In land plants, leaves absorb mostly red and blue light in the first layer of photosynthetic cells because of chlorophyll absorbance. An exception to the predominance of chlorophyll is autumn, when chlorophyll is degraded (because it contains N and Mg) but the accessory pigments are not (because they only contain C, H and O) and remain in the leaf producing red, yellow and orange leaves. Accessory pigments such as carotenes and xanthophylls harvest some green light and pass it on to the photosynthetic process, but enough of the green wavelengths are reflected to give leaves their characteristic color. Bottom: PAR action spectrum (oxygen evolution per incident photon) of an isolated chloroplast.Ĭhlorophyll, the most abundant plant pigment, is most efficient in capturing red and blue light. Top: Absorption spectra for chlorophyll-A, chlorophyll-B, and carotenoids extracted in a solution. Because of their pigments, they form colorful mats of green, red and purple. These bacteria live in environments such as the bottom of stagnant ponds, sediment and ocean depths. Other living organisms, such as cyanobacteria, purple bacteria, and heliobacteria, can exploit solar light in slightly extended spectral regions, such as the near-infrared. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. This spectral region corresponds more or less with the range of light visible to the human eye.
Photosynthetically active radiation ( PAR) designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis.
Photosynthetically active radiation (PAR) spans the visible light portion of the electromagnetic spectrum from 400 to 700 nanometers.
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