Photosynthetically Active Radiation: Range and Uses

Photosynthetically active radiation, which is often abbreviated as PAR, designates the spectral range of solar radiation from 400 to 700 nanometers that the photosynthetic organisms can use in the process of photosynthesis. PAR changes occasionally and varies depending on the latitude and time of the day. Levels are greatest during summer at midday

Read more: Importance of Ecosystem

Key Terms: Photosynthetically Active Radiation, Photosynthesis, Spectrum, Solar radiation

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What is Photosynthetically Active Radiation?

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PAR or Photosynthetically Active Radiation is the wave band or spectral range of the sun’s radiation from which organisms use for the process of photosynthesis. Its range is generally from 400nm to 700nm. The spectrum lies in the range of normal human vision.

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Range of Photosynthetically Active Radiation

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The solar radiation that is absorbed to carry out the photosynthesis process is in the range of 400-700 nm wavelength. And this is the optimal range which is not harmful for the living organisms. If the range of wavelength is less than 400nm to 700 nm, the cells and tissues of organisms would burn from the intensity.

If the range is higher, there is not much energy left to draw from for the process of photosynthesis. This range is applicable to almost all photosynthetically capable organisms other than cyanobacteria, heliobacteria, etc can take in slightly higher wavelengths as well.

• The process of photosynthesis is carried out by plants using sunlight.
• However they only employ a certain wavelength range of solar radiation, known as PAR (Photosynthetically Active Radiation), which is between 400 nm and 720 nm.
• Only 2-10% of this PAR can be converted into chemical energy by plants.
• The majority of solar energy that reaches the Earth is reflected off of surfaces like leaves and other objects before being absorbed by other molecules, which heats up the planet.
• Plants and microorganisms that can make pictures are only able to catch between 1 and 10% of PAR

Read more: Food chains

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Pigments Absorbing Photosynthetically Active Radiation

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Chlorophyll is a pigment present in plants which is the reason plants have colors, just like the pigment melanin in humans. It is an extremely essential component required for the process of photosynthesis. Only plants containing this particular pigment can produce food in the presence of light. The reason it is so important is that it is abundantly available and captures both red and blue components of the sunlight. 

• Chlorophyll ‘a’ and chlorophyll ‘b’ are the major photosynthetic pigments present in higher plants.
• Chlorophyll absorbs blue and red light.
• Chlorophyll ‘a’ and chlorophyll ‘b’ display slightly different absorption patterns. is an important pigment.
• However, there are other pigments that are not as abundant as chlorophyll. These pigments are called accessory pigments.
• Examples of these carotenes and xanthophylls.
• Carotenes give out the orange colour we usually see in carrots, etc.
• These accessory pigments can also use green components of light and also pass it on for the process of photosynthesis.
• Since the component is not as abundant as chlorophyll, the amount of green wavelength absorbed is very less. 

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Uses of Photosynthetically Active Radiation

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A few uses of PAR are mentioned below--

• In Agriculture: In agriculture, one of the most important features of productivity is the adequate amount of Photosynthetically Active Radiation. It is used to evaluate the agricultural potential for investment.
• In Forestry: In forestry, there are sensors placed under canopies at various levels just to measure the pattern of availability of PAR to organisms. It also helps in measuring the utilization by Plants.
• In Oceanography: PAR is extremely important in the study of oceans as it helps in the calculation of euphotic depths.

The euphotic or photic zone forms the uppermost layer of the ocean receiving the sunlight using which the phytoplanktons undergo the process of photosynthesis.

Most aquatic life resides in this zone as it is the layer with abundance of sunlight.

Read more: Ecosystem - What are its components?

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Measurement Units of PAR 

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The process of photosynthesis is a chemical process that is dependent on the number of photons available in a specified time. PAR is measured in irradiance or the radiant flux received by a surface per a given surface area. The units of PAR are:

• PPF - Photosynthetic Photon Flux.
• PPFD - Photosynthetic Photon Flux Density (measured in moles per m² per second)
• YPF - Yield photon flux (It measures the different weightage for different wavelengths)
• YPFD - Yield photon flux density 

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Second Law PAR Efficiency 

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The quality of radiation reaching the plant is as important as calculating the amount of PAR reaching the plant. The radiation reaching the plant has entropy along with energy. Entropy is a state of disorder, randomness, or uncertainty. We have to combine both energy and entropy to get energy.

• The exergy represents the amount or measure of useful work and its analysis is called exergy analysis or second law analysis. 
• One of the advantages of working with the exergy is that it does not only depends on the temperature of the emitter (the Sun), but also on the temperature of the receiving body (the plant), i.e.,
• It includes the fact that the plant is emitting radiation.
• The conversion factor of the organism will be different depending on its temperature, and the exergy concept is more suitable than the energy one.

• In the diagram above, we can find the exact range of PAR being absorbed by plants and other organisms for the completion of the process of photosynthesis.

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Things to Remember

• PAR or Photosynthetically Active Radiation is the wave band or spectral range of the sun’s radiation from which organisms use for the process of photosynthesis. 
• Photosynthetically active radiation range is generally from 400nm to 700nm.
• PAR is measured as Photosynthetic Photon Flux Density (PPFD) as the process is dependent on the number of photons available in the specified time.
• PAR measurement is very important to evaluate agricultural land.
• It is also used in forestry and oceanography. 
• PAR is measured in irradiance or the radiant flux received by a surface per a given surface area.