Photosynthetically Active Radiation (PAR)

Definition - What does Photosynthetically Active Radiation (PAR) mean?

Photosynthetically active radiation (PAR) is the available light required for photosynthesis to take place. The wavelengths of photosynthetically active radiation that plants can use are generally in the range of 400-700 nanometers (nm).

Photosynthetically active radiation is absolutely essential for a plant to survive, and plants grow fastest in conditions of higher PAR. Plants germinate, grow, bloom, and produce fruit during the summer months when PAR is at its highest. But as the levels of PAR begin to drop towards autumn, plants slow their growth, dieback and eventually go dormant.

Photosynthetically active radiation is generated by the sun and commercially available plant grow lamps.

MaximumYield explains Photosynthetically Active Radiation (PAR)

For photosynthesis to occur, plants require a certain amount and intensity of light measured as photosynthentically active radiation (PAR).

The amount of naturally occurring PAR is affected by the weather, season, and time of day. PAR is highest at midday during the summer months and reaches its lowest levels during the short days of January.

PAR values can vary between 0 to 3,000 millimoles per square meter per second (µmol (photons) /m2/s). PAR may be zero at night, but during the midday sun in the summer it can reach 2,000 to 3,000 µmol/m2/s.

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