Photosynthetically active radiation

Photosynthetically active radiation (PAR) is solar radiation in the 400–700 nanometer wavelength range that photosynthetic organisms can use to drive photosynthesis.

Photosynthesis: Photosynthesis converts light energy into chemical energy, commonly by using carbon dioxide and water to produce carbohydrates and oxygen. PAR supplies the wavelengths that power photosynthetic reactions.
Daily light integral: Daily light integral is the total photosynthetic photon flux received per unit area over a day. It accumulates PAR exposure into a measure useful for crop production.
Photon: A photon is a quantum of electromagnetic radiation that carries energy and momentum. PAR is often quantified as a flux of photons rather than radiant energy.
Ultraviolet radiation: Ultraviolet radiation is electromagnetic radiation with wavelengths shorter than visible light and longer than X-rays. It falls below PAR's conventional wavelength range, though it can affect photosynthesis and plant physiology.
Photosystem II: Photosystem II is a protein–pigment complex that uses light energy to extract electrons from water in oxygenic photosynthesis. Its pigments absorb PAR photons to initiate electron transport.
Leaf area index: Leaf area index is the one-sided leaf area per unit ground area in a vegetation canopy. Together with PAR, it helps quantify light interception by a canopy.
Wavelength: Wavelength is the distance between successive corresponding points of a wave, such as adjacent peaks. The 400–700 nanometer interval defines conventional PAR.
Far-red radiation: Far-red radiation is light at wavelengths just beyond the red end of the visible spectrum, commonly about 700–750 nanometers. It lies outside conventional PAR but can influence photosynthesis through interactions with red light.
Photosystem I: Photosystem I is a protein–pigment complex that uses light energy to transfer electrons toward the production of NADPH. It absorbs PAR to re-energize electrons during photosynthesis.
Canopy photosynthesis: Canopy photosynthesis is the combined carbon fixation of leaves across a plant canopy. PAR distribution through foliage constrains total canopy carbon uptake.

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