One of the first biology lessons we are taught in school is that plants need sunlight to perform photosynthesis to “feed” themselves and live. However, the relationship between plants and light is more complex. In this post we will describe how different wavelengths are employed to control the growth or the flowering of plants.

A more formal definition of photosynthesis would be the process by which plants employ sunlight, water and carbon dioxide (CO₂) to produce the oxygen and the glucose they need. The part of the spectrum that it is used by plants for this process is called photosynthetically active radiation (PAR) and it covers approximately from 400 nm to 700 nm, which more or less matches the visible spectrum region.

Light is absorbed by a pigment named chlorophyll, which is located in the membranes of the chloroplasts (one of the organelles in the plant cells). Chlorophyll mostly absorbs blue (400 – 500 nm) and red light (600 – 700 nm) while it minimally absorbs green light (500 – 600 nm) and reflects most of it, being responsible for the green color of plants.

Nevertheless, although only PAR wavelengths are used for photosynthesis, ultraviolet (UV) light (250 – 400 nm) and infrared light (700 – 850 nm) also affect plants. The effect of the different wavelengths, from shorter to longer, over plants, is the following:

• Ultraviolet (UV) light (250 – 400 nm): in general, it is considered harmful for plants. However, UV light stimulates the production of proteins that protect the plants against pests and diseases as well as antioxidants, which also protect the plant and increase its nutritional value. It can also enhance leaf coloration and thickness.

• Blue light (400 – 500 nm): it is associated with the activation of the photoreceptors cryptochrome and phototropin and it can be employed to control the growth of the plant. A large amount of blue light produces shorter stems and thicker leaves, while reducing the plants exposure to it will lead to longer stems and a larger leaf surface area, although a minimum amount of blue light is always required to ensure the plants proper development. Blue light also controls the orientation of plants, as they tend to grow in the direction of the blue light source. This behavior is known as phototropism and is controlled by phototropin. Blue light can also enhance leaf coloration.

• Green light (500 – 600 nm): leaves reflect between 10% and 50% of green light. Nevertheless, green light is still important for photosynthesis. Red and blue light tend to be absorbed by the leaves located in the higher parts of the plant but green light is still available for the lower leaves, enabling them to perform photosynthesis.

• Red (600 – 700 nm) and infrared (700 – 850 nm) light: they are associated with the activation of photoreceptor phytochrome. There are two forms of phytochrome, one absorbs red light (Pr) and the other infrared light (Pfr). Pr is converted to Pfr when absorbing red light and Pfr is converted into Pr when absorbing far red light. The proportion between the two forms of phytochrome tells the plant which type of light is receiving. In environments with many plants, red light will be used for photosynthesis while most of the far red light will be reflected. An important proportion of the plants, especially those in the shade, will receive more far red than red light. These plants will sense that they require more light for photosynthesis, triggering stem elongation and leaves expansion in order to capture more light and resulting in taller plants. This behavior is known as shade avoidance response. On the contrary, taller plants that receive more red light will focus on expanding their branches instead of vertically growing.The proportion between red and infrared light received by the plant, which depends on how long the night is, also determines flowering. When Pfr concentrations are lower and Pr concentrations are higher, short day plants flower while when the situations is the opposite, long day plants flower.

Previous paragraphs tell us how different wavelengths can help to potentiate or mitigate plants development and can even be strategically employed to trigger growth or flowering, for instance. Here,  the use of LED and multiLED light sources at specific wavelengths could be helpful for plant stimulation and has recently emerged as an important technique to control plant production.

Written by J.J. Imas

Bibliography

Plant morphology and spectrum: How plants respond to light quality, Jillian Whitehead, P. L. Light Systems

The Definitive Guide to Grow Light Spectrum, Lumigrow

The effect of light spectrum on plant development, Canna gardening

The effect of red and far red light on flowering, Canna gardening

Photosynthesis, Resource Library/Encyclopedic entry National Geographic