Providing sufficient light quality and quantity for indoor plants has long been one of the more complex aspects of hydroponic gardening. Light powers photosynthesis, which in turn provides the energy required for plant growth and development. However, few growers understand the potential that can be unleashed from spectrum control and manipulation of light wavelengths for different crops and stages of growth.
The irradiance spectrum of the artificial lighting indoor plants are exposed to can have specific effects on a wide range of plant responses, ranging from photosynthesis to photomorphogenesis, to phototropism to photonasty. At a much deeper biochemical level, researchers are starting to discover certain wavelengths of light that can influence factors such as resistance to pests and diseases, the production of flavor, color, volatile and aromatic compounds, and increased concentrations of antioxidants, with many more influences yet to be discovered.
The future of spectrum manipulation of indoor plant lighting is an exciting possibility for hydroponic growers, particularly with the development of new lamp types such as LEDs that can more accurately provide specific wavelength combinations for different purposes while not just focusing on intensity and photosynthesis. Much of these developments are still in the early stages as plant physiological and biochemical reactions to light of different wavelengths are still being investigated. There is much potential yet to be unlocked.
Nature of Light: Wavelengths & Energy
Plants evolved under the full spectrum of light wavelengths, and while they largely use the energy from the red and blue bands for photosynthesis, plants also use light quality to sense and respond to their environment.
Light is part of the electromagnetic spectrum made up of a mixture of wavelengths of a number of colors including violet, indigo, blue, green yellow orange and red. It is the length of the waves that determines the energy and color of the light. The higher-energy end of the light spectrum contains the shorter wavelengths—around 400 nm—in the purple and blue range, while the rest of the rainbow decreases in energy to the lowest-energy red light at 700 nm. The visible portion of the spectrum that the human eye can distinguish is within the 380-740-nm range. Our eyes see green and yellow wavelengths fairly brightly, whereas plants sense and respond to a much wider spectrum, including light down at the ultraviolet range and up at the far-red wavelengths.
Plants mostly use wavelengths between 400 and 700 nm to carry out the vital process of photosynthesis. These wavebands are called the photosynthetically active radiation (PAR) spectrum. This is why indoor grow lamps use a measure of radiant energy in the 400-700-nm wavelength band, commonly referred to as PAR watts. A more scientific measure is moles or micromoles of PAR per square meter per second, and these are the units often seen in research reports on the effects of artificial light sources on plant growth.
In the past, indoor gardens have relied on specific types of artificial lighting to provide the red and blue wavelengths needed to power photosynthesis, including high pressure sodium (HPS) and metal halide (MH) lamps, which are both rich in the photosynthetic spectrum. While these provide a high output of PAR, they largely exclude other wavelengths, which, although they are not used in photosynthesis, play a role in other plant processes and responses.
Full-spectrum HID lamps that provide a wider smorgasbord of wavelengths more similar to sunlight have been popular amongst indoor growers in recent years for this reason. However, with the development of LEDs, growers can customize lighting systems to suit the needs of specific plant species, purposes and growth stages so they can manipulate plant metabolism and produce functionalized food. LED grow lights can use different light wavelengths to induce higher concentrations of antioxidants in certain fruits and vegetables, increase the bioactive compounds in medicinal plants, or improve sweetness and flavor.
Red, Blue & Green Wavelengths
Artificial light in an indoor garden needs to provide plants with both the energy and information required for development. Energy, largely in the form of the red and blue light wavelengths, determines the rate of photosynthesis. Red light (650-665 nm) fits perfectly with the absorption peak of chlorophylls and provides more energy for photosynthesis, but red light alone tends to lead to plant elongation and unbalanced growth in many species. Adding in a certain percentage of blue light (460-475 nm) not only boosts the energy available for photosynthesis, but it also helps prevent stretching, leading to more balanced and compact vegetative growth.
Plants have several blue and red light receptors that control different aspects of growth and development. The UVA/blue-light-sensing phototropins control several light responses and are responsible for the optimization of photosynthetic yields. Phototropins control chloroplast movement, leaf expansion and stomatal openings, and are thought to optimize photosynthesis by helping capture light energy efficiently and reduce photo damage. The UVA/blue-light-sensing cryptochromes and the red/far-red-sensing phytochromes control the internal circadian clock and the transition from vegetative to reproductive growth. It is likely there are other photoreceptors as well, absorbing in the UVA and green regions of the light spectrum.
With the use of LEDs, where light wavelengths can be further mixed and controlled, introducing up to 24% green light, along with the traditional red and blue, enhances growth in some species. While this has exciting potential for indoor growers who are solely reliant on artificial lighting, researchers have found this response to different light combinations varies amongst commonly grown species, making overall optimal recommendations for mixed crop gardens difficult. For large-scale commercial growers, customizing LEDs with a certain blend or ratios of light of different range of wavelengths designed to achieve certain goals on specific species is increasingly being used in crop production.
Apart from the effect of wavelength combinations on photosynthesis, red and blue light ratios also affect other aspects of plant metabolism. Red light (658-660 nm) increases the antioxidant content in vegetables such as lettuce and cabbage. Additional red light, applied a few days pre-harvest, may vastly improve the health quality of certain food plants. In tomato plants, adding additional red LED lights (668 nm) triggered a significant increase in some of the compounds that play a role in fruit flavor as compared with white LED lights, showing a potential for naturally increasing the compositional quality of fruit.
Red Light and Sensing Day Length
Another way light influences plant growth is the effect it has on day length and flowering induction or prevention. This mostly has to do with a plant’s response to red wavelengths. While far-red light (700 nm) has little effect on photosynthesis, it does play a major role in a plant’s response to day length and hence is an important part of the light spectrum provided to indoor crops. The plant pigment phytochrome is responsible for light-mediated responses in plants.
This pigment exists in two forms: one that absorbs red light and one that absorbs far-red light. Red light with a wavelength of 660 nm causes a response in the phytochrome pigment that puts it into an active form and triggers a number of processes in plants, including allowing a plant to measure the length of the day or light period.
Since some plants require specific day lengths to induce flowering, using red wavelengths, which plants use to measure the duration of the light cycle, becomes important for crop production. Indoors, we have the option of changing the light spectrum during different stages of growth. Typically this has been done by using blue, light-rich MH lamps for the vegetative phase, which boosts photosynthesis and keeps plants strong and compact. Later, many growers switch to HPS lamps to induce flowering by providing the correct day-length cycle.
Other Plant Processes: UV and Short Wavelengths
There are a number of other physiological processes in plants that respond to wavelengths of light but don’t play a role in the production of assimilate for growth. Plants are responsive to wavelengths down in the UV range of 280-400 nm, and up in the far-red range of 700-800 nm—wavelengths that don’t have a large effect on photosynthesis but have been found to be important for other biological functions.
One of the major functions of exposure to natural UV light is activation of plants’ defense mechanisms. Researchers have found that with exposure to UV light, plants can produce 15 different defense proteins, which gives them added protection against pests and diseases. Plants also increase the rate of antioxidant compounds to protect themselves from UV-light damage, and many of these, such as carotenoids, influence the nutritional value of the plant material.
Researchers have also found that the short wavelengths in the UV band of 290-310 nm assist with prevention of plant elongation, and they also appear to play a role in the development of flavors, colors, fragrances, aromatics and volatiles in some plant species. Other wavelengths appear to be used for information transfer within the plant that controls a wide range of responses.
Non-plant Advantages of Different Wavelengths
While different light wavelengths are implicated in a range of biochemical and metabolic processes in plants, light can also influence the growing environment and plant health in other ways. Algae, pests and diseases also respond to light, although the ways in which they do so are still under investigation. Certain red wavelengths help suppress downy mildew disease infection of basil and similar plants, making its use in greenhouses a potential preventative measure against this disease, which is becoming an increasingly serious problem for hydroponic producers. The response of certain insect pests to UV light is another area of study that may lead to improved pest control under protected cultivation.
Understanding the role of certain light wavelengths, ratios and intensities on various plant responses and metabolism will, in the coming years, lead to a huge revolution in indoor plant lighting. Focus is already moving away from just providing sufficient intensity for optimal photosynthesis, but also using a wider range of light wavelengths as a way of manipulating plant form, quality and functionality. With the development of LED technology and the exact ratios of different wavelengths, the future of indoor gardening looks bright.