Light Emitting Diodes (LEDs) have a great potential to become the sole source lighting system for plants and crop production. LED’s small size, long operating lifetime, durability, cool emitting surfaces, wavelength specificity, and linear photon output ability with electrical current input makes them an ideal light source for an indoor plant lighting system.
There has been one challenge in designing the most suitable lighting system i.e. to determine the essential wavelengths for growing plants/crops. NASA’s Kennedy Space Center has been working on normal plant growth requirements, including the required blue light proportion and the optimal red and far-end red wavelength ratio.
To maximize the amount of growth of the plant per amount of energy consumed measured in watts, LED grow lights have to target 6-8 wavelengths of light necessary for photosynthesis. Targeting more wavelengths will result in less efficient grow lights which produce plants that are not as healthy nor are able to produce as much flowers as grow lights which are optimized with 6-8 wavelengths.
Introduction to LED Grow Lights
LEDs are the semiconductors that produce a narrow spectra, non-coherent light when forward voltage is applied through them. There are various types of LEDs that range in wavelengths from ultraviolet to visible light to far red and infrared. The diodes are also available in different packages, ranging from less than 1Watt to 10W chips. The best configurations for LED grow lights are with 3W chips and 8 bands of light.
In 1962 the first visible spectrum LED was created. 1990 lead to the development of first high power LEDs. The light created by LEDs is through a semiconductor rather than a superheated element, arc discharge, or ionized gas. Materials used in making the semiconductor junction determine the wavelength of light emitted.
As compared to incandescent, fluorescent, and HID lamps, LEDs produce more light/electrical watt, have no hazardous material, and also have a longer lifetime. Heat is not directly radiated by LEDs but they do produce some heat, which must be removed for maximum performance.
A current DC power source is constantly required by LEDs so it can convert AC power to a DC source. A properly designed LED lighting system has the capability of performing for several years.
Early Findings and the Potential of LEDs in Plant Production
LED grow lights were first tested at University of Wisconsin with a report concluding: lettuce plants that were grown under LED grow lights in supplement with blue fluorescent lamps showed similar growth as kept under white fluorescent along with incandescent lamps. The successful findings of this group also showed how red LEDs resulted in elongated hypocotyls and cotyledon of lettuce seedlings, which was also shown to be reversed by adding a blue 15 μmol·m−2·s−1 light.
These findings inspired further development of LED grow lights and their usage in small plant chambers of NASA’s Space Shuttle. These chambers were used for growing Brassica seedlings, wheat, leaf cuttings, potato leaf cuttings, soybeans, and Arabidopsis thaliana. LEDs potential in terrestrial plant growth also continued where red LED was tested on xenon arc illuminated kudzu leaves. The leaves showed a different response in stomata conductance but photosynthetic photon flux and CO2 response was similar.
Thermal advantages of LEDs in plant growth were measured through the spectral measurement of red LEDs plus BF, red (660 nm) LED, red LEDs plus far red (735 nm) LEDs, and metal halide lamps that showed higher levels of long wave radiation from MH lamps and similar phytochrome photostationary states. The most recent studies have shown higher leaf photosynthetic rates in rice plants under the 8-band formulation, which is heavily dominated by the blue and red LED combination.
Importance of Blue LED Light
One of the most important wavelengths of the visible region of light is blue light. As mentioned above, the importance of blue light and its photomorphogenic role as tested and approved include: stem elongation (Cosgrove, 1981), stomatal control (Schwartz and Zeiger, 1984), and phototropism (Blaauw-Jansen and Blaauw, 1970).
The following tests by various study groups further show the importance of blue light in plant growth:
- Studies by Wisconsin Group demonstrated the use of high output red LEDs with some blue light mixed with other wavelengths of light gave accelerated plant growth.
- Studies at Kennedy Space Center proved how wheat seedlings failed to develop chlorophyll under red light only and a supplement of blue light restored chlorophyll synthesis.
- Studies by Goins (1997) revealed that by adding blue light to red LEDs, larger plants were produced with greater number of seeds.
- Studies by Yorio (1998) showed the effect of a balanced 8 color system which includes mostly red plus blue light was ideal for yielding a perfect harvest of spinach, lettuce, and radish crops.
- The studies by far have shown that leaf morphology of plants under red LED alone resulted in downward curling of leaf margins with spiral growth of rosette, while addition of blue light restored normal leaf growth. When a perfect balance of primary, secondary and tertiary wavelengths were utilized, the 8 color system maximized plant output.
Tests of Blue Light with Far-Red and Infrared LEDs
Studies by Schuerger (1997) showed the changes in leaf anatomy under different color combinations of light. The work was carried out on pepper plants. The results showed the thickness of leaves increased and the amount of chloroplasts per cell increased by adding blue light into the overall ratio. Without the blue light, the results had low leaf cross sectional area.
Light quality has a great significance in crop productivity when growing indoors. The potential to actively implement dynamic lighting techniques in controlling plant development and growth has a great importance for plant cultivation. However, in addition to the importance of the right light, the distance of light source from the plants determines the rate of photosynthetic process of plant productivity.
LED grow lights should be kept a distance of one to four feet from the plants, depending on the intensity of the light. The higher the wattage the more intensity and the greater distance needed for healthy growth. The lower the wattage of the light, the closer it must be to the plants to be effective. The LED alone has numerous insights to offer for a successful harvest. LEDs have been tested and approved by numerous indoor growing market experts. The benefits of LEDs coupled with the fact that they are very environmentally friendly, are some of the reasons why they are quickly becoming the technology of choice for indoor horticulture lights. Look into grow lights made with LEDs and discover why indoor growing is enhanced by utilizing this amazing technology.