HOW DOES THE SPECTRUM OF LED GROW LIGHT AFFECT PLANT GROWTH?

How Does the Spectrum of LED Grow Light Affect Plant Growth?

How Does the Spectrum of LED Grow Light Affect Plant Growth?

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The spectrum of LED grow lights plays a critical role in plant growth and development, as different wavelengths of light trigger specific physiological responses in plants. By mimicking or manipulating the natural sunlight spectrum, growers can optimize photosynthesis, enhance plant morphology, and increase yields. Below, we explore how various light spectra influence plant growth, drawing from established insights in horticulture.


Understanding the Light Spectrum and Plant Growth


Light spectrum refers to the entire range of wavelengths of electromagnetic radiation that we can perceive as light, including both visible and non-visible light. This spectrum extends from gamma rays, which have the shortest wavelengths and the highest energy, to radio waves, which have the longest wavelengths and the lowest energy. For plants, the most relevant part of the spectrum is the Photosynthetically Active Radiation (PAR) range, spanning 400-700 nanometers (nm), which plants use for photosynthesis.


Plants rely on the process of photosynthesis to convert light energy into chemical energy (in the form of glucose) and oxygen that fuels plant growth. Photosynthesis primarily occurs in specialized cell structures called chloroplasts, where pigments, such as chlorophyll, capture light energy. However, not all wavelengths of light are created equal when it comes to photosynthesis. Certain spectra are more effective in driving this essential process, and understanding them is vital for optimal plant growth.


The Role of Different Light Spectra




  1. Blue Light (400-500 nm)
    Blue light, with wavelengths between approximately 450 and 490 nm, is one of the most crucial parts of the light spectrum for plant growth. It has a short wavelength and high energy, making it especially effective in driving several key developmental processes. Blue light plays a central role in photosynthesis by exciting electrons in chlorophyll molecules, powering the light-dependent reactions that convert light energy into chemical energy. While it doesn't directly increase chlorophyll production, it is efficiently absorbed by chlorophyll, making it crucial for energy capture. Blue light affects plant development by influencing the distribution and activity of auxins. A larger proportion of blue light has an inhibitory effect on cell elongation, which leads to shorter stems and thicker leaves. Conversely, a decrease in the amount of blue light will cause a larger leaf surface area and longer stems. Too little blue light will negatively affect the development of plants. Many plants need a minimum amount of blue light, which ranges from 5 to 30 μmol/m2/s for lettuce and peppers to 30 μmol/m2/s for soybean.

  2. Red Light (600-700 nm)
    Red light, with wavelengths ranging from approximately 600 to 700 nanometers, is a critical component for plant growth. Red photons are the most photosynthetically efficient of all and therefore indoor growers want to maximize the amount of red in the grow light spectrum. Red will be about 30 - 40% of any white LED spectrum output. To increase the proportion of red photons in a grow light, deep red LEDs with a peak wavelength of 660nm can be added. Not only are 660nm red LED diodes photosynthetically efficient, but they are also electrically efficient. Red light is known to be the most effective light spectrum to encourage photosynthesis as it’s highly absorbed by chlorophyll pigments. In other words, it sits in the peaks in chlorophyll absorption. Red light wavelengths (particularly around 660nm) encourage stem, leaf, and general vegetative growth – but most commonly, tall, stretching of leaves and flowers. A balanced pairing with blue light is necessary to counteract any overstretching, like disfigured stem elongation. It’s important to consider that while red is the most responsive light spectrum for plants, its efficacy really steps in when in combination with other PAR wavelengths.

  3. Green Light (500-600 nm)
    A lot of people think green light is less important than other wavelengths, as chlorophyll can’t actually absorb this light as well as others. But it’s actually this lack of absorption that gives your plants their zesty green colour. Green light has also been found to be helpful for photosynthesis in the lower leaves of a plant. Once the plant has had enough of blue and red light, green light can still activate the chloroplast deep within the leaves of the lower canopy. It can also help to increase crop yields. Researchers believe that because plant cells don’t absorb green light as easily, it is able to penetrate deeper into the canopy before being absorbed. This provides light to plant cells that were being blocked from receiving red/blue photons by cells higher in the canopy, thus allowing them to contribute to photosynthesis and increasing the plant’s total yield. They have also found that green light can result in a healthier plant structure.

  4. Far-Red Light (700-850 nm)
    Far-red light also falls outside the PAR range. With a wavelength of 700-850 nm, far red falls just between red and infrared light. Interestingly, your plants are extremely sensitive to the difference between red and far red light, and the balance between the two can give your crop very specific instructions on how to grow. Your plants perceive red and far red light with the help of photoreceptors called “phytochromes.” When your plants “see” more far red light than red light, they think they’re in the shade. A higher proportion of far-red light signals the plant that it’s shaded, prompting a shade avoidance response that causes the stem to grow taller in search of more light. There are a few ways far-red can affect plant growth – one is to initiate a shade-avoidance response. A cultivator can shift relative percentages of red and far-red light to achieve the optimum height and leaf ratios as the plants develop their vegetation.

  5. Ultraviolet (UV) Light (100-400 nm)
    Ultra-violet is outside the window of PAR, and its effect on plants is yet to be fully understood. That said, an easy way to think of UV’s impact on plants would be similar to humans. Ultraviolet (UV) light has an effect on plants, too, causing compact growth with short internodes and small, thick leaves. However, too much UV light is harmful for plants, since it negatively affects the DNA and membranes of the plant. Photosynthesis can be hampered by too much UV light. Research shows that this happens at UV-values higher than 4 kJ/m2/day.


Full-Spectrum LED Grow Lights


After seeing how different wavelengths are responsible for different plant reactions, it is easy to see why full-spectrum lights are the best for plant growth. Full-spectrum light most closely mimics the natural sunlight by using a combination of all colors at all stages of growth. Excluding certain wavelengths that contribute to plant growth can negatively affect yields. When horticulture LED grow lights were first introduced in the market, they only included produced light in the red and blue wavelengths which led to them being known as “smurf” lights. The focus on red and blue light came from the idea that the cells in plants absorbed these spectrums far better than they do green light. While this is true, more recent studies have shown that adding green light to an LED grow light actually increases crop yields compared to fixtures focused entirely on red and blue light. Full spectrum LED lights best mimic the light of the sun. Daily light intervals are a fancy way of saying when is the light on and when is the light off – or when are your plants “sleeping” and when are they “awake.” How much light your plants need changes as they grow and mature. Younger plants need more light for longer periods of time and more mature plants can get by with less light – but they need more intense light.


Practical Applications for Growers


The ideal grow light spectrum for plants depends on several factors. These include how specific plants use PAR-spectrum light for photosynthesis but also the wavelengths outside of the 400-700nm range. This light can help accelerate flowering, increase nutrition, speed up rate of growth, etc. If the light source is sole (indoors) or supplementary (greenhouses) also affects which grow light spectrums should be used. Generally, photosynthetic efficiency occurs at the red and blue peaks which means plants absorb these spectrums most when growing. You might think the ideal grow light spectrum is equal to sunlight – after all, it’s had millions of years of experience – however, it’s more detailed than this. Sunlight produces a lot of greens, yellows, and oranges – they’re the most readily available spectrums of light. In fact, studies tell us how green light, while not absorbed by chlorophyll as well as red and blue, it’s absolutely critical for photosynthesis.


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Frequently Asked Questions (FAQs)


1. What is the best light spectrum for plant growth?
Full-spectrum light most closely mimics natural sunlight by using a combination of all colors at all stages of growth. Both blue and red light are critical, with blue promoting vegetative growth and red encouraging flowering and fruiting. Green light also contributes to healthier plant structure and higher yields by penetrating deeper into the canopy.


2. Why is blue light important for plants?
Blue light (400-500 nm) is essential for chlorophyll production, root growth, and leaf thickness. It inhibits cell elongation, leading to shorter stems and thicker leaves, which is ideal for compact, healthy vegetative growth.


3. How does red light affect plant growth?
Red light (600-700 nm) is highly efficient for photosynthesis and promotes stem, leaf, and vegetative growth. It also regulates flowering, germination, and dormancy, making it crucial for plants in the flowering stage.


4. Is green light necessary for plants?
While plants absorb less green light (500-600 nm) compared to red and blue, it penetrates deeper into the plant canopy, aiding photosynthesis in lower leaves and contributing to overall plant health and yield.


5. Can too much UV light harm plants?
Yes, excessive UV light (100-400 nm) can damage plant DNA and membranes, hampering photosynthesis. However, controlled UV exposure can promote compact growth and enhance crop quality.


6. What is the role of far-red light in plant growth?
Far-red light (700-850 nm) can trigger a shade avoidance response, causing plants to grow taller in search of light. It’s useful for manipulating plant height and leaf ratios but should be used carefully to avoid excessive stretching.


Conclusion


The spectrum of LED grow lights significantly influences plant growth by targeting specific wavelengths that drive photosynthesis and photomorphogenesis. Blue light fosters compact vegetative growth, red light boosts photosynthesis and flowering, green light enhances canopy penetration, and far-red and UV light offer nuanced control over plant morphology. Full-spectrum LED lights, which combine these wavelengths, provide the most versatile and effective solution for indoor and greenhouse growers, mimicking natural sunlight to optimize plant health and yield.

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