LED Spectrum Effects in Horticultural Trials

Sections:

  1. The Role of Horticultural LEDs
  2. Product and Purpose
  3. Empirical Methods and LED Trial Configurations

The Role of Horticultural LEDs

Whether you are a commercial greenhouse operator, small-scale market farmer, or home gardener, you already know the importance of light availability and quality for successful plant production. From precarious winter windowsills to vast fields of hoop houses to multiple levels of indoor hydroponic facilities, the need for light control is paramount.

Any of the established grow light technologies will suffice, and sunlight certainly isn’t obsolete. HID (HPS and MH) and fluorescent lights will do the trick, and they offer control over intensity and coverage. LEDs, though, give growers greater flexibility in spectrum manipulation, and this opens up new options for controlling plant morphology and chemistry. They also improve efficiency, quality, timing, and production flexibility.

<<< Note that in all cases, performance and testing is very much specific to an individual plant species. Plants find every possible lighting environment in nature to thrive in. Growing conditions in controlled lighting trials should generate data for a single species unless it is the growing condition that remains static while the data collection concerns mixed or layered plantings.

Here’s a quick guide to some present and future LED spectrum concepts, and a sampling of related Digi-key products.

White Horticultural LEDs

These are designed to be general purpose lights suitable for many, but not all, continuous growing environments. They are also used to provide a base spectrum which can be enhanced intermittently with other wavelengths. A white LED module can still be physically positioned to change intensity, coverage, and frequency of exposure, but it doesn’t offer spectrum-specific control unless it’s combined with other colors. These white LEDs may be listed Cool White, Neutral White, etc., but it’s best to find a graph titled ‘spectral distribution’, ‘spectral power distribution’, or anything similar that shows the relationship between power and wavelength. An example for a LUXEON SunPlus 2835 series is shown below.

Luxeon SunPlus 2835 Series Spectral Power Distribution

Trial variations are limited for white horticultural LEDs, but changing the intensity and duration of this lighting can control growth speeds or growth shapes (morphology), and scheduled flowering. Also, white LEDs can provide a baseline for one group of plants while others receiving the same levels of water and nutrients undergo some type of specific wavelength treatment. If nothing else, trials can be set up for different brands of white LEDs within the same configuration.

Here is a list of white horticultural LED components: [click here https://www.digikey.com/short/z9709q ]. This category can be found at Product Index > Optoelectronics > Led Lighting - White

Here are some white horticultural LED strips or modules: [click here https://www.digikey.com/short/z970h9 ]. This category can be found at Product Index > Optoelectronics > LED Lighting -COBs, Engines, Modules, Strips.

Far-Red Horticultural LEDs

The wavelength specified for ‘Far Red’ (FR) will vary depending on the topic. It may be shown as 700nm to 850nm on a generic spectrum graph, but it may be listed as 700nm to 800nm or a similar range when looking at documents for a certain brand or series of horticultural LEDs. It’s best to find a wavelength chart for an individual product to see an accurate depiction of its power vs. wavelength spectrum.

FR light will often be referred to as “shade light”, and it is most associated with morphology and flowering. A common plant response to FR signals is extended growth (elongation) and increased leaf size. As noted earlier, not all plants will react the same, but many see FR as shade and will try to grow higher to reach sunlight, or they may produce larger leaves to capture more light. Some plants also use FR light as a signal to begin their flowering stage.

There are many trial options involving FR LEDs. It can be used in combination with other wavelengths in continuous or intermittent applications to compare plant morphology, flowering behavior, and coloration. There may be points in production where elongation is desired, larger leaves are required for aesthetics, and flowering needs to follow a fixed schedule (holidays, for example). Plants relying mostly on natural sunlight may benefit from calculated exposures to FR light to achieve desired effects. Be sure to keep a base group that doesn’t receive FR light emissions, or divide the plants into separate test groups according to the length, intensity, or timing of exposure.

FR LED products were not well represented on our site at the time of this post, but many manufacturers list them as options within a series. Find the orderable part number by looking at the series datasheet or the supplier home site. Below is an example from the aforementioned LUXEON SunPlus 2835 Line where an LED is offered in the 720nm to 740nm range.

Datasheet: [click here https://www.lumileds.com/wp-content/uploads/files/DS237-luxeon-sunplus-2835-line-datasheet.pdf ]

Here is a short list of far-red horticultural LED components: [click here https://www.digikey.com/short/z939qj ]. This category can be found at Product Index > Optoelctronics > LED Lighting - Color.

Here is a link to a far-red horticultural LED strip or module: [click here https://www.digikey.com/short/z939zp ]. This category can be found at Product Index > Optoelectronics > LED Lighting -COBs, Engines, Modules, Strips.

Red Horticultural LEDs

Generally, red LED wavelengths are listed in the full range from 600nm to 700nm. This may be split between Red and Deep Red. An example is shown below for OSRAM LED Engin LuxiGen products. A peak wavelength is often listed in product documentation.

OSRAM LED Engin LuxiGen Red and Deep Red

Source: [click here https://www.osram.us/ledengin/light-for/horticulture/index.jsp ].

This range of red light affects plant growth in ways that are similar to FR, but it is considered much more efficient at stimulating photosynthesis than FR. In graphic form, this is usually shown as a ‘relative quantum efficiency curve’, and the purpose is to show which wavelengths are most readily used by plants. It can’t be said often enough, though, that not all plants fit neatly along that line. There are multiple structures within plants that are capable of enabling the photosynthetic process, and from the varying degrees of concentration and placement within the plant, different wavelengths and intensities are utilized at different efficiencies.

Trials with red LEDs can be similar to those of FR LEDs involving morphology and flowering, but they can also be combined with other wavelengths—notably blue or the white horticultural range—to study growth rates, plant volumes, and to some degree, internal plant chemistry. There is a direct relationship between red and FR involving phytochromes (photoreceptors), but it’s a topic best left to a separate article. This relationship can be an important trial subject, though, if the grower has the means to test and monitor the presence of active (Pfr) and inactive (Pr) forms of these phytochromes.

Here is a link to various red and deep-red horticultural LED components: [click here https://www.digikey.com/short/z97vtb ]. This category can be found at Product Index > Optoelctronics > LED Lighting - Color.

Here is a link to a few red horticultural LED strips or modules: [click here https://www.digikey.com/short/z97v1f ]. This category can be found at Product Index > Optoelectronics > LED Lighting -COBs, Engines, Modules, Strips.

<<< Note that many existing horticultural LED modules will already contain a mix of red and blue LEDs. Try to find the individual and overall specs to see if it will work for any given application or trial.

Blue Horticultural LEDs

Blue light will generally be listed in the range of 400nm to 500nm, and blue horticultural LEDs will often have a typical value near 450nm with a range above and below. Like the red wavelength, it is a primary driver of photosynthetic reactions, but not to the same efficiency. For many plants, a portion of the higher energy of blue light is lost. Blue light is often used as a control factor with varying levels of intensity applied. It has a strong influence on morphology, especially in conditions where 100% artificial light is used. It has little to no effect as a supplement to normal levels of outdoor greenhouse light.

Along with morphology (shorter stems and smaller leaves, for example), blue horticultural wavelengths can strongly affect plant chemistry and color, both of which are important aspects of saleable plant products. It has a regulatory role in controlling the stomata (leaf openings), and thus affects water loss and gas exchange.

Trials with blue light are often conducted with ‘mix rates’ where the percentage of blue light compared to other wavelengths is known in each test group. Equally valid options involve intensity and duration, but aside from visible effects, measuring chemical changes requires specific testing capabilities. Intensity levels can be adjusted to promote flowering or suppress flowering in long-day plants and short-day plants, respectively. As always, studying the effects on one plant species under various conditions is preferable unless the point is to find which crops respond best over time to a fixed configuration.

Here is a list of blue horticultural LED components: [click here https://www.digikey.com/short/z9t3hc ]. This category can be found at Product Index > Optoelctronics > LED Lighting - Color.

Here are some blue horticultural LED strips or modules: [click here https://www.digikey.com/short/z9t3rm ]. This category can be found at Product Index > Optoelectronics > LED Lighting -COBs, Engines, Modules, Strips.

<<< Note that many existing horticultural LED modules will already contain a mix of red and blue LEDs. Try to find the individual and overall specs to see if it will work for any given application or trial.

Green Horticultural LEDs

An often used, but inaccurate, statement regarding green wavelengths and plants is that, “Leaves don’t use that color, so that’s why they appear green.” With a general wavelength range between 500nm to 600nm, green light presents an opportunity for energy input that plants cannot ignore. It is simply not used as efficiently or in the same manner.

Depending on the species, plants reflect a significant portion of green light, but some is captured by chloroplasts further within a leaf structure or further down in the overall canopy of leaves. When chlorophyll reaches a saturation point from red and blue wavelengths, chloroplasts in these non-primary locations can still utilize this green wavelength of light that has passed through the upper leaf layer or canopy, and this contributes to greater photosynthetic efficiency. Green light also has a role in regulating plant stomata.

Trials using green horticultural LEDs mostly use them in a supplemental role, so a base group of LEDs using the full photosynthetically active range without green could be compared to another with varying levels of intensity and duration of green wavelength exposure. It isn’t commonly used in outdoor greenhouse operations that utilize natural light, but a placement methodology using a layered crop canopy presents an opportunity to test green light effects on the lower levels.

Horticultural light trials have been conducted for decades, but LED usage is relatively new, and therefore so is the testing. Look for new information regarding applications of green wavelengths to specific crops. The following article addressing Lumileds LUXEON SunPlus series lime LEDs is an intriguing example. [click here https://www.lumileds.com/wp-content/uploads/files/WP34.pdf ].

Here is a short list of green (lime) horticultural LED components: [click here https://www.digikey.com/short/z9tjvn ]. This category can be found at Product Index > Optoelctronics > LED Lighting - Color.

Ultraviolet (UV) Horticultural LEDs

UV light is outside the photosynthetic active radiation (PAR) range of 400nm to 700nm, but it has a significant role in plant development. UV radiation can be harmful to plants in the same way that it can damage human tissue. However, some effects are beneficial, and others are useful within the production goals for a specific plant species.

Plant chemistry may be altered by UV exposure, so if the purpose of the cultured plant is to harvest a compound through distillation, for example, an increase in the level of that compound due to UV radiation is more important than the physical mass of the crop. UV light can also illicit a color response in plants, and this can be essential for both landscaping and food crop aesthetics as well as the essential nutrient levels within some food market plants.

UV light can also affect plant morphology, disease susceptibility (especially fungal agents), and harmful insect damage. UV exposure can also be used to condition plants grown in greenhouses since the sudden exposure to natural sunlight can cause damage once the transition to an outdoor planting is made. Many greenhouse materials block UV light, and some indoor growing facilities may not provide any UV light at all, so there are possible benefits to using UV LEDs.

There are currently no UV products listed as ‘horticultural’ on the Digi-key site, but they can be found in a few categories under Product Index > Optoelectronics.

Empirical Methods and LED Trial Configurations

It is important to follow some guidelines when conducting horticultural LED trials. This generally falls under the label of empirical testing. In other words, the experiment should not be conducted with the goal of proving a preconceived idea. It’s ok to start with a hypothesis, but the tests are done to produce data or observations from which conclusions can be made. These may or may not support the original theory.

The trial should not be conducted in a manner which produces results based on incomplete or manipulated data. This isn’t the same as cheating. For example, an experiment where there was no comparison group, or where conditions across test groups were not consistently controlled, would produce invalid results. Another example is letting human nature interfere. It’s easy to think along the lines of, “Oh, I see that I should have set that up differently, so I’ll just change it before the test is over.” Those results are also invalid.

With that in mind, here are some basic scenarios to use when testing horticultural LEDs.

  • Use a base group for comparison, or use a preset standard or database which can be used to measure the results. One group of plants could receive natural light or a white LED source, and other groups could receive a different wavelength or combination of wavelengths. In this scenario, all plant species should be the same, and other conditions (water, fertilizer, etc.) should also be equal.

  • A variation of this base group concept is to use a fixed set of LEDs for all the plants, but change the other inputs across the test groups. Similarly, use the fixed set of LEDs, but try different plant species for each test group. In both of these cases, the LEDs are the base which doesn’t change. This is useful when looking for crops that work best when the lighting can’t be altered.

  • Repeat the trials. Good experiments can, and should, be replicated to establish consistency or provide a large enough data set to analyze. Over time, repetition also removes any errors in the empirical testing. For example, if trial numbers 01 through 10 had consistent results except for number 06, the experiment might still be valid, but it’s also a reason to check the setup. Maybe one or more light fixtures had the wrong LEDs in trial number 06.

  • When the LEDs are the focus of the trial, separate each test run or test group by a single feature if possible. Some of those features or qualities were noted earlier. They include wavelength, intensity, positioning (coverage), ratios, brands, timing, etc. Try to keep the number of varying features down for each test group so that results are easier to observe and analyze. Otherwise, it’s difficult to understand cause and effect.

Given the number of trial variables that are available, it’s easy to see that there is no limit or restriction on innovation. Research involving horticultural LEDs is very fresh, and each new study shows that this is a very dynamic field of study. It is a technology that can both work on its own or be adapted to established methods. Don’t be left out in the dark. Grab a light and go!

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