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 photoreceptors is the last in a 4-part series.
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: Photoreceptors
Photoreceptors are non-pigment molecules that respond to changes in light intensity, quality, direction, and duration. Photoreceptors are not found in the LHC, but rather in other areas of the cell like the nucleus and mitochondria (Figure 1). Photoreceptors allow the plant to sense its environment and modify plant growth accordingly (Figure 2). For example, a plant can tell that a neighboring plant is shading it because the neighbor’s leaves block certain wavelengths of light, causing an altered light spectrum to reach the plant (Figure 2). Photoreceptorsfall into five broad categories, and we will give a short description of each along with the wavelengths of light it responds to:
1. Phytochromes convert between red-absorbing (Pr; 600–700 nm) and far-red absorbing (Pfr; 700– 750 nm) forms (Figure 3). The change is reversible: with sufficient far-red light, Pfr converts back to Pr (Figure 3). The ratio of red:far-red light is more important than the absolute amounts of each type of light. Phytochromes control seed germination, chlorophyll synthesis, stem elongation, the size, shape, and number of leaves, as well as the timing of flowering1. Generally, a low red:far-red (i.e., a high amount of far-red light) causes stems to elongate, leaves to grow longer and wider, and chlorophyll content to increase. A low red:far-red can also cause flowering to happen earlier2.
2. UV Resistance locus 8 (UVR8) is a photoreceptor that responds to UV-B wavelengths (280–315 nm). UVR8 regulates flavonoid biosynthesis and the circadian clock. Flavonoids have diverse functions: they can transmit chemical signals, regulate physiological responses, and modulate the cell cycle. UVR8 also regulates epidermal cell expansion (expansion of the plant “skin”), number of stomata (air pores), and it improves tolerance to UV-B light3. Relatively little information is known about the interaction between UVR8, wavelength, and plant development. So far, we know that increased UV-B light causes flavonoid content to increases, epidermal cells to expand, and stomata number to increase.
3. Chryptochromes respond to blue light (390–500 nm) and control many aspects of plant growth and development. Chryptochromes control stem elongation (etiolation), flowering time, the circadian clock, stomatal opening, and anthocyanin production. Generally, increased amounts of blue light reduce stem elongation, open stomata, and increase anthocyanin content. Blue light can promote flowering of long-day plants and inhibit flowering of short-day plants.
4. Phototropins respond to blue (390–500 nm) and UV-A (320–390 nm) light. They control many of the same responses as chryptochromes. Phototropins control stem elongation, stomatal opening, phototropism (bending of the plant towards the light), leaf solar tracking, and chloroplast migration (movement of chloroplasts to prevent mutual shading). Generally, increased amounts of blue light promote phototropism, leaf solar tracking, and chloroplast migration.
5. Zeitlupe family photoreceptors respond to blue light (390–500 nm) and regulate the circadian clock and flowering. Very little is known about how these photoreceptors function. We know that during long days, zeitlupe photoreceptors can induce flowering.
Figure 1: While chlorophylls and carotenoids are usually found in chloroplasts, phytochromes are typically found in the nucleus or mitochondria.
Figure 2: Plants can sense their environment and modify their growth accordingly. A tall plant receives the full spectrum of light from the sun. A shorter plant is shaded by its neighbor and so does not get the full spectrum of light. Instead it gets a spectrum that is low is blue and red light. Photoreceptors in the leaves of the short plant receive this altered light spectrum. The photoreceptors respond accordingly and signal to the short plant to grow a taller stem.