The correct light coupled to the correct daylength (the length of time the lights remain on per day) can yield healthy, show-quality plants. The incorrect lighting for a particular plant can yield plants with bleached leaves (too much blue) or hyper-elongation of petioles (too much red or far-red). Learning how to interpret a light source’s spectra will enable you to grow the plant the way you envision it. Another blog detailing the impact of each major portion of the spectra is forthcoming. This article is an expansion to my 2020 AVSA Virtual Show program.
Chlorophyll a and b are the key components to photosynthesis, which converts light energy into carbohydrates. They have max absorption at various peaks throughout the spectrum, shown in the upper panel. Carotenoids, another plant pigment, also absorb light predominantly in the blue to cyan range. The original review with the spectral absorption pictured for the three pigments can be found here.
Natural light, or the sun, is rich with light that contains UV, visible, far red, and infra-red.
Artificial lights, notably fluorescent and LEDs, have sufficient coverage to encompass a plant’s photosynthetic needs between 400 (blue)-700 (red) nm, more so in the blue spectrum. However, note that the red portion of these lights do not fully and efficiently cover the red portion of the plant’s needs. Whether fluorescent/LEDs, commercially available lights often report color temperatures such as 3000K (warm), 4000K (cool), or 5000-6500K (daylight). Warmer lights have more red and emit a yellowish glow, while daylight appears very bright white due to more blue emission. The light you use can impact how your plant grows and develops, and even the coloration of the foliage and blooms.
The 400-700 nm visible region is termed Photosynthetic Active Radiation (PAR). PAR is determined by Photosynthetically Photon Flux Density (PPFD), jargon for the amount of energy that reaches the plant. This energy is measured in micromoles per square meter per second (umol/m2/sec).
Though scientists/botanists will often use umol/m2/sec, this standard has not been widely adopted by the commercial lighting industry. Most commercial lights express their output as Lumens or Lux, sometimes Foot-candles, all which can converted amongst each other and to umol/m2/sec.
The one drawback to the use of PAR meters restricted within the 400-700 nm visible range is that research in the last two decades have shown that far red (700-750 nm), infra-red (>750 nm-1000 um), and ultra-violet (10-400 nm) can impact plant growth and development. For more on this topic, please refer to forthcoming blogs.
NOTE: all spectral analyses were done using the Lighting Navigator spectrometer. Natural light measurement was done on a cloudy November 2019 day at high noon. Four-feet long T5HO Fluorescent and 5000K retrofit LED bulbs powered by Sunblaster, and Mieemclux pink 1500w LED lights were used.
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