Light is the single most important environmental variable concerning plant growth. Plants are autotrophs that evolved the ability to use light energy from the sun to produce a food source through the process of photosynthesis. The process is quite complex, and for the sake of this guide, we can use this simplified definition of photosynthesis: using light energy to split water (H 2 O) and fix carbon dioxide (CO 2 ) to form carbohydrates (CH 2 O) and oxygen (O 2 ) ( Figure 1 ). The purpose of this guide is not to discuss the biochemical light reactions of photosynthesis, but to discuss the different properties of light and their influence on photosynthesis. Quality (spectrum), quantity (intensity), and duration (photoperiod) are separate but related light properties that influence photosynthesis. The science behind each property will be defined, then the application of the properties will be discussed in regards to horticulture lighting systems
Spectral Light Quality
Photosynthetically active radiation (PAR) is the major driver of photosynthesis in plants. However, not all wavelengths of light are equally efficient at driving photosynthesis. Two plant scientists by the names of Dr. McCree and Dr. Inada performed several studies in the 1970s to determine the influence of light spectra on photosynthesis. This research resulted in the creation of a photosynthetic response curve that is now appropriately known as the McCree curve. If you refer to the McCree curve ( Figure 2 ), you will notice that red light (600-700 nm) is almost twice as effective as blue light (400-500 nm) at driving photosynthesis, with green (500-600 nm) light in between the two. Prior to this research, it was a common misconception that since chlorophyll absorbs light mainly in the red and blue parts of the visible light spectrum (leading to the green color of plant leaves) that green light was not used by plants for photosynthesis. However, precise and independent measurements of photosynthetic activity under different wavelengths by McCree and Inada demonstrated that light in the green spectrum (500 â 600 nm) is nearly as effective as blue light for a considerable number of plant species. The short explanation for this experimental fact is that higher plants have evolved both
biochemical and biophysical solutions (e.g. accessory pigments) to utilize green light. These accessory pigments (mainly carotenoids) can be thought of as storage molecules for photons that are not directly absorbed by chlorophyll.
Spectral light quality is a key component that goes into the design of horticulture lighting systems, especially in sole-source (absence of sunlight) lighting applications. Traditional high-intensity discharge (HID) lighting systems (high-pressure sodium and metal halide) have always had a limitation when it came to modifying the spectral light quality. Using light-emitting diodes (LEDs) for horticulture lighting systems allow manufacturers the ability to create custom spectral light qualities along with many other advantages over conventional lighting systems, including: high photoelectric conversion efficiencies, low thermal output, and adjustable light intensities. Light quality not only influences photosynthesis, but it also influences the morphology of plants, which is known as photomorphogenesis.
Light Intensity
The number of photons that are absorbed by specialized photoreceptors known as chloroplasts directly influences the rate of photosynthesis. As light intensity (PPFD) increases, so does the rate of photosynthesis, until a saturation point is reached. Every plant species has a different light saturation point where photosynthetic levels plateau based on the light environment they evolved in. Light saturation occurs at much lower intensities in plants that evolved in shade conditions than those that evolved under full sun conditions. However, light saturation normally occurs (especially in sun plants) when some other factor (normally CO2) is limited (Figure 3). Another important consideration regarding light intensity is known as the light compensation point. Plants have a minimum light intensity required to promote maintenance growth to keep plants alive. As you would expect, the light compensation point occurs at higher light intensities for sun plants than shade plants. Providing adequate light intensities with the correct spectral light quality is critical to promote new plant growth.
Horticulture lighting systems can be used in two ways to increase light intensities to promote photosynthesis. Supplemental light can be provided by lighting systems in greenhouse environments, generally during light limiting conditions (eg. winter months in northern latitudes, cloudy conditions, or a combination of the two), or as a sole-source of photosynthetic light in an indoor controlled environment (eg. growth chamber, warehouse, grow tent, etc.) where sunlight is not being used as a source for photosynthesis. One of the key benefits of using LEDs for either application is the low thermal output at the surface of the diode. Achieving very high PPFD with HID lights has always been limited by the distance that the lamps need to be kept from the crop canopy, since these lamps emit a high percentage of energy as infrared (IR) light. IR light is not photosynthetically active and significantly increases plant temperatures, therefore, one method to mitigate this response is to increase the distance between the light and the crop canopy (which results in decreased light intensities and limits the environments they can be used in to facilities with tall ceilings). With proper thermal management, LEDs dissipate most of their heat from the back side of the diode, therefore, lighting fixtures can be placed at much closer distances from the crop canopy, allowing very high PPFD levels (℠1000 ”mol/m 2 /s) to be provided to plants.
Light Duration
Light duration (photoperiod) is the length of time a plant is exposed to light during a 24-hour period. The length of a photoperiod can influence the overall light intensity that a plant receives in 24 hours, which in turn influences overall growth. This is described as daily light integral (DLI), which is defined as the cumulative PPFD delivered during 24 hours, and is expressed in mol/m2/d. Photoperiod also influences the transition from vegetative to reproductive growth in several plant species. However, it is actually the dark period (skotoperiod) and not the photoperiod that determines when certain species will transition to reproductive growth. The photoreceptor phytochrome is mainly responsible for signaling the transition to reproductive growth in photoperiodic crops (To learn more about this click here to read our photomorphogenesis guide). Short day (long night) plants flower when the phytochrome perceives an uninterrupted long night (generally ℠12 hours). Long day (short night) plants flower during short nights (generally †12 hours). Alternatively, several plant species are day neutral, where photoperiod does not influence flowering.
Horticulture lighting systems can be used to provide photoperiodic light to extend the day length to either promote flowering of long-day plants or suppress flowering of short day plants regardless of the season or climate. Traditionally, HID, incandescent, or fluorescent lights have been used to provide photoperiodic lighting in greenhouses. However, these technologies are relatively inefficient at converting electrical energy into PAR. Fluence lighting systems convert electrical energy into PAR more efficiently than these lighting technologies. See this publication from Utah State University for more information.
Conclusion
The work of Drs. McCree and Inada was fundamental in understanding the influence of spectral light quality on photosynthesis; however, the study of photobiology is still in its infancy and rapid advances in LED technologies have allowed researchers the ability to expand on the work of previous plant scientists. There is still a great deal of work to be done in the study of photobiology, and Fluence Bioengineering is working with world-renowned plant scientists and commercial growers to continue to explore the interaction between life and light.
We have known that light is responsible for driving plant growth via photosynthesis for many years; however, the influence of light on plant development has only become well understood in the last century. The color of light (spectral light quality) is not only an important variable for photosynthesis, but also acts as a packet of information to signal light mediated developmental responses in plants, such as: seed germination, stem elongation, and flowering. The term used to describe these responses in plants is photomorphogenesis (photo = light, and morphogenesis = the process that causes an organism to develop its shape). Plant morphology (plant architecture) is extremely important in controlled environment agriculture where vertical or horizontal growing space may be limited. Depending on the plant architecture you desire, there may be other aspects of horticulture lighting systems to consider beyond providing a source of photosynthetic light.
Photosynthetically active radiation (400 â 700 nm) is mainly used for photosynthesis , however, plants can sense wavelengths ranging all the way from UV-C (260 nm) to far-red (730 nm) using separate photoreceptors that are not utilized for photosynthesis. These photoreceptors direct an adaptive response in plants under changing environmental conditions to regulate key stages of plant development which depend strongly on the spectrum of light, and in some cases timing, periodicity, and the overall exposure. The latter is usually called fluence, and is measured in micromoles of photons per square meter of surface. There are very low, low, and high fluence responses, with the corresponding sufficient light levels ranging from those of star light (for very low) to direct sunlight (for high). The purpose of this article is to describe photomorphogenic responses in plants to help you consider
Red And Far-Red Light Responses
When it comes to photomorphogenesis, the most understood developmental processes are those controlled by red and far red light (for the purposes of this discussion, we will refer to red Âź light as the spectral region around 660 nm and far red (FR) light around 730 nm). In order to better understand the influence that these two spectral regions have on plant development, you need to first understand the significance of the pigment known as phytochrome, which is responsible for R and FR light mediated responses.
Phytochrome is a pigment protein which exists in two interconvertible forms â a red light absorbing form (P r ) and a far red absorbing form (P fr ). Phytochrome converts from one form to another upon absorbing the corresponding light until an equilibrium is established (phytochrome photoequilibrium), with the relative amount of each form depending primarily on the ratio of R to FR light in the light spectrum. To put this another way, when P r absorbs R light it is converted into P fr , and when P fr absorbs FR light it is converted into P r (There is some overlap in in the spectra of both forms, and phytocrome does absorb some blue light as well, but for the sake of this guide, this will not be discussed). The prevalence of one form or the other (which depends on the R/FR spectral ratio) can stimulate or inhibit a number of developmental processes such as: seed germination, leaf unrolling, chlorophyll formation, and stem elongation. Additionally, phytochrome is the controlling factor of promoting (or suppressing) flowering in photoperiodic plant species. For the sake of brevity, and to discuss important applications related to horticulture lighting systems, we will focus on the influence that phytochrome has on flowering and stem elongation.
Photoperiodism
There are long-day plants (which require short nights to flower) , short-day plants (requiring long nights), and day-neutral plants which have no specific requirement for the photoperiod. This dependence on the photoperiod is referred to as photoperiodism, however, it is actually the length of the dark period (skotoperiod) that regulates flowering of photoperiodic plant species. In the absence of light, P fr slowly converts to P r , and as the skotoperiod increases, so does the relative amount of P r . Long day plants (which have a short skotoperiod) will not flower if P fr converts to P r during the skotoperiod, while short day plants (which have a long skotoperiod) will only flower if P fr converts to P r during the skotoperiod. Photoperiodic phytochrome responses occur in the low fluence range (as low as 1 ”mol/m 2 ), so it can only take a short flash of R light to during a skotoperiod to revert P r back to P fr . For example, flowering of a long-day plant may be induced by night interruption, using a series of short flashes of red light with photon flux levels as low as a few ”moles/m 2 /s. Conversely, short-day plants may be induced to flower by a single flash with pure FR light at the very beginning of the dark photoperiod, after turning off all other lights. This effectively adds a couple of hours to the dark period for the purpose of flowering, which can be used to extend the light period for growth and optimize plant yields as a result. Switching the above methods for plants with opposite photoperiod requirements would delay flowering, which may also be desired sometimes (e.g. to provide the best quality flowers on schedule for certain holidays).
A good energy-saving (and thus, cost-saving) strategy is to use one set of lights for growth and another for photoperiod control when necessary. Since phytochrome response is in the low fluence range, the number of fixtures needed for photoperiod control may be much smaller than that of fixtures needed for growth. In addition, the operating time needed for photoperiod control can be much shorter, such as only minutes at a time. Since FR light is only partially photosynthetically active, its use in horticulture lighting is often limited for reasons of energy efficiency.
Shade Avoidance Response
Another important R and FR photomorphogenic response important to horticulture lighting systems is called the shade avoidance response. Far-red light is transmitted through leaf tissue more so than red light, which causes an enrichment of far-red light, relative to red light, for plants grown under canopies. When a low R:FR ratio is perceived by phytochrome pigments, a shade avoidance response is activated to elongate hypocotyls or stems in an attempt to out-compete neighboring plants. This is very important when it comes to the spectral light quality of horticulture lighting systems. Photoperiodic lights that provide a low R:FR ratio to promote flowing may also induce a shade avoidance response in plants, which may result in an undesirable growth habit (especially if a compact growth habit is preferred).
##Blue Light Responses
Two important blue light photreceptors are cryptochromes and phototropins. Blue light is important for a variety of plant responses such as: suppression of stem elongation, phototropism (bending towards a light source), chloroplast movement within cells, stomatal opening, and activation of gene expression (some of these are morphogenic and others arenât). Stomatal opening and height control are of particular relevance to horticulture lighting systems. A low overall content of blue light in the growth spectrum (e.g. less than 10% of the total photon flux) can lead to leaf edema (swelling of the leaves) and developmental problems in several plant species. The absolute content of blue light has a progressively stronger effect for plant height reduction. This may be desirable in some cases (e.g. to produce more compact seedlings and reduce transportation costs) but generally leads to a lower photosynthetic efficiency of the light with respect to energy consumption. A high relative content of blue light reduces the plant leaf area and may be undesirable for that reason. Near UV light has an effect similar to blue light, with further reduced photosynthetic efficiency, especially below 400 nm (although the other effects may be stronger by comparison). It also affects the biosynthesis of compounds responsible for the flavor of certain fruits, increased anthocyanin concentration, as well as that of other compounds which are not directly produced by photosynthesis alone. Whenever the use of near UV light is necessary to control a corresponding sensory mechanism or the production of a specific molecule of interest by the plant, an overall efficiency trade-off may have to be reached, similarly to that for the use of far red light.
Green Light Responses
The least understood spectrum related to photmorphogenic responses in plants is green light (500 â 600 nm). The control effects of green light are generally opposed to those of red and blue light. For example, green light has been shown to reverse blue light induced plant height reduction and anthocyanin accumulation. The phytochrome and cryptochrome photoreceptors mentioned earlier are also responsive to green light, though to a significantly lesser extent than to red or blue light. So far, all efforts by researchers to find photoreceptors responding primarily to green light have given no definitive results. However, it should be mentioned that the addition of green light into the spectrum of horticulture lighting systems has demonstrated to be beneficial to the growth of several plant species. Similar to far-red light, green light penetrates deeper into leaves and canopies than red or blue light, and can significantly increase the rate of photosynthesis. The addition of green light also significantly improves the color rendering index (CRI) of a horticulture lighting systems, which allows growers to effectively monitor crops for disease or nutrient deficiency/toxicity symptoms, without the use of specialized glasses.
