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 anthocyanins and betalains is the third in a 4-part series. Click here to read about chlorophylls. Here to read about xanthophylls and carotenes. And here to read about photoreceptors.
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 Wavelengths for: Anthocyanins and Betalains
Anthocyanins and betalains are pigments that range in color from orange to red to purple to blue. It is these pigments that give berries, beets, and autumn leaves their colours. Anthocyanins are found in the vacuole of plant cells, which is where most water is stored (Figure 1 and 2). Anthocyanins and betaliains never occur in the same plant – it’s either one or the other (but usually it’s anthocyanin). Anthocyanins protect plant cells from various environmental stresses, including excess light, nutrient deficiencies, and salt stress. Anthocyanins may also act as a deterrent to herbivores such as aphids1.
Which wavelengths of light do anthocyanins and betalians absorb? Many of these pigments absorb in the yellow, green, UV-A, and UV-B wavelengths (280–400 and 500 – 550 nm; Figure 3). Providing plants with excess light at these wavelengths typically will not enhance photosynthesis because anthocyanins and betalains mainly function in a protective capacity.
However, like carotenes, anthocyanins and betalains can positively contribute to the appearance and health benefits of plant products. For this reason, some growers may wish to enhance anthocyanin content to give a plant a more purple or red colour. Modifying light spectrum is one method of altering anthocyanin content in a plant. Growers can increase anthocyanin content by increasing total light levels, as well as increasing levels of green and UV light. Inducing a stress on the plant (such as drought, temperature, or nutrient stress) can also stimulate anthocyanin and betalain production.
Figure 1: Each plant cell contains a large vacuole that function in storing water, minerals, and other water-soluble compounds. The vacuole also functions in keeping the plant cell from collapsing in on itself.
Figure 2: Red anthocyanins are stored in the vacuoles of these petal cells from a geranium flower petal. The vacuole typically occupies most of the space within a plant cell.
Figure 3: 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.