PAR stands for Photosynthetically Active Radiation; it represents the fraction of sunlight with a spectral range from 400 nm to 700 nm, usually expressed in µmol (photons) m-2 s-1 though it is also expressed in micro Einsteins OR it is the amount of light which is available for Photosynthesis. “Photosynthetically active” means the portion of the spectrum that plants need for Photosynthesis.
A measure of PAR is a measure of the amount of those wavelengths available to plants. In the process of Photosynthesis, plants use energy from sunlight to convert carbon dioxide and water into molecules of sugar (the simplest units of food). They need a certain threshold level of PAR to do this, which differs for different species of plants.
Photosynthetically Active Radiation (PAR) is measured by a Silicon Photovoltaic Detector.
This spectral region corresponds more or less with the range of light visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by protective compounds in leaves when plants are grown in the presence of UV light. Photons at longer wavelengths do not carry enough energy to allow Photosynthesis to take place.
PAR is needed for Photosynthesis and Plant Growth.PAR measurements are also used to calculate the Euphotic depth in the ocean. In oceans and lakes, the limits of PAR mark the lower boundary of the Euphotic Zone. Below that depth, plants can’t photosynthesize more food than they use up in respiration. How far PAR reaches depends on the clarity of the water.
Increase in PAR enhances the Light Reactions of Photosynthesis before reaching a saturation point. Beyond that saturation point, any further increase in PAR will not result in more light-fixation. The photosynthetic response to different levels of PAR varies with plant species and leaf position. Shade plants and shaded leaves harvest PAR more efficiently at low light levels but less efficiently at high light levels compared with sun plants and sunlit leaves. Light levels measured by PAR are very useful in agronomic and agricultural research.