What Is An Action Spectrum

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Last Updated on October 31, 2022

If you are wondering what an action spectrum is, we’ve got you covered here. Getting to know about the action spectrum in various plants will give you an idea of how plants make use of different lights for their development.

The lights we see consist of various wavelengths and light has a comprehensive relationship with plants. Plants will either absorb or reflect the different colors of the light spectrum. This absorption or reflection affects the process of photosynthesis that will determine how your plants process their food.

Now, let’s have a discussion about the role of the light absorption spectrum, light action spectrum, and how they are connected to photosynthesis. We will also talk about various plant pigments, their functions and, many more. So, let’s begin.

Absorption And Action Spectrum

Both absorption spectrum and action spectrum play a significant role in the plant process of photosynthesis. Let’s go further by looking at what these two entails and discuss more details on them.

What Is An Action Spectrum?

Firstly, let’s define what the action spectrum is. An action spectrum is an illustration (usually showed in a graph) that reveals the rate of photosynthesis at various light wavelengths. Thus, it illustrates the wavelength that propels photosynthesis.

Action spectrum was first discovered and titled by T.W. Engelmann when he was researching on algae Cladophora. The action spectrum gives an insight into the nature of the photoreceptor involved. It also helps us understand the differences between the absorption spectrum of a known pigment. Action spectrum will illustrate the usefulness or effectiveness of light in photosynthesis.

Maximum photosynthesis usually takes place in the red and blue light spectrum. Next, let’s take a look at what the action spectrum of photosynthesis is.

Photosynthesis Action Spectrum

The action spectrum of photosynthesis in any given plant will indicate high light absorption for red or violet and orange or red color spectrum. It will also show maximum light absorption in the blue region. However, it will indicate a low absorption rate for the green or yellow color spectrum.

The colors blue and violet on the light spectrum have the shortest wavelength and the highest energy. On the other hand, red has the longest wavelength and has the least amount of energy.

Absorption Spectrum

The absorption spectrum will indicate all the colors of light absorbed by a given plant. It describes the electromagnetic radiation range of plant absorption. It relies on the cellular and molecular structure of a plant. So, it varies depending on the plant species.

Absorption And Action Spectrum


The pigment is a common name used for a molecule that takes in or absorbs light with color. Also, we can term pigment as any chemical compound that assimilates visible radiation between 380 nm (which is violet) and 760 nm (which is red).

These pigments assimilate light rays from the visible part of the electromagnetic spectrum known as photosynthetically active radiation (PAR).

Now, any type of colored substance generated by a plant is known as plant pigment. There are different pigments contained in plants and they result in the different colors we see. These plant pigments give rise to leaves’ color, flowers, as well as fruits. Now the most crucial function this pigment offers is control over the photosynthesis process with plants’ growth and development.

Plant pigments will only absorb the wavelength of light ranging from 700 nm to 400 nm. Now, this is the ideal PAR range for plants.

Below are the 3 major plant pigments that determine plant color:


This pigment is one of the most important in plants because it is directly involved in the plants’ photosynthesis. Their absorbing wavelength spectrum is 429 nm and 659 nm. They are also responsible for the violet-blue and orange-red colors.

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They are accessory pigments. Involved indirectly in photosynthesis. Even though plants can’t photosynthesize with carotenoids, this pigment can still transfer the light energy they get down to chlorophyll.

Carotenoid pigments are photo-receptive agents that prevent harmful reactions. They are also responsible for the orange, bright yellow, and red colors in various plants, vegetables, and fruits.


This pigment plays a crucial role in the reproduction of plants. It as well helps by attracting pollinators and seed disperses. They are, however, not directly involved in the photosynthesis process. But just like carotenoids, they also help shield plants from harmful rays.

Absorption Spectrum Of Chlorophyll

Chlorophyll is a type of pigment and in nature, we have 6 types of chlorophyll. However, in plants, there are 2 major types of chlorophyll which are chlorophyll A and chlorophyll B. The region of the spectrum that plant chlorophyll absorbs differs for both chlorophylls A and B.

Chlorophyll A and B have different roles they play in photosynthesis. Chlorophyll A is the main pigment that is involved in photosynthesis. On the other hand, chlorophyll B is a supplementary pigment that receives light energy which it will pass to chlorophyll a.

Absorption Spectrum Of Chlorophyll

The color spectrum that chlorophyll A absorbs is violet and orange. On the other hand, the color spectrum that chlorophyll B absorbs is blue and yellow. Both chlorophyll a and b hardly absorb the green spectrum. The green light is reflected rather than being absorbed. So, this results in the green color of most plants.


What does action spectrum of photosynthesis indicate?

Photosynthesis is the process that provides the energy necessary for growth and development of plants and algae. The primary role of photosynthesis is to fix atmospheric CO~2~ into organic molecules, such as sugar or starch. Carbon is stored in plant tissues during the day and it is released at night to fuel respiration. Photosynthesis uses solar energy (in the form of photons) to create chemical energy from water and carbon dioxide.

Photosynthetic organisms absorb light energy from the environment in order to convert it into chemical energy. In plants, this is realized by the light-harvesting complex (LHC) II and the reaction center (RC). The RC is the part of LHCII that contains the primary quinone electron acceptor Q~A~.

The action spectrum is the distribution of photons that excite a biological reaction or process. The spectrum of the light that reaches the Earth's surface is called the solar spectrum, and its shape is modified by absorption and scattering in the atmosphere and the Earth's surface .

What action spectrum indicates is the rate of photosynthesis.

How does the light spectrum affect plant growth?

Plants need a certain amount of energy in order to grow and thrive. This energy is provided by the sun. The energy that plants receive from the sun varies depending on how the light spectrum is. This means that plants grown under different light spectrums will have different growth rates. Sunlight is composed of different wavelengths of light. The spectrum of sunlight can be broken down into three main categories: red, green and blue. These wavelengths are all present in sunlight but are more or less prominent depending on the time of day and the season. In general, the more intense the wavelength, the greater the energy of the light. The spectrum of sunlight has an effect on the growth rate of plants in a variety of ways. For example, plants grown under a red spectrum of sunlight grow faster than those grown under a blue spectrum. Red light causes the plant’s photosynthetic pigments to absorb more of the light, which leads to increased plant growth. The opposite is true for blue light. Blue light inhibits the photosynthetic pigments of plants, which decreases plant growth.

Additional Say On What Is An Action Spectrum

We hope by now you have been enlightened on what an action spectrum is and you can see the role they play in a plant’s photosynthetic process and developments.

Remember, the absorption spectrum of chlorophyll that determines photosynthesis lies in the blue and red regions. Also, light wavelengths within the range of 400 nm to 700 nm are utilized by plants for photosynthesis and this is the ideal PAR range.

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