow band lighting, the most important things to consider are whether or not your plants are already exposed to broad spectrum light (solar for greenhouses, or a broad-spectrum fixture for sole-source lighting applications) and which crops you are growing. When plants are already exposed to broad spectrum lighting from a sole-source fixture, it makes sense to supplement with narrow band lighting only if there is a desired photomorphogenic effect that your crop can’t achieve without being exposed to a specific waveband. However, if you are growing under solar radiation and a high DLI, your crop may not be as responsive to changes in light spectrum, as solar radiation is quite broad already and may drown out the photomorphogenic benefits of narrow band lighting. Another aspect to consider when growing under solar radiation is whether or not you need to increase your DLI. If you supplement with a narrow band fixture as a method of increasing DLI, you may see inconsistencies in product quality as solar radiation increases and decreases throughout the year exposing your crops to differing amounts of sunlight and narrow band light. If your DLI is constant and you only wish to induce a photomorphogenic response such as coloring, compactness, or rooting, it may make sense for you to supplement more blue light. However, DLI often has more of an impact on desired traits than spectrum. If you are growing a flowering/fruiting crop and only wish to encourage more flower/fruit growth (with a sufficient DLI and photoperiod) it may be beneficial to supplement more red light, as 660 nm light encourages phytochrome responses in many species, which sends signals throughout the plant to encourage reproductive growth.
Narrow band lighting can provide acceptable growth for many species, so long as there are no fluctuations in DLI independent of the portion supplied by the light fixture. However, plants use several different photoreceptors and pigments that cooperatively regulate growth and development. Plants developed these photomorphogenic responses under broad spectrum light and it is very rare for a certain species to express a response to narrow band lighting that cannot also be achieved by broad spectrum lighting given sufficient DLI. For consistent product quality and capability to produce a broad array of crop species without complications due to lighting, broad spectrum fixtures are a safer choice. Different species can have varying responses to changes in light spectrum. Research is constantly underway that helps us understand how individual crops respond to different light spectra, and in some cases there is clear evidence about what type of lighting is best for a crop. If you are uncertain about the response your crop may have, supplementing with broad spectrum light has a proven record of improving crop quality, consistency, and yield.