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Why is the color of light important for photosynthesis?

Why is the color of light important for photosynthesis?

The basics of photosynthesis

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create energy in the form of carbohydrates. This process is vital for life on Earth as it provides the energy that fuels plant growth. Additionally, photosynthesis produces oxygen as a byproduct which is essential for most other forms of life.

At its core, photosynthesis relies on capturing light energy from the sun and converting it into chemical energy. This light energy is absorbed by chlorophyll, a green pigment found in plant cells. When a photon of light hits a chlorophyll molecule, it excites an electron which then travels down an electron transport chain creating energy compounds like ATP and NADPH. These energy carriers provide the power needed to fuel the rest of the photosynthetic process.

Overall, the general equation for photosynthesis is:

6 CO2 + 6 H2O + Light Energy -> C6H12O6 + 6 O2

Where carbon dioxide and water are combined using light energy to produce glucose sugar and release oxygen. Understanding this process and the factors that influence it has important implications for plant growth and agricultural productivity.

The color of light matters

While photosynthesis relies fundamentally on capturing light energy, not all wavelengths (colors) of light are utilized equally. Chlorophyll is specifically optimized to absorb blue and red wavelengths while reflecting green wavelengths – which gives plants their characteristic green appearance.

Within the visible light spectrum, blue light has a wavelength of about 475 nm and red light has a wavelength of about 680 nm. These regions correspond to peak absorption bands for chlorophyll where the molecule is most efficient at capturing photons and exciting electrons.

In contrast, green light around 550 nm falls within a range where chlorophyll absorbs very little. Therefore, when white light containing the full spectrum shines on a leaf, the green portion is reflected while the blue and red portions are absorbed and used for photosynthesis.

This differential absorption of select colors underlies the importance of light color for photosynthesis. Blue and red wavelengths drive the process much more effectively than greens and other intermediate wavelengths.

Action spectra demonstrate differential responses

The concept of preferential absorption of blue and red light has been conclusively demonstrated through action spectra studies. An action spectrum examines how the rate of a biological process responds to different wavelengths of radiation.

In the case of photosynthesis, researchers can shine monochromatic light of varying wavelengths on a leaf and measure the rate of oxygen production as an indicator of photosynthetic activity. This allows an action spectrum to be constructed with photosynthetic response plotted against wavelength.

Multiple experiments using this approach have shown that photosynthetic activity peaks in the blue and red regions, with minimal response in the green and yellow wavelengths – matching the absorption profile of chlorophyll.

For example:

Wavelength (nm) 400 425 450 475 500 550 575 600 625 650 675
Photosynthetic Response 0.2 0.4 0.6 0.9 0.8 0.4 0.3 0.5 0.7 0.9 1.0

This data demonstrates that photosynthetic activity peaks at ~450 and ~675 nm corresponding to blue and red light respectively.

Overall, action spectra provide definitive evidence that photosynthetic organisms like plants have evolved to maximize capture of blue and red light wavelengths that are most useful for driving photosynthesis.

Pigment absorption spectra

The selective light absorption described above arises from the characteristics of chlorophyll pigments embedded in plant cell chloroplasts. Each type of photosynthetic pigment has a specific absorption spectrum that indicates which wavelengths it can efficiently capture.

Chlorophyll A and chlorophyll B are the primary pigments responsible for absorption in plants. The spectrum below compares the absorption profiles of these two forms of chlorophyll:

Wavelength (nm) 400 425 450 475 500 550 575 600 625 650 675 700
Chlorophyll A 0.1 0.3 0.6 0.9 0.8 0.4 0.2 0.3 0.7 1.0 0.5 0.1
Chlorophyll B 0.0 0.1 0.3 0.6 0.8 0.5 0.3 0.2 0.4 0.8 0.9 0.3

As seen, chlorophyll A has peak absorption in the blue (~450 nm) and red (~675 nm) regions with minimal absorption in the center green region (~550 nm). This closely matches the overall action spectrum for photosynthesis. Meanwhile, chlorophyll B fills gaps by absorbing intermediate wavelengths.

The combined absorption capacity of all photosynthetic pigments allows plants to harness light across much of the visible spectrum. But the predominance of blue and red wavelengths absorption explains why those colors play a primary role.

Complementary chromatic adaptation

Another line of evidence demonstrating the importance of blue and red light comes from complementary chromatic adaptation studies. This term refers to changes in pigment production and ratios observed when plants are grown under light sources lacking certain wavelengths.

For example, when plants are grown under only blue or only red light, they change their production of chlorophyll A and B to compensate for the missing wavelengths. This allows them to adapt and still absorb light efficiently for photosynthesis.

Conversely, plants grown under green light do not thrive and show much less morphological change. They are unable to adequately compensate because green wavelengths do not strongly drive photosynthesis in the first place.

The capacity of plants to adapt to blue or red light, but not green light, shows that they can detect which wavelengths are most useful and make internal changes to maximize absorption of those beneficial colors of light.

Effects on plant morphology and growth

In addition to internal biochemical responses, variable wavelength light sources induce morphological and developmental changes in plants. Exposing plants to different colors of light can affect things like stem elongation, leaf expansion, stomatal opening, and more.

For example, blue light generally inhibits stem and leaf growth, causing plants to grow more compactly. In contrast, red light promotes stem elongation and leaf expansion. Both colors influence stomatal opening with blue light inducing greater opening than red light during the day phase.

These morphological effects allow plants to appropriately shape their architecture and physiology based on the predominant light spectrum in the environment. The contrasting responses again demonstrate that plants recognize blue and red as the most useful wavelengths driving photosynthesis and growth.

A summary of these morphological effects is shown below:

Plant Response Blue Light Effect Red Light Effect
Stem elongation Inhibited Promoted
Leaf expansion Inhibited Promoted
Stomatal opening Promoted Inhibited

The ability of blue and red light to trigger these contrasting developmental responses allows plants to properly tune their morphology.

Applications for horticulture

The knowledge that blue and red wavelengths play special roles in photosynthesis has important applications in horticulture and agriculture. Techniques leveraging this information can optimize plant growth in greenhouses and indoor facilities.

Specific strategies include:

– Using supplementary electric lighting to increase intensities of blue and red wavelengths. High-power LEDs emitting narrow bandwidth light are well-suited for this purpose.

– Adjusting ratios of blue to red light to find an optimum balance of stem elongation and leaf expansion. Generally a red:blue ratio between 2-5:1 is recommended.

– Implementing photoperiodic lighting schedules with blue light present during daylight periods to promote stomatal opening and red light during night periods to inhibit elongation growth.

– Screening young plants under blue light to limit stem stretching and promote compact growth.

– Shifting toward blue light and away from red light as plants mature to control excessive elongation and induce flowering/fruiting.

Properly leveraging these applications allows growers to manipulate plant morphology and development to maximize productivity, growth rate, and plant quality.


In summary, blue and red wavelengths play a predominant role in photosynthesis due to the absorption characteristics of chlorophyll pigments. Preferential absorption of blue and red light has been conclusively demonstrated through action spectra studies and complementary chromatic adaptation experiments. These colors also trigger morphological changes that allow plants to adapt their form to the surrounding light environment. Knowledge of the importance of blue and red light has provided plant growers with strategies to optimize horticultural practices and productivity. Overall, the ability of plants to differentially sense and respond to these key wavelengths of light provides a fascinating look into the evolution and tuning of photosynthetic efficiency.