Plants require light for photosynthesis, the process by which plants convert light energy into chemical energy that is stored in the bonds of glucose molecules. Not all wavelengths or colors of light can drive photosynthesis equally. Plants utilize specific wavelengths or colors of light more efficiently for photosynthesis.
Introduction
The primary function of light for plants is photosynthesis. Photosynthesis consists of light-dependent reactions and carbon fixation (Calvin cycle). In the light-dependent reactions, plants absorb photons of light to power the conversion of carbon dioxide and water into energy-rich molecules like ATP and NADPH. In the Calvin cycle, plants utilize the ATP and NADPH to fix carbon from carbon dioxide into sugar molecules.
Plants cannot perform photosynthesis without light. However, not all wavelengths or colors of light work equally well. Plants preferentially absorb red (600-700 nm) and blue (400-500 nm) light. Meanwhile, green light (495-570 nm) is least efficient for photosynthesis. This selectivity is due to the presence of specialized pigment molecules like chlorophyll that strongly absorb red and blue wavelengths.
Absorption spectrum of photosynthetic pigments
Plants possess specialized pigment molecules that absorb specific wavelengths of visible light. The main pigments involved in photosynthesis are chlorophyll a, chlorophyll b, and carotenoids.
Chlorophyll a has absorption peaks at 430 nm and 662 nm, corresponding to blue and red wavelengths. It reflects green light, giving leaves their green color. Chlorophyll b absorbs maximally at 453 nm and 642 nm. Meanwhile, carotenoids like beta-carotene absorb maximally at 449 nm and 475 nm, also corresponding to blue wavelengths.
When looking at an absorption spectrum showing the relative absorption of different wavelengths of visible light, chlorophylls and carotenoids show enhanced absorption in the blue (400-500 nm) and red (600-700 nm) portions of the spectrum. Green wavelengths (495-570 nm) are minimally absorbed and instead reflected, giving plants their green color.
Action spectrum of photosynthesis
An action spectrum depicts the relative effectiveness of different wavelengths of radiation at powering a biological process, which for plants is photosynthesis. Action spectra have revealed that plants maximally harness blue and red light for photosynthesis.
Specifically, an action spectrum shows that photosynthetic activity is greatest between 400-500 nm and 600-700 nm. Blue light induces photosynthesis slightly more effectively than red light. Meanwhile, green light from 495-570 nm is least effective at driving photosynthesis.
The action spectrum of photosynthesis closely matches the absorption spectra of chlorophylls and carotenoids. This indicates the vital role these pigments play in capturing light energy for photosynthesis.
Why plants absorb blue and red light
Plants preferentially absorb blue and red wavelengths of light due to the presence of chlorophyll and carotenoid pigments tuned to harness these wavelengths.
Chlorophyll’s role is to absorb light energy and funnel it into the biochemical reactions of photosynthesis. Its structure includes a network of alternating single and double bonds that enable electrons to become excited upon absorption of blue and red photons.
Meanwhile, carotenoids like beta-carotene extend a plant’s ability to harvest blue light. Carotenoids absorb excess energy from chlorophyll to prevent damage and also aid in light-collection and transfer of energy to chlorophyll.
Without these pigments, plants would be unable to perform photosynthesis. The pigments grant plants the ability to absorb the predominant wavelengths of light that reach Earth’s surface.
Do different plant species absorb specific wavelengths?
Most plants possess the same core set of photosynthetic pigments, including chlorophyll a, chlorophyll b, and various carotenoids. Therefore, the range of maximally utilized wavelengths does not drastically differ between plant species.
However, some variations can exist. Certain accessory pigments may fine-tune a plant’s light absorption. For example, red algae possess phycoerythrin which enhances absorption of green light. Meanwhile, kelps have fucoxanthin for maximized absorption of blue-green wavelengths.
Additionally, plants adapted to specific light environments may exhibit some differences. Plants native to shaded conditions often have increased levels of chlorophyll b, enhancing their shade tolerance. Plants from sunny environments may possess more photoprotective carotenoids.
How do different lights impact plant growth?
Different light wavelengths and intensities can impact plant growth in varied ways:
| Light Color | Effects on Plants |
|---|---|
| Blue |
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| Red |
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| Green |
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| Far-red |
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In general, blue light keeps plants short and compact while red light promotes flowering and fruiting. Exposure to only one color can cause abnormal growth. Instead, a balance of wavelengths optimizes plant health and development.
Light intensity
Light intensity, or flux density, also impacts plant growth. While photosynthesis increases linearly with light intensity at low levels, at higher intensities photosynthesis plateaus due to saturation.
Too little light can result in small, weak plants. Excessively high light causes photoinhibition, where plants close chloroplast stomata to prevent damage. Moderate light intensity typically optimizes growth.
When do plants absorb the most light?
Plants preferentially absorb light during the daytime to power photosynthesis. However, absorption patterns throughout the day depend on the type of photosynthesis a plant utilizes.
Plants such as soybeans use only C3 photosynthesis, absorbing light most actively early in the morning. Meanwhile, plants like corn employ C4 photosynthesis, absorbing light intensely in the late morning to early afternoon.
Some plants like sugar cane possess CAM photosynthesis, absorbing CO2 at night to use it for growth during the day. Such plants take in light energy primarily in the late afternoon and early evening.
How does light absorption vary by season?
Light absorption by plants follows seasonal variations correlated with changing day length. In the northern hemisphere, plants absorb maximal light in the summer when days are longest. Light duration and intensity decrease into fall and winter.
Seasonal light changes trigger growth and reproductive transitions in plants. Shortening days of late summer provide a signal for plants to transition into fall. Changes in light also spur flowering in spring and going dormant in fall/winter.
Despite seasonal shifts, plants aim to absorb as much useful light as possible for photosynthesis at a given time of year. Their pigments and photosystems are adapted to capture available light.
Interaction between different wavelengths
Plants respond to interactions between red and far-red light through the phytochrome system. Phytochromes exist in two forms, an inactive Pr form and active Pfr.
Red light converts phytochrome into the active Pfr form, triggering responses like seed germination and seedling development. Far-red light converts phytochrome back to the inactive Pr form, inhibiting development.
The ratio of red to far-red light provides plants with information about their light environment. Plants grow more compactly under far-red enriched shade to avoid competition for sun.
Blue and UV light serve as signals
In addition to photosynthesis, plants also utilize some wavelengths of light as signals. Blue light perceived by cryptochrome photoreceptors regulates growth and development. It inhibits stem elongation so plants grow short and compact.
Ultraviolet (UV) light, while harmful in excess, serves as a signal in small doses. UV-B light signals plants to produce protective pigments and antioxidants. It also regulates development and metabolism.
Artificial lights for plant growth
Artificial lights can be used to grow plants indoors where natural light is insufficient. Light emitting diodes (LEDs) that emit specific wavelengths are often used.
LEDs emitting red, blue, and/or white light can support plant growth. Combinations of red and blue LEDs are frequently used, since these wavelengths drive photosynthesis. Far-red LEDs can also supplement growth lights.
Grow lights emitting UV are not commonly used since UV can damage plants. Green lights are also unnecessary since plants barely absorb green wavelengths.
Conclusion
Plants preferentially absorb blue and red wavelengths of light through their photosynthetic pigments. Chlorophylls and carotenoids optimize light capture in the 400-500 nm and 600-700 nm spectral regions. Consequently, plants depend on blue and red light for photosynthesis and healthy growth.
While most plants absorb similar wavelengths, some specialty pigments can alter absorption patterns. Overall light intensity and duration also influence plant development and morphology.
Understanding how plants utilize light enables optimized lighting strategies for horticulture and agriculture. Tailoring light quality, intensity and duration to a plant’s needs can boost productivity and plant health.
