Photosynthetically Active Radiation (PAR)

Light intercepted by a leaf may be absorbed, reflected, or transmitted; the fraction absorbed depends on the spectral content of the radiation and the absorption spectrum of the leaf.

Photosynthetically active radiation (PAR) is light of wavelengths 400-700 nm and is the portion of the light spectrum utilised by plants for photosynthesis. Photosynthetic photon flux density (PPFD) is defined as the photon flux density or PAR. If PAR is low for a given species of plant, growth and carbon assimilation is limited, while too much PAR may damage the photosynthetic apparatus.

The spectral responses of all quantum sensors deviate from the ideal response to some degree. Spectral error occurs when measuring a light source that has a different spectral output than the light used to calibrate the sensor. This error occurs because no quantum sensor can perfectly match the ideal quantum response, which is defined as an equal response to all wavelengths of light between 400 and 700 nm.

The Apogee SQ-500 Full Spectrum Quantum Sensor has a response closer to that of an ideal quantum sensor than the SQ-110.

Plants sense light using photoreceptors, such as phytochrome, and use wavelengths outside of the PAR range – mainly within the UV and far-red light spectrums to sense and respond to their environment.

The plant canopy selectively absorbs red wavelengths (approximately 660 nm) more than far-red wavelengths (approximately 730 nm) resulting is a decrease in the red: far-red ratio of light toward the base of the canopy, such changes in light quality result in photomorphogenic changes in plant growth.

In agricultural production systems an understanding of these responses is central to optimising planting density and canopy management.

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