White Light

White Light

White Light


Horticulture lighting technologies have improved dramatically over the past century, but manipulation of the light spectrum is fairly new. Since plants tend to absorb red and blue light most strongly, other wavelengths have been ignored for plant growth, pink/purple light fixtures flooded the horticulture lighting market, as LED technology allowed individual spectra.

However, further research on the interactions between plants and light confirmed that plants need more than those 2 individual spectra for optimal growth, such as benefits from light in the 550 - 600 nm range as well as in the far-red and ultraviolet wavebands.

Spectral power distribution chart

High Pressure Sodium Lamps
Measurements of Normalized Photosynthetic Photon Flux


How do Plants use Light

Although humans and plants perceive much of the light through the same molecules, our eyes are often fooled. Narrow band red, blue, and green light in a certain proportion are perceived as white light to the human eye, but a plant knows that it is receiving 3 individual spectra, as each individual wavelength may promote a different growth habit and photo-morphogenic response.

Depending on the source, broad spectrum white light comes in many forms. Human eyes perceive these different spectra as cooler (blue) or warmer (orange/red), depending if it is a metal halide or high-pressure sodium lamp, or what type of phosphor coating is used with LEDs and fluorescent bulbs.

Ceramic Metal Halide lamps
Measurements of Normalized Photosynthetic Photon Flux

Incoming photons are absorbed by pigments, which absorb light as energy, and photo-receptors which perceive light as a signal. In case of the chlorophyll pigments, photons are used to drive photosynthesis and growth.

However, just chlorophyll, with its wide-ranging absorption spectrum, is not enough to efficiently harvest light. The “antenna complex” is a concept that describes how accessory pigments, such as carotenoids, assist in capturing light that chlorophyll does not absorb, and dissipate excess light as heat (non-photochemical quenching), when photosynthetic reaction centers are overloaded with incoming energy.

T5 Fluorescent lamps
Measurements of Normalized Photosynthetic Photon Flux


Accessory pigments

Chlorophill-Magnesium molecule structure

These are primarily carotenoids such as beta-carotene, lutein, zeaxanthin, antheraxanthin, and violaxanthin. These pigments are yellow to orange in color and absorb most strongly in the range of 450 - 550 nm. Some of these pigments change forms, based on lighting conditions, through processes called epoxidation and de-epoxidation.

Fluence is the sum of the energies contained in all particles - photons and neutrons - per unit area, with which a material is irradiated. If this fluence is too high, damage can occur to the photosynthetic apparatus. Therefore, it is critical for a plant to manage this incoming energy, by adjusting the antenna complex to accept or dissipate light during each 24 hr natural light/dark cycle.

When fluence is low, violaxanthin will capture photons and transfer this energy to chlorophyll, thereby improving the efficiency of light absorption. When fluence is high, violaxanthin is de-epoxidated (converted) into zeaxanthin which then dissipates excess photons as heat.

Beta-carotene functions similarly to violaxanthin, and lutein functions like zeaxanthin but without this interconversion process called the “Xanthophyll Cycle.” This flow of energy between pigments occurs spontaneously as they become “excited” by photons.

Carotenoids serve similar functions within the eyes of humans and many animal species. There are several other plant pigments not associated with the photosynthetic light-harvesting complex, such as anthocyanin and lycopene.

They absorb light, but their main function is to protect cells and DNA from damaging UV radiation, and scavenge “free radicals”, like H2O2, to prevent further cellular damage.

Metal Halide
Measurements of Normalized Photosynthetic Photon Flux


Photo/Light Receptors

In most cases they are proteins paired with a “chromophore” that absorbs certain wavelengths of light and then sends a signal to the plant that influences photo-morphogenesis. There are several different types of photo-receptors, whose light absorption ranges overlap. Some of them are:

  • Cryptochromes use light in the range of 300 - 500 nm, whereby it absorbs 350 nm (UV-A) and 450 nm (blue) most strongly. When excited by light, this receptor prevents elongation of the main stem of seedlings, and mediates flowering and light-period in some species.
  • Photo-tropins are also blue/UV-A absorbing photo- receptors but with a much stronger absorption peak at 450 nm and are thought to regulate:
    • light-tropism, the process in which plants move in response to light,
    • stomatal aperture, opening and closing,
    • movement of chloroplasts (photosynthetic organelles) within leaf cells, and
    • inhibition of leaf expansion.
  • Phytochromes can strongly influence flowering. They absorb light from 300 - 800 nm, are constantly changing form, and reach a “photo-equilibrium” that is regulated by spectral ratio and PPFD present in the growing environment. Depending on this equilibrium, different signals are sent to metabolic pathways within the plant that regulate many processes including:
    • flowering,
    • light-period,
    • germination,
    • leaf expansion,
    • stem elongation,
    • seedling establishment,

Different ratios of light received by a plant will dictate how the plant develops in terms of compactness, flower size, flower number, etc.

Because of this overlap, most photo-morphogenic responses are co-regulated, such as turning on/off by one receptor. The expression of that response can be amplified by another receptor.

The perplexing “circadian clock” is a culmination of activity from multiple photo-receptors entraining a rhythm of growth patterns based on light-period, light spectrum, and PPFD.

This rhythm of growth patterns within a plant strongly influences photo-morphogenic outcomes however, just like photosynthesis, there is an action spectrum for all photo-morphogenic responses that is dictated by a mixture of signals from these photo-receptors and does not necessarily mirror the absorption spectrum.

Broad spectrum vs supplementing specific wavelengths

When considering using narrow band lighting, the question is:

  1. which crops you are growing,
  2. are your plants exposed to broad spectrum light:
    • solar radiation for greenhouses, or
    • a broad-spectrum fixture for sole-source lighting applications.

In case of a sole-source fixture, supplementing with narrow band lighting makes only sense if one needs a photo-morphogenic effect that the crop cannot achieve without it.

In case of solar radiation and a high Daily Light Integral (DLI is the daily amount of Photosynthetically Active Radiation (PAR) received as a function of light intensity and duration): Solar radiation is quite broad it may drown out the photo-morphogenic benefits of narrow band lighting.

If you supplement daily natural solar radiation with a narrow band fixture so as to increase DLI, inconsistencies in product quality may occur as solar radiation increases and decreases throughout the year, thus exposing crops to differing amounts of sunlight and narrow band light.

To solely induce a photo-morphogenic response such as coloring, compactness, or rooting, with a constant DLI, it may make sense to add more blue light.

If you are growing a flowering/fruiting crop and only wish to encourage more flower/fruit growth (with a sufficient DLI and photo/light-period), it may be beneficial to add more red light, as 660 nm light encourages phytochrome responses in many species, which sends signals throughout the plant to encourage reproductive growth.


Plants use several different photo-receptors and pigments that cooperatively regulate growth and development. Plants developed these photo-morphogenic responses under broad spectrum light and it is very rare for a certain species to express a response to narrow band lighting that cannot also be achieved by broad spectrum lighting given sufficient DLI.

Research shows how individual crops respond to different light spectra, and what type of lighting may be best for a crop.

Narrow band lighting can provide acceptable growth for many species, so long as there are no fluctuations in DLI, apart from the light supplied by the fixture.

However, supplementing with broad spectrum light has a more proven record of improving crop quality, consistency, and yield.

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