What light is for
Photosynthesis
Light powers sugar from water and carbon dioxide, then starches, cellulose, and (with nitrogen) amino acids
Life-cycle signals
Light also sets direction of growth and whether the plant stays vegetative or starts flowering
Usable band
Cannabis draws energy from about 400 nm (blue) to 730 nm (far-red), wider than what people see well
Cost note
Blue photons cost more electricity to make than red, but the plant gets the same energy per photon, so mostly-red light is cheaper to run
Chlorophyll and helper pigments
Chlorophyll A
Peak absorption near 430 nm (blue) and 662 nm (red)
Chlorophyll B
Peak absorption near 453 nm (blue) and 642 nm (orange-red)
Chlorophyll build
Synthesis itself peaks near 435 and 445 nm in blue, and 640 and 675 nm in red
Carotenoids
Orange pigments grab blue light near 450 nm and blue-green near 475 nm, feed photosynthesis, and shield chlorophyll from excess light
Flavonoids
Anthocyanin and related pigments take blue and UV so chlorophyll is less likely to be photo-destroyed
Xanthophyll heat bank
Hidden yellow
Xanthophyll works from about 400-530 nm and only shows as bright yellow when chlorophyll fades, such as after nitrogen shortage
Dawn form
At low light it sits as violaxanthin, with peak reactions near 480 nm and 648 nm
High-light dump
As intensity rises toward a damaging level, violaxanthin soaks extra photon energy and becomes zeaxanthin
Night release
When light drops, zeaxanthin returns to violaxanthin over minutes to hours and releases stored heat
Energy share
Neoxanthin, lutein, and zeaxanthin can each pass more than half the energy they catch to chlorophyll
PAR versus what you see
PAR window
Photosynthetically active radiation is 400-700 nm and is counted as photons, not as human brightness
Human peak
People see yellow-green near 550 nm best, which is among the least useful bands for the plant
Full sun
Clear midday sun is about 2,000 µmol per square meter per second, varying with latitude and season
After absorption
A chlorophyll photon can become heat, fluorescence, or a chemical handoff that drives photosynthesis