Photosynthetically Active Radiation

Photosynthetically Active Radiation, also abbreviated as PAR, is the radiation absorbed by the plants to carry out photosynthesis. It is a spectral range of wavelength of the solar radiation that is absorbed by the plants for photosynthesis. The PAR range lies within the visible spectrum of the solar radiation which is 400 nm - 700 nm, a spectral range that can be observed by a naked human eye. Although lying in a specific spectral range, the amount of PAR wavelength absorbed changes depending on the time of the day and the altitude at which the plant is growing. 

Why a Specific Wavelength For PAR Range?

As given above, the PAR wavelength lies in the spectral range of 400 nm - 700 nm, which is the visible wavelength range of the solar radiation. This range of the photosynthetically active radiation absorption, is the optimal range as it is not harmful by either being in excess or lower than required by the living organism. 

Any wavelength shorter than the PAR wavelength is an excess of energy for the plants. Shorter wavelengths tend to be characteristics of photons with intense energy which can result in the damaging of the cells and the tissues of the plants. But usually, such photons with shorter wavelengths are filtered out the ozone layer in the stratosphere. Going beyond the other end of the PAR wavelength, lies photons with long wavelengths and less energy. They do not carry enough energy to allow the photosynthesis reaction to take place. Thus, the plants have evolved to utilise the given spectrum of 400 nm - 700 nm of the solar radiation as the photosynthetically active radiation (PAR) which provides sufficient energy without causing any damage to itself.

Exceptions to this reasoning include a few cyanobacteria, purple bacteria, and heliobacteria. These bacteria are capable of absorbing extended wavelengths of the solar radiation as PAR such as the near-infrared region. These bacteria live in environments such as the bottom of stagnant ponds, bottom of sediments and many times the bottoms of the oceans. Sometimes at such extreme places, the reach is mostly only of the longer wavelengths like the near-infrared. Hence, the PAR range for such bacteria extends beyond the visible spectrum as well. 

Absorption of Photosynthetically Active Radiation (PAR)

To carry out the photosynthetic process, plants and cyanobacteria make use of pigments that help them absorb the photosynthetically active radiation. 

The main pigment, as quite well-known, utilised by the plants to absorb the energy in the PAR form from the solar radiation is the chlorophyll. It is the main pigment for absorption of the PAR in higher plants and the algae. Other pigments that also aid the plants in the absorption of solar energy, known as accessory pigments, include examples of carotene and xanthophylls. There are two types of chlorophyll present in the higher plants identified as chlorophyll ‘a’ and chlorophyll ‘b’. Chlorophyll absorbs blue and red wavelengths of the visible spectrum. Both the forms of chlorophyll show a slightly different absorption pattern from the spectrum. Also, the reason for the absorption of red and blue light, is because the rate of photosynthesis is higher at these wavelengths.

Plants also absorb some of the green wavelengths with the help of accessory pigments but most of the green light is reflected back. The speciality of the green wavelengths is that they can penetrate into the interior of the plants and stimulate photosynthesis and the growth of the plants. This facilitates the growth of the plants beneath their canopy. Along with the green wavelengths the yellow wavelengths also transmit through the chlorophyll to the entire plant.

The percentage absorption by the different pigments and the percentage of rate of photosynthesis at different wavelengths is given below:

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The Reaction Centres of Photosynthesis

There are hundreds of pigments present in the plants that help it to absorb the wavelengths falling in the PAR range. These pigments altogether form large light-harvesting complexes or the reaction centres of photosynthesis. These light-harvesting complexes are capable of absorbing all the different wavelengths of the solar radiation for an efficient photosynthetic process. The center stage is taken by the chlorophyll ‘a’ in the reaction centre as one chlorophyll ‘a’ molecule is present in the center of each of the reaction centre or the light-harvesting complex. 

These light-harvesting complexes consist of two photosynthetic systems. They are the photosystem I (PSI) and photosystem II (PS II). PS I usually absorbs maximum light at 700 nm and PS II at 680 nm. Thus, PS I is known as P700 and PS II is known as P680.

Photosynthetically Active Radiation Measurement

It is clearly known that the photosynthetic process is a chemical process. It is dependent on the pigments and the number of photons available in a given amount of time. Because it is dependent on the number of photons it can be measured quantitatively as Photosynthetic Photon Flux Density (PPFD). The unit of PPFD is M/(m2.s) (moles per meter2 per second) which is the moles of photons received per unit area per unit time.

Another way the PAR form can be measured is Yield Photon Flux (YPF). It is a quantitative and qualitative form of measurement of PAR form as it takes into account the different weightage of different wavelengths of the solar spectrum.

PAR measurement is very important in the evaluation of agricultural land. Adequate availability of PAR is a prerequisite condition for the productivity of a farm. The measurement of the rate of photosynthesis by PAR can also be used in forestry and oceanography. 

Conclusion on PAR

From the above article, it is learnt that PAR is an essential component of the photosynthetic process of the plant and hence, there needs to be a scientific system in place for the measurement of photosynthetically active radiation as in some manner it is the basis of all life on earth as it forms the part of the primary self-producing organisms in the food chain.