Part 2: Which Wavelengths do Xanthophylls and Carotenes Absorb?

growing tomatos

In order to choose the best light for growing your plants, it’s essential to understand which wavelengths of light are required for normal plant growth.  Plants are experts at capturing light energy and converting it into sugars through the process of photosynthesis.  The first step of photosynthesis is the absorption of light by specialized molecules called pigments that are found in plant cells.  In addition to pigments, plants have a number of other light receptor molecules known as photoreceptors.  We will explore the range and function of key plant pigments and photoreceptors and identify the wavelengths of light they absorb and respond to.  This article, which will cover xanthophylls and carotenes is the second in a 4-part series.  Click here to read about chlorophylls.  Here to read about anthocyanins and betalains.  And here to read about photoreceptors.

Plant cells have chloroplasts that convert light energy into sugar.  Each chloroplast has many light harvesting complexes (LHC) that absorb this light energy.  LHC have two main parts: the reaction center and the antenna.  The reaction center is a single chlorophyll A molecule (Figure 1).  The antenna is a mix of many pigments such as chlorophylls, xanthophylls, and carotenes (Figure 1).  A packet of light energy (photon) is captured by the pigments in the antenna and transferred to the reaction center where it is converted into two electrons.  These electrons are essential for photosynthesis.  Most pigments in the LHC are chlorophylls (about 65% of them)!  There are also xanthophylls (about 29%) and carotenes (about 6%)1.

Words of caution: a complex network of factors control plant growth and development.  This article focuses on just one of these factors: light spectrum.  When deciding which wavelengths of light will be best for your plants, consider how all factors (light intensity, temperature, soil, etc.) interact together.  It’s also important to remember that most of what we know about pigments and photoreceptors is derived from studies with the model plant Arabidopsis (the plant equivalent of the lab mouse) and much remains to be learned about other species.  Different plant species have variations in the chemical composition of their pigments and photoreceptors.  For this reason, pigments and photoreceptors from different species can have slightly different absorption peaks than the values listed here.

light harvesting complex with a beam of light energy
Figure 1: Diagram of a light harvesting complex (LHC) within the chloroplast of a plant cell. The LHC is made up of the reaction centre (a single chlA molecule) and the antenna (a mix of chlorophylls, xanthophylls, and carotenes).

Light Wavelengths for: Xanthophylls and Carotenes

In addition to chlorophylls, antenna complexes also contain xanthophylls and carotenes, which are two classes of pigments within the carotenoid group.  Typically, xanthophylls are yellow while carotenes are orange.  It is these pigments that give carrots, yellow peppers, and pumpkins their color.  Xanthophylls and carotenes absorb wavelengths of light that chlorophylls cannot absorb.  They also function as a sunscreen for the plant by protecting it from the potentially harmful effects of high light2.  Lastly, some carotenes are also important for the long-distance flow of electrons within the LHC3.

Which wavelengths of light do xanthophylls and carotenes absorb? Many of the carotenes and xanthophylls absorb in the wavelength range of 425 to 475 nm4.  Beta-carotene has the highest absorption at 450 nm while the xanthophylls lutein and vioxanthan absorb the most at approximately 435 nm (Figure 2)4.  To optimize photosynthesis, your plants should be provided with light that satisfies the wavelengths requirements of these pigments.  Although these pigments absorb in the UV region of the spectrum, growers must be very cautious when growing with UV lights.  These wavelengths can cause oxidative damage and modify DNA of plants and humans.

Some xanthophylls and carotenes contribute to the smell and flavor of a plant and have positive effects on human health5.  For this reason, some growers may wish to increase the concentration of xanthophylls and carotenes in their plants.  Synthesis of these pigments depends on several factors, to include light.  In particular, blue and UV light can is known to increase carotenoid content in some species6.  Our most popular grow light, the Optilux, has a spectrum designed to satisfy the absorption requirements of xanthophylls and carotenes.

Figure 2: Light absorption for various plant pigments and photoreceptors. Chlorophyll A is the most abundant pigment and chlorophyll B is the second most abundant. Xanthophylls (lutein and vioxanthan) and carotenes (beta-carotene) are the next most abundant pigments. Anthocyanins (malvidin, pelargonidin, and chrysanthemin) and photoreceptors (phytochrome) are also essential for how a plant senses its environment.

Dr. Vanessa Nielsen completed her Ph.D. at the University of Toronto, Canada in plant biology and physiology.  She uses this background and fascination with novel lighting technologies to research the impact of light on plant yield at URSA Lighting.  Her background and extensive experience in a plant biotechnology lab offers a unique perspective on lighting for the cannabis industry.  Vanessa is always happy to share the best industry practice in cannabis growth and the latest discoveries of how to optimize lighting conditions for your plants.

One Comment

  1. canadian bacon

    Really interesting, helps me understand alot more about the way plants reap that good sunshine

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