Measuring Photosynthesis & Consumer Interpretation: PAR, Lumens, PPFD & DLI

Words and images by Richard Jones.

Part 1: PAR

Photosynthesis is the biochemical process by which plants use light to generate fuel and activate various growth mechanisms. The spectral range of light that induces photosynthesis is called Photosynthetically Active Radiation (PAR). PAR is visible light that is measured from 400 to 700 nanometers (nm) and includes Violet, Blue, Green, Yellow, Orange and Red. Of these colors, Violet to Blue (400 to 500 nm) is associated with vegetating growth and Orange to Red (600 to 700 nm) is associated with flowering growth.

Although many plant pigments are not fully understood, the Horticulture Industry has developed a general understanding of wavelengths that are critical for vigorous terrestrial growth:

  • As is indicated by the absorption signature of Chlorophyll a and b, terrestrial plants primarily absorb photons in the vegetating and flowering range of light.
  • Carotenoids (Lutein, beta-Carotene, Lycopene, etc) are ancillary receptors that typically produce yellow and orange pigments in plants.
  • Flavoproteins control plant functions like phototropism, stomata activity and circadian rhythm.
  • Anthocyanin develops purple, blue and red pigments among some species and strains.
  • Phytochromes control plant functions like flowering, germination and circadian rhythm.
  • The Emerson Enhancement Effect accelerates growth by combining two photosystems, Red and Far Red.

Consumer Interpretation of PAR

If you’re shopping for a full-spectrum grow light that can take your crop from seedling/clone to full maturity, there are a few general details you may want to consider:

  • Horticulture light vendors should offer spectral images of their products. If a grow light company does not display genuine spectra or they provide clumsy cartoons, assume they are not truly serious about their grow light business.
  • Ideally, a grow light should detail the PAR spectrum with a logical explanation of photosynthetic coverage. For example, the Zenith Bud Cultivator spectrum is complemented by a precise overlay of critical absorption pigments.
  • The most effective full-spectrum grow lights are those that devote more energy to flowering than to vegetating. More Orange, Red and Far Red light will cause stretching and promote the development of larger flowers.
  • Absorption peaks need not be covered by a light in order to effectively grow plants. Rather, the adaptability of plants is such that they merely require solid coverage of their various photoreceptors. As is detailed by the Zenith Bud Cultivator graph, the strong vegetating signature effectively covers Chlorophyll a, b, Carotenoids and Flavoproteins.
  • Although Far Red (700 to 750 nm) is not part of the PAR spectrum, Far Red significantly increases the rate of photosynthesis by inducing the Emerson Enhancement Effect. The best grow lights emit about 10% of their energy as Far Red light.
  • Whether HID or LED, white light is important, because it widens overall PAR coverage. This expanded bandwidth typically includes Green and Yellow light which will penetrate the canopy, improve quantum yield and stimulate unique photoreceptors like Anthocyanin.

Part 2 explores Lumens.

Happy Growing from Zenith LED Grow Lights.

Words and images by Richard Jones.

Copyright © 2016   Zenith LED Grow Lights & ActionGroove Media Limited.

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