The role grow lights play in an indoor garden is an important one, one that horticultural researchers are constantly considering and improving. It all has to do with the photosynthesis process, during which plants use water, carbon dioxide and light as the source of energy to produce glucose and oxygen.
In the past, greenhouse growers used either natural sunlight, high pressure sodium (HPS) or fluorescent lamps to illuminate their crops, but there were certain disadvantages to using these light sources.
For example, natural sunlight is only available during the daytime; and HPS lights consume a lot of energy and run at high temperatures, which prevents them from being placed close to plants. Several types of grow lights also contain mercury, which can be problematic upon disposal.
The development of light-emitting diodes (LEDs) in the last few decades has introduced growers to a new source of lighting that provides many superior advantages.
First of all, plants need wavelengths in the visible region (400-700 nm) in varying proportions. Photosynthetic photon flux (PPF) designates the intensity of visible spectral radiation, which plants use in the photosynthesis process.
Plants use more red and blue light for photosynthesis than they do green, and the absorption spectrum of plants can effectively be matched by using the right combination of LEDs.
LEDs as an illumination source in an indoor garden are much more suitable than other grow lights whose peak emissions widely differ from the absorption spectrum of plants.
LEDs allow growers to pick the spectrums of light they want, rather than relying on whatever colors the phosphors happen to make, or what color sodium glows at when it gets really warm.
Some wavelengths of interest for growers using LEDs, within the 400-700 nm range applicable to plants growth, are:
- 439 nm is the blue absorption peak of chlorophyll a.
- 450-460 nm is the royal blue that is absorbed by one of the peaks in beta-carotene. It is a readily available LED wavelength commonly used to excite the remote-phosphor in white LED lamps.
- 469 nm is the blue absorption peak of chlorophyll b.
- 430-470 nm is a range that is important for the absorption of chlorophyll a and b, which is key for vegetative growth.
- 480-485 nm is the second absorption peak of beta-carotene.
- 525 nm (green light) is a phototropic activator that researchers are still trying to find the chromophore of. Green light isn’t important for photosynthesis, but it is apparent that plants are gaining direction and environmental signals from it, and that it affects internodal spacing. This is also the wavelength of GaN or InGaN green LEDs commonly used in RGB and tunable applications.
- 590 nm is key for carotenoid absorption. Carotenoids are starch-storing, structural and nutritional compounds.
- 590 nm is additionally the phycoerythrin absorption wavelength. Phycoerythrin is a red protein-pigment complex from the light-harvesting phycobiliprotein family, present in red algae and cryptophytes, and is an accessory pigment to the main chlorophyll pigments responsible for photosynthesis.
- 625 nm is the phycocyanin absorption peak. Phycocyanin is a pigment-protein complex from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is also an accessory pigment to chlorophyll.
- 642-645 nm is the peak absorption point of chlorophyll b.
- 660 nm is often called the super-red LED wavelength and is important for flowering.
- 666-667 nm is the peak red absorption point for chlorophyll a.
- 700 nm light is to be avoided. It confuses the phytochrome recycling systems in green plants. Phytochromes are photoreceptors that are sensitive to light in the red and far-red region of the visible spectrum.
- 730 nm, often referred to as far-red, is important for phytochrome recycling. It is needed for all kinds of morphogenic (shape-forming) processes. A few minutes of 730 nm light treatment after the full light cycle is over will revert the phytochrome chromophore from activated to inactive. This resets the chemistry for another lights-on cycle and may be useful in shortening the classic dark side of the photoperiod. This color is important to plants but is not considered in PPF as it is outside of the 400-700nm PPF range.
LEDs provide growers the unique opportunity to use a light spectrum that can be tailored to provide maximum benefit to the plants and minimize wasted energy. Several LEDs at different wavelengths can be combined to provide an ideal illumination source that follows the plant-sensitivity curve. Aside from this, there are several other advantages of using LEDs in horticulture, including:
- Geometry: Since radiation falling on a plant is inversely proportional to the square distance between the source of radiation and the plant, it is advantageous to bring plants closer to the light source. LED lights can be placed closer to plants than is possible with other lamps because LEDs run cooler than other lights that produce a lot of heat and will burn leaves at close distances.
- Efficiency: The electrical efficiency of LEDs is much higher than other grow lights, which helps growers save on their electrical bills.
- Durability: The lifetime of an LED is defined as how long it takes for it to drop to 70% of its original value. This is about 50,000 hours—much longer than the typical lifespan of fluorescent or HPS lights.
- Spectral quality: Spectral quality of a carefully chosen LED illumination source can have dramatic effects on plant anatomy, morphology and pathogen development.
- Small size: The small, compact size of today’s LED fixtures allows more options for installing the light source, and more space for plants to grow.
Several researchers have experimented with using different intensities and wavelengths to grow different crops. It is important to understand that different crops may behave differently under different illumination levels, and different light recipes may be needed for each crop, but overall, an increased PPFD causes an increase in plant growth.
Although red light is sufficient for plant growth, blue light is important for increased leaf thickness and number of chloroplasts. For example, rice plants grown under a combination of blue and red LEDs showed higher photosynthetic rates than those grown under red illumination alone.
It’s also worth noting that although a combination of red and blue LEDs is useful for better crop growth, the presence of both these colors in a growroom makes it difficult to observe plants visually and check for disease symptoms.
The addition of a few single green light bulbs, although not as essential for plant growth as blue or red as mentioned earlier, makes it easier to visually assess the plants for damages.
Along with the benefits of growing with LEDs, another important issue for researchers is the development of metrics for quantifying PPFD and light absorption by crops.
Growers need to calibrate their LED light sources and find the optimal light recipe as far as flux efficacy, appropriate wavelengths for different crops and optimal geometry of illumination is concerned.
There are affordable spectrometers and PAR meters that can be used to measure light output and intensity.
Smartphone apps that are used in conjunction with a phone’s camera are also being developed that will help give growers a rough idea of how intense their lights are. The PPFD measurement is simply done by pointing the device at the light source and pressing a button.
Some spectrometers are also being designed to work in conjunction with smartphones to provide even more accurate readings. The software behind these apps also records data on a day-to-day basis, and monitors the growth of the plants.
The resulting plant journals, or logs, will help growers closely monitor what’s going on in their growroom, and what the best course of action should be if things go sideways or otherwise require attention.