Does the Wavelength of Light Affect a Plant's Growth?

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Light is a critical factor in plant growth, influencing processes like photosynthesis, photomorphogenesis, and flowering. But does the wavelength of light—its color or spectral quality—play a role in how plants develop?

The Role of Light Wavelengths in Plant Growth

Light is an essential factor in maintaining plants. The rate of growth and length of time a plant remains active is dependent on the amount of light it receives. Light energy is used in photosynthesis, the plant’s most basic metabolic process. When determining the effect of light on plant growth there are three areas to consider: intensity, duration and quality. ... Light quality refers to the color (wavelength) of light. Sunlight supplies the complete range of wavelengths and can be broken up by a prism into bands of red, orange, yellow, green, blue, indigo and violet. Blue and red light, which plants absorb, have the greatest effect on plant growth. Blue light is responsible primarily for vegetative (leaf) growth. Red light, when combined with blue light, encourages flowering. Plants look green to us because they reflect, rather than absorb, green light.

Plants primarily respond to wavelengths from 400-700 nanometers(nm) for photosynthesis; light within this range is referred to as photosynthetically active radiation (PAR). The various wavelengths of light in the spectrum can trigger morphological responses. Light spectrum in terms of plant growth and morphology is often referred to as light quality, and collectively these responses to light are called Photomorphogenesis. ... Light from the red (600-700nm) and far-red (700-750nm) wavelengths are responsible for enabling the photoreceptor Phytochrome. The pigment phytochrome allows plants to detect light and regulate morphological processes such as flowering, vegetative growth and set the plant’s circadian rhythm.

Specific Wavelengths and Their Effects

Blue Light (400-500 nm)

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.

Chlorophyll accumulation, leaf expansion and positioning can also be triggered by blue light. An increase or decrease in blue light can control the opening and closing of stomata that affects photosynthesis and transpiration. Blue light is necessary, even in low intensities, for healthy plants. Plants require blue light for full functioning photosynthesis and a lack of blue light may lead to further developmental problems such as blistering on leaves and stems. The red colour of lettuces can be enhanced under blue light spectrum.

Red Light (600-700 nm)

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.

Red light affects phytochrome reversibility and is the most important for flowering and fruiting regulation.

Green Light (500-600 nm)

When combined with blue, red and far-red wavelengths, green light completes a comprehensive spectral treatment for understanding plant physiological activity. But what color light is best for photosynthesis? The function of green light is less well understood than the other spectrums, and there are only certain species of plants that require green light for normal growth. Its effects appear to be very strain specific. The pigments that can absorb green are found deeper in the leaf structure. It is thought that because green light reflects off of the Chlorophyll in leaf surfaces, and thus reflected deeper into the shaded areas of the canopy than Red and Blue which are readily absorbed, that green may actually be mostly absorbed through the undersides of the leaves as it bounces around in the shaded depths of the canopy.

Green light (500-600nm) responses appear to be triggered under low light intensities. It has been studied that high far-red light with high green light can cause an increase in the shade avoidance response. While far-red light is not useful for photosynthesis, green light is. And one potential advantage to green light is since it can penetrate the lower canopy better the lower leaves can continue to photosynthesize.

Ultraviolet (UV) Light (280-400 nm)

Although damaging in large quantities, UV light can have important benefits such as producing different defense proteins that give them protection against pests and disease. Plants are also able to increase antioxidant compounds to protect themselves against UV light damage, and many of these also add to the nutritional value of the plant. Researchers have found that UV-B wavelengths in the 280-315nm range can also play a role in the development of flavonoids and phenolic acids.

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.

Far-Red Light (700-750 nm)

Far-red will also have the opposite effect to blue light on root to shoot ratio, resulting in higher shoot to root distribution. Yet, as with all elements of the light spectrum, there is a balance to be struck between a beneficial amount of far-red light and too much. Plants grown under high levels of far-red light will appear tall and stretched, with lower chlorophyll content resulting in yellowing of the leaves, which is perhaps unfavourable from a marketability perspective. In addition to direct effects, the ratio of red to far-red light is also an important mechanism for governing plant responses. Far-red penetrates the canopy more than red light, so plants receiving a higher amount of far-red relative to red will interpret this as a shading effect, and increase shade avoidance responses such as increased upwards growth.

Practical Implications for Growers

After seeing how different wavelengths are responsible for different plants 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. Both VOLT Grow®’s LED grow lights have white, full-spectrum light. 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.

Many growers take advantage of LED lights to help scale plant production due to their full light spectrum capabilities, low heat waste and maintenance, and extended lifespan. And given a plant’s physiology and morphology are strongly affected by specific spectrums, LED grow lights can efficiently promote growth in crops at specific times in the growth cycle. With the ability to closely monitor quality, energy output can be easily evaluated for scaling crop production.

Conclusion

The wavelength of light profoundly affects plant growth, with each spectral band triggering specific physiological and morphological responses. Blue light promotes vegetative growth, red light drives flowering and photosynthesis, green light aids lower canopy photosynthesis, UV light enhances defense mechanisms, and far-red light influences shade avoidance. By understanding and manipulating light spectra, growers can optimize plant development, making full-spectrum LED lights a powerful tool for modern horticulture.

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Frequently Asked Questions (FAQ)

1. What is the best light spectrum for plant growth?

2. Why are blue and red light most important for plants?

Blue and red light, which plants absorb, have the greatest effect on plant growth. Blue light is responsible primarily for vegetative (leaf) growth. Red light, when combined with blue light, encourages flowering. Red light is known to be the most effective light spectrum to encourage photosynthesis as it’s highly absorbed by chlorophyll pigments.

3. Does green light contribute to plant growth?

While far-red light is not useful for photosynthesis, green light is. And one potential advantage to green light is since it can penetrate the lower canopy better the lower leaves can continue to photosynthesize. The function of green light is less well understood than the other spectrums, and there are only certain species of plants that require green light for normal growth. Its effects appear to be very strain specific.

4. Can UV light harm plants?

5. How does far-red light affect plant growth?

Far-red will also have the opposite effect to blue light on root to shoot ratio, resulting in higher shoot to root distribution. ... Plants grown under high levels of far-red light will appear tall and stretched, with lower chlorophyll content resulting in yellowing of the leaves, which is perhaps unfavourable from a marketability perspective.

6. Why are LED grow lights popular for controlling light spectra?

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DOES THE WAVELENGTH OF LIGHT AFFECT A PLANT'S GROWTH?

Does the Wavelength of Light Affect a Plant's Growth?